Cosmochemical studies have proposed that Earth accreted roughly 5%–10% of its mass from carbonaceous
(CC) material, with a large fraction delivered late via its final impactor, Theia (the Moon-forming impactor).
Here, we evaluate this idea using dynamical simulations of terrestrial planet formation, starting from a standard
setup with a population of planetary embryos and planetesimals laid out in a ring centered between Venus and
Earth’s orbits, and also including a population of CC planetesimals and planetary embryos scattered inward by
Jupiter. We find that this scenario can match a large number of constraints, including (i) the terrestrial planets’
masses and orbits; (ii) the CC mass fraction of Earth; (iii) the much lower CC mass fraction of Mars, as long
as Mars only accreted CC planetesimals (but no CC embryos); (iv) the timing of the last giant (Moon-forming)
impact; and (v) a late accretion phase dominated by non-carbonaceous (NC) bodies. For this scenario to work,
the total mass in scattered CC objects must have been ∼ 0.2−0.3 M_⊕, with an embryo-to-planetesimal mass ratio
of at least 8, and CC embryos in the ∼ 0.01 − 0.05 M_⊕ mass range. In that case, our simulations show there are
roughly 50-50 odds of Earth’s last giant impactor (Theia) having been a carbonaceous object — either a pure
CC embryo or an NC embryo that previously accreted a CC embryo. Our simulations thus provide dynamical
validation of cosmochemical studies.

How to cite: Branco, D., Machado, P., and Raymond, S.: Dynamical origin of Theia, the last giant impactor on Earth, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-101, https://doi.org/10.5194/epsc2026-101, 2026.

Contact binaries are common among small bodies throughout the Solar System. Based on radar images,  ~10-30% of all NEAs larger than ~200 m are thought to be contact binaries [1,2], though their origin remains poorly explained. NASA’s Lucy mission recently encountered the main-belt asteroid 52246 Donaldjohanson (DJ), revealing its bilobate shape. DJ’s cratering age and YORP spin-down timescale are both consistent with the age of its collisional family (Erigone, ~150 Myr) [3,4], suggesting that its bilobate shape could be a direct product of catastrophic disruption and reaccumulation.

We perform smoothed-particle hydrodynamics (SPH) simulations of the collisional disruption of a 100 km asteroid followed by the subsequent gravitational reaccumulation of fragments using a N-body code with the soft-sphere discrete element method (SSDEM) for interparticle friction forces. With recent algorithmic improvements and GPU acceleration, we can simulate asteroid family formation using ~10⁸ particles, roughly ~3 orders of magnitude beyond previous studies, resolving the spins and shapes of km-sized fragments. A preliminary low-resolution simulation is shown in Fig. 1, where we find roughly ~20% of the reaccumulated fragments form bilobate shapes as a result of hierarchical growth of debris, leading to frequent low-speed mergers of similarly sized objects. In addition to explaining the formation of asteroid Donaldjohanson, this high contact-binary fraction suggests that catastrophic disruptions may be the dominant formation mechanism for contact binaries observed among NEAs. We will present what are, to our knowledge, the highest-resolution simulations of asteroid catastrophic disruptions to date, and explore how contact-binary properties depend on material properties and impact conditions (friction, impact energy, target rotation, etc.).

Figure 1: (a) A snapshot showing the gravitational reaccumulation of particles 4 hours after a collision between a 25 km projectile and a 100 km target at an impact speed of 5 km/s and angle of 45°. (b) Renderings of four km-scale contact binaries ~50 hours after the collision. Particle colors indicate their source location within the parent body. 

References
1. Benner, et al., in Asteroids IV, Radar observations of Near-Earth and Main-Belt Asteroids, 2015, 165-182.
2. Virkki, et al., PSJ, Arecibo Planetary Radar Observations of Near-Earth Asteroids: 2017 December–2019, 2022, 3:222, 36.
3. Marchi et al., PSJ,  A Pre-flyby View on the Origin of Asteroid Donaldjohanson, a Target of the NASA Lucy Mission, 2025, 6:59, 19.
4. Bierhaus et al., EPSC-DPS Joint Meeting 2025, An overview of the geology on the C-type Main Belt asteroid (52246) Donaldjohanson from NASA's Lucy flyby.

How to cite: Agrusa, H., DeMartini, J., Meier, T., and Michel, P.: Catastrophic disruptions as the origin of contact-binary asteroids, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-202, https://doi.org/10.5194/epsc2026-202, 2026.

Large impact basins provide key insights into planetary evolution by excavating and redistributing deep material. The South Pole–Aitken (SPA) Basin on the Moon and the Hellas Basin on Mars are among the largest known impact structures in the Solar System. Observations indicate that both impact events significantly modified crustal structure and exposed deep crustal and potentially mantle material (e.g., [1-4]). Numerical modelling allows these observations to be used to constrain basin formation and improve our broader understanding of basin-scale impact processes across planetary bodies.

3D numerical simulations of the SPA and Hellas basin formation were conducted using iSALE-3D [5-6]. Simulations explored impact velocities of 10–20 km s⁻¹, oblique impact angles of 20–45°, and a range of target thermal gradients representative of early lunar and martian interiors. Targets were modelled as spherical layered bodies and impactors as differentiated spheres, with materials defined by ANEOS-derived equations of state [7-10] and strength models from [11]. Simulations were run up to 5 hours post-impact, until crustal motions stabilised. Simulated crustal thickness, topography, and material distributions were compared with observation-based constraints, including basin morphology and crustal structure datasets [1, 12].

Best-fit scenarios for both SPA and Hellas favour moderately oblique impacts at an angle of 30° to the tangent plane, which produce elongated basin planforms that are consistent with the observed basin shapes (Figure 1a-b). This supports the idea that, for impacts large enough for planetary curvature to influence basin-forming dynamics, moderately oblique impacts produce elliptical basins.

Figure 1: Left: a best-fit SPA basin-forming simulation employing a 200km diameter impactor striking a target with a 30K/km near-surface thermal gradient and 1700K mantle at 11.1km/s and a 30° impact angle.  Inner and outer ellipses [13] and the centre of the basin are plotted. A northwards impact trajectory is assumed. Right: a best-fit Hellas basin-forming simulation employing a 480km diameter impactor striking a target with a 30K/km near-surface gradient and 1400K mantle at 10km/s and a 30° impact angle. Here, the crust is still deforming and has not yet reached its final state at 5 hours post-impact. An ellipse shows the approximate outline of the observed basin, and an eastward impact trajectory is assumed.  In the top figures, the change in crustal thickness induced by the simulation is shown, with areas of thickened crust in red and thinned crust in blue. In the lower figures, the integrated thickness of mantle above or entrained in the crust and within 5km of the model surface is shown. Each map shows the simulation at 5 hours after impact.

Basin-scale impacts are highly sensitive to target thermal structure and rheology, yet the pre-impact thermal states of both bodies are not yet well constrained. Simulation results show that impacts into warm, thermally weakened crust provide the best-fit for both SPA and Hellas, suggesting early formation times when lower crust and mantle temperatures were close to the solidus. In cases employing a cooler target, a thickened crust surrounds the basin whilst the mantle remains exposed at the basin centre. To produce a thin crust in the basin consistent with present-day observations, these scenarios would require a secondary buoyant crust to be produced from the mantle melt pool.

The preferred warmer scenarios produce extensive material redistribution, including crustal rafting that covers mantle exposed at the basin centre (Figure 2e-h) and asymmetrical, downrange “butterfly” patterns of ejecta (Figure 1c-d). Mantle material is excavated during both the excavation stage and central uplift collapse. However, continued post-impact crustal motion and inward crustal rafting bury or entrain much of this mantle beneath and into the redistributed crust. Thus, a large portion of the excavated mantle becomes mixed within the upper crust rather than remaining exposed at the surface. The precise trajectories and therefore the ultimate distribution of the material excavated by each of these basin-forming impacts remain uncertain.

Figure 2: Snapshots from an iSALE-3D simulation at different timesteps for a 200 km diameter impactor striking a 30 K/km Moon-like target with a 1700 K mantle at a 30° impact angle and 11.1 km/s velocity. The impact direction is toward the upper left (arrow) and the approximate outer rim of the basin (dashed outline) is shown in the final snapshot.

Ongoing and future work will investigate the effects of lateral variations in crustal thickness and thermal structure on basin formation and material redistribution. This will help better constrain impact direction and the fate of excavated material, while providing new insights into the role of basin-forming impacts in the early evolution of the Moon and Mars.

Acknowledgments: CB is funded by the Kentfield Scholarship. This work used the DiRAC Data Intensive service at the University of Leicester, managed by the University of Leicester Research Computing Service on behalf of the STFC DiRAC HPC Facility. This DiRAC service was funded by BEIS, UKRI and STFC capital funding and STFC operations grants. DiRAC is part of the UKRI Digital Research Infrastructure.

References : [1] Wieczorek, M.A. et al. (2013) Science 339: 671-675. [2] Melosh, H.J. et al (2017) Geology, 45 (12), 1063-1066. [3] Moriarty, D. et al. (2021) JGR: Planets, 126(1), e2020JE006589 [4] McNeil, J., et al. (2025),  PSC–DPS Joint Meeting 2025, Helsinki, Finland, Abstract EPSC-DPS2025-617. [5] Elbenhausen, D. et al. (2009) Icarus, 204:716-731. [6] Elbenhausen, D. & Wünnemann, K. (2011) 11th Hypervelocity Impact Soc. Symposium. [7] Thompson, S. L. & Lauson, H. S. (1972). Sandia National Laboratory Report, SC-RR-710:309p. [8] Benz, W. et al. (1989). Icarus, 81:113–131. [9] Pierazzo, E. et al. (1997). Icarus, 127:408–423. [10] Ivanov, B. A. et al. (2010). Geol. Soc. Spec. Pap. 465:29– 49. [11] Potter, RWK et al. (2012) Icarus, 220, 730–743. [12] Wieczorek, M. et al. (2022), JGR: Planets, 127, e2022JE007298. [13] Garrick-Bethell &  Zuber (2009),  Icarus, 204(2), 399–408. https://doi.org/10.1016/j.icarus.2009.05.032.

How to cite: Bill, C., Collins, G., Davison, T., Johnson, B., Wakita, S., Sokolowska, A., Coombes, O., Kallenborn, D., Grindrod, P., and McNeil, J.: 3D Numerical Modelling of the South Pole-Aitken and Hellas Basin-Forming Impacts, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-1156, https://doi.org/10.5194/epsc2026-1156, 2026.

Earth’s co-orbitals are asteroids in mean motion resonance with the Earth. They are of interest because of their dynamics that is typically stable and because they represent low Δv targets for asteroid mining companies. Recently, spectroscopic analysis of the co-orbital asteroid Kamo‘oalewa, and of two objects temporarily captured by the Earth, have indicated the possibility of a lunar origin for these bodies. Several subsequent studies addressed this possibility by means of numerical integration, especially for the case of Kamo‘oalewa, and attempted to identify the crater that could have launched it.

Here we compute the steady-state number of Earth’s co-orbitals deriving from lunar ejecta by computing their frequency and lifetime by means of long-term numerical simulations (about 50 My), paying attention to their co-orbital regime (quasi-satellite, horseshoe, tadpole or compound) and orbital eccentricity and inclination. The number and size are estimated by employing scaling laws that characterize the impact processes. In parallel, we compute the population of Earth’s co-orbital that the Main Belt can yield using NEOMOD3 [2]. Both outcomes are compared with the known population obtained from JPL Horizons [3].

The results show that the co-orbitals that the Moon can provide have a lower average eccentricity and inclination than those that the Main Belt can provide. Moreover, the existing population can be explained by the NEOMOD3 model. With NEOMOD3 the co-orbitals’ provenance is mainly from the inner Main Belt and we find that about half of them are in the tadpole regime while only 2 Earth Trojans are officially recognized, likely due to observational bias. The tadpole population in our results are eccentric and inclined, and we identify 3 additional objects as Trojans, namely, 2005 UH6, 2005 QQ87 and 2024 JR16.

Finally, the main source of uncertainty is given by impact process’ laws and thus we suggest that a systematic spectroscopic and dynamical characterization of Earth’s co-orbital can help reduce the uncertainty on those scaling laws.

The work that we will present is based on [1].

References

[1] Alessi, E.M., Jedicke, R., 2026. The steady-state population of Earth’s co-orbitals of lunar provenance. Icarus 455, 117109.

[2] Nesvorný, D., Vokrouhlický, D., Shelly, F., Deienno, R., Bottke, W.F., Fuls, C., Jedicke, R., Naidu, S., Chesley, S.R., Chodas, P.W., Farnocchia, D., Delbo, M., 2024. NEOMOD 3: The debiased size distribution of Near Earth Objects. Icarus 417, 116110.

[3] https://ssd.jpl.nasa.gov/horizons/

How to cite: Alessi, E. M. and Jedicke, R.: Can the Moon be a source of Earth’s co-orbitals?, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-587, https://doi.org/10.5194/epsc2026-587, 2026.

Lunar impact flashes (LIFs) provide direct constraints on the flux and physical properties of meteoroids impacting the Earth–Moon system. Conventional monitoring is mainly performed in the visible wavelength range and is strongly limited by lunar phase, sky brightness, and observing geometry, resulting in sparse temporal coverage and a low probability of detecting rare energetic impacts.

We present the Twin Impact Lunar Telescope (TILT) network, a new infrastructure dedicated to monitoring LIFs in the short-wave infrared (SwIR). Each station consists of twin robotic 0.4 m telescopes operating simultaneously in the J band (1.25 μm) and Z band (0.9 μm), allowing rejection of false positives and multi-wavelength characterization of impacts. The systems use high-speed cameras operating at 60-100 FPS together with dedicated acquisition software (pyfcam) enabling on-the-fly compression and FITS cube storage.

The telescopes incorporate elongated carbon-fiber tubes and dedicated baffles allowing daylight observations of the Moon, substantially increasing observing time. TILT-1 is currently installed at Calern Observatory (Observatoire de la Côte d’Azur), while TILT-2 and TILT-3 are under construction. The objective is to deploy at least three stations at separated longitudes to maximize lunar coverage and increase detection of energetic LIFs.

The TILT network will support the Advanced ERC project LISTEN FLASH, which aims to combine telescopic observations of LIFs with lunar seismic detections. This unique approach will provide new constraints on the luminous efficiency of hypervelocity impacts, the flux of small bodies in cislunar space, and the properties of the lunar regolith and crust’s thickness.

We will also present some results from the TILT observations of the Geminids meteor stream in Dec 2025.  We detected 53 potential impact flashes and confirmed 11 of them through multi-frame observations, and independent detections by other observers. Our confirmed events range between magnitude +7.5 and +10.4, primarily in the V- and R-band. We obtained a higher rate of observations per hour than the expected values from existing campaigns, highlighting the importance of high-ZHR meteoroid streams for observing LIFs.

How to cite: Delbo, M., Lognonne, P., Sheward, D., Froissart, P.-Y., Avdellidou, C., Girard, P., Mauclert, N., Herrier, L., Rivet, J.-P., Mongellaz, B., Parra, T., Anfosso, N., Maeght, E., Saliby, C., Ferrero, A., Giacometti, F., and Angelini, M.:  The Twin Impact Lunar Telescope network and Geminids 2025 observations, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-664, https://doi.org/10.5194/epsc2026-664, 2026.

How to cite: Sheward, D., Avdellidou, C., and Delbo, M.: Statistical Analysis of Lunar Impact Flashes, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-1021, https://doi.org/10.5194/epsc2026-1021, 2026.

Introduction.

The Moon has long been considered a laboratory for collecting additional data to investigate impact craters, lacking an atmosphere and erosive processes caused by wind, water, and active plate tectonics that quickly modify landforms.

Past space missions, like the Lunar Reconnaissance Orbiter (LRO, [1]), have allowed to improve our understanding of impact structures and their formation, and the lunar surface more in general. For example, the LRO camera (LROC [2]) system can provide images in the panchromatic broad filter with a resolution up to 0.5 m (Narrow Angle Cameras / NACs), which can be furtherly combined to derive accurate digital terrain models (DTMs) of specific lunar features. These DTMs could allow the detailed analysis of the morphology of impact craters, and provide both constraints on the surface stratigraphy and ground-truth of numerical models of the formed impact structure [3, 4].

Currently, a new mission, “LUMIO” (LUnar Meteoroid Impacts Observer), dedicated to the observation of the impact process itself, is under development. It is an ESA 12U form-factor CubeSat mission for the lunar exploration [5, 6]. It aims to monitor and quantify the flashes produced by meteoroid impacts on the far side of the Moon from an L2 orbit, using its optical LUMIO-Cam designed for visible and near-infrared observations.

The collected data will be more accurate than the ground-based telescopes, due to the CubeSat closer to the observing event and unaffected by the influence of the terrestrial atmosphere. Thus, these will allow to outline the first accurate dynamical model of the meteoritical flux in the lunar environment. The lightcurve of the flash allows to derive the integrated energy E_remitted within the spectral interval Δλ on the lunar surface, and once known the luminous efficiency η, to derive the kinetic energy EK as E_r/η.

The combination of these two sets of measurements will offer a natural validation of impact models, in addition to constrain the lunar stratigraphy. This work shows the results of systematic modelling that investigates how the impact mass and velocity, as well as the near-surface target properties, can affect the final crater morphology, in the framework of the scientific activities conducted within the LUMIO mission.

Methods. A systematic numerical investigation has been carried out using iSALE shock physics code (https://isale-code.github.io/, e.g., [7, 8, 9, 10]). Strength and porosity were modelled using a Drucker-Prager model and the ε-α-porosity compaction model, respectively.

For this initial investigation, we simulated projectiles of increasing diameters (from 1 µm to 1 m) impacting at 8.5 km/s to 71 km/s on the lunar surface. The target is assumed as an infinite half space, made of a basaltic regolith-like material, with 12% porosity. Different values of cohesion, friction coefficient, and porosity were tested. In particular, we varied cohesion (from 5 Pa to 0.5 MPa), friction coefficient (0.55, 0.6, 0.7), and porosity (12%, 20%, 40%), to evaluate their influence on crater morphology.

Results.

The first results have highlighted that craters exhibit a bowl-shape morphometry and a scattered ejecta deposit when forming in higher cohesion targets, whereas a decrease in cohesion lowers the aspect-ratio. This difference is shown in Figure 1. For the impact velocity of 9 km/s, passing from one extreme to the other of the tested range of cohesion (1 kPa and 100 kPa, cf. Figure 1), the diameter of the final crater can increase up to a factor of three. The depth-to-diameter ratio varies from 0.22 to 0.47. This is comparable to the effect of increasing impact velocity from 8.5 km/s to 71 km/s with cohesion held constant, which increases crater diameter by a factor of about 2.5; however, increased impact velocity has a minimal effect on depth-to-diameter ratio, remaining between 0.22 and 0.26.

Figure 1. Final time step of a 1 m basaltic projectile impacting at 9 km/s on the surface, with temperature set to 293 K. The left and right panels show the Total Plastic Strain distribution and the temperature variations, respectively. Cohesion is set to 1 kPa and 100 kPa, respectively in the left and right panels.

Generally, for a given projectile velocity, the crater sizes scale linearly, but different trends were found for different impact speeds (cf. Figure 2).

Figure 2. Comparison between the depth-to-diameter ratios of impacts occurring on one-layer regolith-like surface with various cohesions, at two different impact speeds (9 and 13 km/s).

Finally, μm- and mm-scale craters show a slightly larger depth-to-diameter ratio than m-scale craters (0.3 vs. 0.25), and once normalized to projectile size they have greater dimensions (Figure 3).

Figure 3. Comparison between impacts occurring at the same 9 km/s speed, for different values of projectile size.

Future work. Future simulations will consider impacts on layered targets, which are a more realistic representation of planetary terrains (e.g., [11, 12]).

Acknowledgements.

We gratefully acknowledge the developers of iSALE‐2D/Dellen version (https://isale-code.github.io/), including Gareth Collins, Kai Wünnemann, Dirk Elbeshausen, Tom Davison, Boris Ivanov, and Jay Melosh. Some plots in this work were created with the pySALEPlot tool developed by Tom Davison.

This work has been funded by the Italian Space Agency through the agreement n. F43C23000340001 entitled “Supporto scientifico alla missione LUMIO”.

References.

[1] Keller et al. (2016) Icarus 273, 2−24.

[2] Robinson et al. (2010) Space Sci Rev 150, 81–124.

[3] Martellato et al. (2017) Meteorit Planet Sci 52, 1388−1411.

[4] Prieur et al. (2017) J Geophys Res: Planets 122, 1704−1726.

[5] Cipriano et al. (2018) Front Astron Space Sci 5, 29, 23 pp.

[6] Topputo et al. (2023) Icarus, 389, 115213.

[7] Amsden et al. (1980) Los Alamos Nat Lab Rep LA−8095, 101 pp.

[8] Collins et al. (2016) iSALE-Dellen manual, figshare.

[9] Collins et al. (2004) Meteorit Planet Sci 39, 217−231.

[10] Wünnemann et al. (2006) Icarus 180, 514−527.

[11] Hopkins et al. (2019) J Geophys Res: Planets 124, 349−373.

[12] Martellato et al. (2020) J Geophys Res: Planets 125, e2019JE006108.

How to cite: Martellato, E., Rice, P., Luther, R., and Borin, P.: Impact Flashes: exploring the far side of the Lunar surface, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-1276, https://doi.org/10.5194/epsc2026-1276, 2026.

Introduction: The Chicxulub meteorite impact crater, located on the Yucatán Peninsula in Mexico, is the remnant of a significant short-term deformation process [1, 2]. The crater with a diameter of 200 km, contains a morphological peak ring 80 to 90 km in diameter [2, 3] (Fig. 1a). Peak rings are hallmarks of many large impact structures in the Solar System, formed toward the end of cratering [2, 4]. Elucidating the deformation kinematics of peak-ring formation is of particular significance for better understanding the formation of this class of complex impact craters. The IODP-ICDP Expedition 364 drill core revealed that shocked target rock of the Chicxulub peak ring is pervasively affected by decimeter-scale brittle shear faults (Fig. 2), apparently formed toward the end of peak-ring formation [5]. These shear faults lend themselves to infer the deformation kinematics under which the peak ring formed.

Figure 1. Drill site and lithological column of IODP-ICDP Expedition 364 drill core M0077. (a) Drill site and crater location on the Yucatán peninsula. Line labeled A/B shows the location of the cross section in Fig. 3. Background colors: Sandwell gravity anomaly map [7] (b) Schematic stratigraphy of the recovered lithologies (modified from [4]). The lower peak ring rocks exhibit pervasive evidence of brittle deformation.

Figure 2. Brittle shear fault surfaces in the lower peak ring rocks from drill core M0077. Striations indicate the slip direction, marked by a red arrow for the hanging wall.

Methods: The orientations and slip senses of 602 exceptionally well-preserved mesoscopic shear faults (Fig. 2), extracted from the drill core, were analyzed kinematically. This structural data set and its quality are unique among drill-core-based structural studies and offer a most original contribution to the field of impact cratering studies. The data, initially measured on core segments in a horizontal position, was rotated back to its in-situ position prior to extracting the principal kinematic axis orientations for selected fault subsets. The kinematic inversion was then conducted using the software FaultKin [6], which produced lower-hemisphere equal-area projections of the kinematic data, most importantly the orientations of the principal kinematic axes. Contouring of the principal kinematic axes and application of a Linked Bingham Analysis allowed us to visualize the dominant orientation of principal kinematic axes for each fault subset. The heterogeneity of shear faulting with depth was elucidated by employing a fault separation method specifically designed for this purpose. This method treated the data according to their occurrence with depth and did not eliminate “unfitting” data, which, unfortunately, is a common practice of other fault-slip inversion programs. Our approach resulted in 17 domains of fault subsets based on the examination of multiple cycles of fault separation, guided by metric, lithologic and structural separation criteria. Subsequent analysis of the domains enabled the identification of prominent deformation regimes.

Results and Discussion: A total of thirteen distinct kinematic regimes were identified with depth along the drill core, four additional zones exhibited ambiguous results. The identification of multiple kinematic regimes indicates that kinematic heterogeneity occurs on the 10 to 100-meter scale. The kinematic regimes are characterized by various principal kinematic axis configurations. Transitions from one regime to another appear to be incremental in most segments of the core, while sudden shifts in the fault kinematics are also observed. The kinematic axes oriented horizontally demonstrate variability in direction. In the case of domains exhibiting horizontal maximum shortening axes, the predominant direction is NNW-SSE (Fig. 3). By contrast, the horizontal maximum extension axes cluster mostly around ENE-WSW, i.e., concentric with regard to the crater center (Fig. 3). These directions do not align with numerically predicted horizontal strain axes.

Our inversion of the fault-slip data revealed two major kinematic regimes, depicting the local complexity of the cratering flow field beneath the peak ring (Fig. 3). Horizontal shortening with vertical extension is the predominant principal kinematic axis configuration (Fig. 3). This kinematic regime is mostly evident from approximately 748 meters below seafloor (mbsf) to approximately 1234 mbsf (Fig. 1b). Consequently, we propose that this regime is the one that ultimately resulted in the formation of the peak ring. However, horizontal concentric extension prevails at greater depths within the drill core, from 1235 mbsf to 1333 mbsf (Fig. 1b, 3). We propose that this occurrence is associated with horizontal dilatation at this depth, where a localized thrust shear zone seems to be present (Fig. 3).

Collectively, the deformation kinematics are consistent with rock flow during peak ring formation. This is marked by the occurrence of two simultaneously operating processes: rollback of the crater margin, and outward displacement of collapsed central peak material. These processes result in a radial velocity discontinuity at the location of peak-ring formation. The findings of this study refine current numerical impact simulations of peak ring crater formation with ground truth data derived from the drill core. Currently, numerical models cannot depict kinematic regimes at the same resolution as our fault-slip analysis does. Our work supports gravity-controlled cratering of the dynamic collapse model. While the impetus for this study was the Chicxulub meteorite impact and the associated IODP-ICDP Expedition 364 drill core, the results demonstrate that impact-induced brittle shear faults can principally offer valuable insights into the deformation kinematics during peak-ring formation through a kinematic fault-slip analysis.

Figure 3. Schematic radial cross section of the crater, indicating major kinematic regimes as red/blue arrows for extension/shortening, respectively. Black arrows are proposed material transport trajectories during late cratering. Morphology at T = 600 s, i.e., after the peak ring formed; green stippled line marks crater morphology at T = 236 s, i.e., at the beginning of central peak collapse (based on [5]).

References:

[1] Melosh H. J. (1989) Oxford University Press.

[2] Gulick S. et al. (2013) Reviews of Geophysics 51:31–52.

[3] Morgan J. et al. (1997) Nature 390:472–476.

[4] Morgan J. V. et al. (2016) Science 354:878–882.

[5] Riller U. et al. (2018) Nature 562:511–515.

[6] Marrett R. and Allmendinger R. W. (1990) Journal of Structural Geology 12:973–986.

[7] Sandwell D. T. et al. (2014) Science 346:65–67.

How to cite: Bahr, L. and Riller, U. and the IODP-ICDP Expedition 364 Scientists: Deformation kinematics of peak-ring formation: unique evidence from brittle fault analysis of the Chicxulub IODP-ICDP Expedition 364 drill core, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-103, https://doi.org/10.5194/epsc2026-103, 2026.

Introduction

Elongated craters on Mars represent a unique class of geological features formed by very low-angle impacts, typically below 10-15° from the horizontal. According to the ‘Moonlets Decay Hypothesis’, some ancient impact structures may have originated from the orbital decay of multiple moonlets formed from an equatorial debris disk generated by a giant impact, such as that thought to have resulted in the formation of Phobos and Deimos [1,3,5]. Thus, this work pursues two main goals: (i) building a database of already identified Martian elongated craters that contains additional morphological measurements (of ejecta, rim) and derived geometric properties (azimuth), and (ii) introducing a new methodology to constrain the direction of impact by investigating the depth variation of the crater floor, hereafter referred to as Deepest Point (DP) analysis.

Updated Database

We performed a systematic revision of prior efforts [8, 9] using CTX imagery acquired by NASA's MRO spacecraft, which provides significantly higher resolution than the Viking and HRSC images used in previous versions [1,2,3]. This procedure led to the removal of 46 misidentified features, including non-impact landforms such as irregular depressions, fractured zones, and double circular impact structures. After identifying and adding new valid craters, this new version of our database consists of 306 craters, covering latitudes from +65° to −65°. Each entry is associated with at least 24 descriptive attributes, including coordinates, ellipticity, azimuthal orientation, degradation level (DL), crater type, ejecta morphology, and deepest point measurements. The Robbins & Hynek (2012) global crater database [6] served as an additional validation tool.

Deepest Point Analysis

Previous studies and laboratory experiments [4,7] have demonstrated that from the sole post-impact distribution of the ejected material, it is possible to retrieve the direction from which the impactor came. We aim to provide the deepest point methodology for further constraining and/or confirming the impact direction from the analysis of the depth variation of the crater floor along its major axis.

During the systematic data collection for each crater in the renewed database, a measurable rim-to-floor elevation difference along the major axis was identified, with the deepest point consistently shifted toward the crater downrange. The deepest point is assumed to be the best approximation of the lowest area of the crater floor, representing a morphological signature not previously exploited as an independent directional indicator.

Using the MOLA DEM (1px/463m), topographic profiles were traced and extracted along the major axis of selected craters in QGIS. For each profile, the positions of both rims and the DP were recorded, and the Deepest Point Shift (DPS) was measured relative to the geometric crater center. A normalization procedure (left rim = 0, center = 0.5, right rim = 1) was applied to enable size-independent comparison. Selection criteria included: recent craters (degradation level DL = 1–2), single-impact morphology [6], major axis > 7 km, absence of a linear central peak, and presence of a clearly identifiable forbidden zone. A subset of 10 older craters was included for comparison. The final sampling consists of 56 craters.

Results

Among the 56 craters analyzed, 47 recent craters (RC) show a DP shifted toward the crater downrange, contrary to numerical predictions [4,7], yielding a reliability rate of approximately 85% (40 correct vs. 6 inconsistent), compared to ~58% for the 10 older craters (OC). These results confirm that the deepest point is a statistically robust directional indicator, mainly for well-preserved elongated craters.

Moreover, we distinguished craters into: (i) clear direction (constrained by well-preserved ejecta distribution), (ii) unclear direction (limited by resolution or degradation), and (iii) old craters (with intrinsically higher directional uncertainty and degradation state).

The correlation between DPS and crater major axis reveals a direct proportionality: the larger the crater, the greater the shift from the crater's center. No significant correlation was found between DPS and ellipticity alone, confirming crater size as the dominant control. Regarding impact angle, a rough correlation is observed between shallower impacts and absolute DPS values, but strong limitations were found due to the estimation of impact angles.

Conclusion

Revising the latest database and adding new features were fundamental steps to accurately distinguish and classify elongated craters in the new catalog version. Additionally, the deepest point finding demonstrates that the crater floor topography of elongated craters preserves a measurable record of impactor trajectory, providing a new morphological tool complementary to the traditional ejecta-distribution-based method. Numerical simulations and experimental analyses are necessary to better understand oblique impact processes and the crustal response at different impact angles.

References

[1] Schultz, P. H. and Lutz-Garihan, A. B.: Grazing Impacts on Mars: A Record of Lost Satellites, Journal of Geophysical Research, Vol. 87, pp. 84-96, 1982.

[2] Barlow, N. G.: Crater Size-Frequency Distributions and a Revised Martian Relative Chronology, Icarus, Vol. 75, pp. 285-305, 1988.

[3] Bottke, W. F., Love, S. G. and Tytell, D.: Interpreting the Elliptical Crater Populations on Mars, Venus and the Moon, Icarus, Vol. 145, pp. 108-121, 2000.

[4] Anderson, J. L. B., & Schultz, P. H. (2006, March). Flow-field center migration during oblique impacts: Implications for curved uprange ejecta rays. In 37th Annual Lunar and Planetary Science Conference (p. 1726).

[5] Craddock, R. A. (2011). Are Phobos and Deimos the result of a giant impact? Icarus, 211(2), 1150–1161

[6] Robbins, S. J., & Hynek, B. M. (2012). A new global database of Mars impact craters ≥1 km: 1. Database creation, properties, and parameters. Journal of Geophysical Research: Planets, 117(E5), 2011JE003966

[7] Elbeshausen, D., Wünnemann, K., & Collins, G. S. (2013). The transition from circular to elliptical impact craters. Journal of Geophysical Research: Planets, 118(11), 2295–2309

[8] Sefton-Nash, E., Faes, Z., Witasse, O., & Buchenberger, B. (2019). Alignment of Mars Elongated Crater Azimuths with Orbit Planes Representing Paleo-Equators. In 50th Annual Lunar and Planetary Science Conference (p. 3252).

[9] Buchenberger, B., Sefton-Nash, E., & Witasse, O. (2021). Analysis of topographic profiles of elongated craters on Mars. In European Planetary Science Congress (pp. EPSC2021-492).

How to cite: Pulvirenti, A., Sefton-Nash, E., and Witasse, O.: An Updated Database of Elongated Craters on Mars: Revised Population, New Attributes, and Deepest Point Analysis, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-121, https://doi.org/10.5194/epsc2026-121, 2026.

INTRODUCTION

Water ice plays a fundamental role in the geological history of Mars, human exploration, and the search for life. Its distribution constrains paleo-climate interpretations, while sub-surface ice concentrations reveal key depositional processes [1,2]. Impact craters serve as probes into deeper layers; their morphology reflects the mechanical properties and volatile content of the target material. Craters in ice-rich substrates exhibit lower depth-to-diameter ratios (d/D) than those in dry regolith due to differences in target rheology and post-impact viscous relaxation [3]. This effect is most pronounced in simple craters, which are highly sensitive to local substrate properties [4]. Impacts in ice-rich, layered substrate could also lead to the formation of terraced craters, indicative of a strength discontinuity/interface in the subsurface (Figure 1).

This study focuses on simple craters with terraced morphologies in selected martian regions. While some of these features have been mapped locally [2], we leverage high-resolution imagery acquired by several orbiters and convolutional neural networks (CNNs) to substantially expand the existing catalogs, building on our preliminary survey [5]. Our improved dataset, combined with surface topographic data, enables a large-scale, accurate analysis of morphometric properties, such as d/D and terrace dip angles, to better characterize the underlying stratigraphy responsible for terraced crater morphologies.

DATA & METHODS

We performed morphometric analysis on 22 craters in Arcadia Planitia (11 with single and 11 with double terraces) using the catalog by [2]. High-resolution DTMs were generated from HiRISE [6] and CaSSIS [7] stereo pairs; specifically, a dedicated observation campaign planned with the CaSSIS instrument (7-14 June 2025) led to the acquisition of four new stereo pairs. The study sites and the 11 intersecting SHARAD [8] radargrams used to investigate the subsurface structure are illustrated in Figure 1. In addition, CTX [9] images were employed to extend our mapping efforts to other martian regions. The positions of the terraced craters analyzed in this study, as well as those already present in [2], have been integrated into the MATISSE tool of ASI [10].

Morphological Analysis: Using QGIS and custom Python scripts, we compared the d/D ratios of terraced craters against a global sample of bowl-shaped craters. To validate the hypothesis of subsurface layering, we are currently using EchoTerraeTrace [11] software to analyze SHARAD data and studies subsurface features.

Deep Learning Algorithm: To expand our current terraced crater dataset, we adopt a supervised CNN-driven mapping approach that is using a YOLOv5x model, following e.g.  [12,13]. The model is fine-tuned on manually annotated CTX images, also incorporating negative examples (e.g., bowl-shaped craters) and data augmentation to prevent overfitting. During inference, the CNN generates georeferenced candidate detections with associated model confidence scores; default Non-Maximum Suppression is applied to remove duplicates. Model performance is validated using a withhold test set and Recall, Precision, and Average Precision (AP) [14].

RESULTS

Morphological Analysis: At the same latitude, terraced craters systematically exhibit lower d/D ratios than bowl-shaped ones, suggesting that subsurface ice reduces target material strength during impact (Figure 2). As all considered craters are smaller than 2 km, gravitational collapse processes are limited, making their morphology particularly sensitive to target material properties and volatile content. Variability in terraces number and arrangement further indicates a stratified subsurface with a range of mechanical properties, while shape and asymmetric terraces distribution can reveal impact angle and direction.

To validate the hypothesis of a layered substrate, we analyze SHARAD radar sounding data to identify subsurface dielectric interfaces (secondary returns); these detections can confirm stratigraphic layering and allow for the estimation of the dielectric constant (ε_r), providing a direct constraint on composition. Figure 3 illustrates a representative radar echo, differentiating surface and subsurface returns associated with distinct material properties.

Deep Learning Algorithm: In our withheld test set of 9 CTX images covering the mid-latitudes (Figure 4), the detector effectively distinguished terraced craters from bowl-shaped ones (and other features). At a model confidence of 0.8 and above, the detector identified a total of 350 candidates in the test set, corresponding to a recall of 0.63 (% of terraced craters found) and a precision of 0.92 (% of detections correct).

CONCLUSIONS & FUTURE WORK

The successful evaluation of our terraced craters detector in this initial analysis validates our overall approach of using deep learning to expand our feature catalog across the full longitudinal extent of the northern and southern mid-latitudes (32°-60°N/S) and potentially globally. The goal is to produce a comprehensive new catalog that will provide a robust foundation for further large-scale geographical, morphological, and morphometric studies, integrating both existing datasets and new high-resolution observations, including the planning of additional CaSSIS and HiRISE stereo pair acquisitions.

In support of the automated detection workflow, we employ an established multi-instrumental strategy that integrates morphometric, stratigraphic, and radar data to interpret the martian subsurface. Based on this integrated approach, we are performing a rigorous statistical analysis to identify significant correlations between terrace parameters, dielectric properties, and latitudinal distributions.

ACKNOWLEDGMENTS: This work has been developed under the ASI-INAF agreement n.2024-40-HH.0

REFERENCES: [1] Bramson, Ali M., et al., Geophysical Research Letters 42.16 (2015): 6566-6574.

[2] Bramson, A. M., Shane Byrne, and J. Bapst., Journal of Geophysical Research: Planets 122.11 (2017): 2250-2266.

[3] Douglass, B. S., & Bell, J. F. (2025) LPI Contributions, 3090, 2677.

[4] Robbins, Stuart J., and Brian M. Hynek., Journal of Geophysical Research: Planets 117.E5 (2012).

[5] Faletti, M., et al., EPSC-DPS2025-729, https://doi.org/10.5194/epsc-dps2025-729, 2025.

[6] McEwen, Alfred S., et al., Journal of Geophysical Research: Planets 112.E5 (2007).

[7] Thomas, Nicolas, et al., Space science reviews 212.3 (2017): 1897-1944.

[8] Seu, Roberto, et al., Journal of Geophysical Research: Planets 112.E5 (2007).

[9] Malin, Michael C., et al., Journal of Geophysical Research: Planets 112.E5 (2007).

[10] Zinzi, Angelo, et al., Astronomy and Computing 15 (2016): 16-28.

[11] 10.5281/zenodo.14728638

[12] Bickel, V.T., Nature Communications 16.1 (2025): 9583.

[13] Bickel, V.T., and Valantinas, A., Nature Communications 16.1 (2025): 4315.

[14] Bickel, Valentin Tertius, et al., IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing 13 (2020): 2831-2841.

How to cite: Faletti, M., Cremonese, G., Martellato, E., Bickel, V. T., Munaretto, G., Tullo, A., Bertoli, S., Costa, N., Marzari, F., Zinzi, A., Thomas, N., and Pommerol, A.: Terraced craters on mars: morphological and subsurface analysis in arcadia planitia and detection in additional regions via deep learning, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-235, https://doi.org/10.5194/epsc2026-235, 2026.

Introduction. New impact craters on Mars, discovered during spacecraft observation, are identified by their extensive "blast zones" of disturbed dust [1]. Distinct albedo features within these blast zones include quasi-spherical halos [2], extensive ejecta rays, and arcuate rays/scimitars [3]. We present a detailed analysis of a new type of feature now termed blast zone (BZ) streaks [4], which differ from the aforementioned. BZ streaks are elongated, unidirectional, and asymmetrical with respect to the crater center. They also have diffuse edges and low albedo compared to the pre-impact surface. Since these features resemble wind streaks formed by wind interaction with topographical obstacles like crater rims [5], we refer to them as "streaks." However, as they are significantly wider and longer relative to the crater rims than typical wind streaks, we hypothesize other formation mechanisms.

 In addition to local wind origins, we test other origins related to the projectile, the impacted target, and various shock wave fronts. Our analysis suggests that BZ streaks are likely of impact origin and indicate the direction of impact. 

Methods. Image data analysis. We analyzed map-projected RED images of 1,203 new impact sites on Mars [3] from HiRISE [6]. Using JMARS [7] and QGIS [8], we measured the azimuthal orientations of various features and the lengths of the streaks.

Local winds. We extracted annual average wind speeds at impact sites through JMARS map sampling [9] and obtained global circulation model data from the Mars Climate Database [10] for a representative climate scenario.

Proxy for prevalent winds. We identified sites where wind streaks or spire streaks [5], indicating strong wind directions, are near the BZ streaks and measured the angles between them to assess alignment.

Proxy for impact direction.  Ejecta asymmetry indicates impact direction [11]. We identified sites where the impact direction could be approximated using ejecta asymmetry and measured the angles between the downrange direction and the streak direction.

Entry & touchdown model. Using the CTH shock physics code for modeling impact and airburst phenomena [12], we simulated a 66-cm diameter meteoroid entering the Martian atmosphere at a 15-degree angle from horizontal. The simulation accounted for meteoroid ablation and surface-atmosphere interactions, excluding crater excavation. We analyzed the geometry and scalar quantities of resulting wind speeds and surface pressures.

Results. Feature identification. Among the 503 craters classified as “single” (indicating an intact projectile) [3], we confirmed BZ streaks at 106 sites (21%). We identified 54 sites (32%) as possible streaks based on diffuse asymmetric albedo features, despite lacking clear elongation. In clusters formed by projectile fragmentation, we found streaks at 76/700 sites (11%). We also noted 20 sites with wind/spire streaks near BZ streaks. Of the 106 single sites with streaks, 36 exhibit asymmetric ejecta, with 16 showing asymmetries allowing us to determine the impact direction within 30deg.

Local winds. No relationship was observed between map-sampled annual average wind speeds and BZ streak lengths, nor between BZ streak length and wind speeds from the Mars Climate Database. Figure 1b shows a lack of alignment between the directions of nearby wind streaks (proxies for prevalent winds) and BZ streaks.

Impact geometry. Lengths of BZ streaks correlate with crater diameter (Figure 2a). The distribution of differences between vectors indicating downrange impact direction and streak direction falls within half a quadrant (Figure 2b). Despite measurement uncertainties (5°-10° for BZ streak azimuth and within 30° for impact direction), the results suggest that BZ streaks predominantly point downrange from the impact direction.

Simulated winds. CTH simulations demonstrated that a vaporized jet from an ablated projectile can couple with the ground and travel over a significant distance. The wind pattern on the surface resembles the BZ streaks (Figure 3). Surface winds reach speeds of several hundred m/s, exceeding the typical wind speeds from the Mars Climate Database(tens of m/s).

Figure 1.a. Examples of local wind-driven streaks (orange) near BZ streaks (pink,ellipses). Arrows indicate directions.(Image: NASA/JPL/University of Arizona).b.The histogram of minimum angular differences between the directions of  these features.

Figure 2.a. Crater diameter and streak length are correlated. b. The histogram of the angular difference between the azimuths of downrange impact direction and BZ streaks.

Figure 3. CTH wind speeds at the surface level . The arrow points downrange to the impact point (0,0).

Conclusions.

Local wind theory. We found no relationship between the lengths of BZ streaks and average wind speeds, nor any clear alignment between the line directions of BZ streaks and nearby wind/spire streaks. Our results indicate that local winds are not the primary drivers of the BZ streaks.

Impact / projectile theories. The correlation between crater diameter and the length of the BZ streak implies an impact origin for the BZ streaks. This may be explained by larger impact energies generating stronger winds. Several potential sources of “impact wind” exist, including the quasi-hemispherical blastwave, the uprange wave shock behind the incoming projectile, and bow shock ahead of a projectile / a vaporized jet from a projectile coupling with the ground. However, the shape and proportions of the surface feature (i.e. pointy, narrow and long) along with their downrange direction, are all consistent with the bow shock or vapor jet dynamics. Our simulation also confirmed that winds generated from this origin can be stronger than local winds. Nevertheless, it remains possible that local wind fronts and “impact wind” fronts may interact with one another.

Implications. Our results indicate that the direction of the BZ streaks may offer a novel method for determining the impact trajectory. This approach is particularly valuable in cases where ejecta blankets are relatively axisymmetrical, obscured by other blast zone features, or influenced by topographic obstacles. Our findings also reveal that blast zone streaks are common, occurring at 21-32% sites, and due to their extended nature, BZ streaks play a significant role in enhancing impact detectability.

References: [1]Malin et al.(2006),10.1126/science.1135156 [2]Bart et al.(2014), j.icarus.2019.03.004 [3]Daubar et al.(2022),10.1029/2021JE007145 [4]Gao et al.(2023), 54th LPSC,2806 [5]Cohen-Zada et al.(2016),10.1002/2016JE005242 [6]McEwen et al.(2007),10.1029/2005JE002605 [7]Chistensen et al.(2009), AGUFMIN22,2009,IN22A-06 [8]QGIS.org [9]Adler et al.(2016),47th LPSC,2981  [10] Forget et al.(1999),10.1029/1999JE001025 [11]Schultz (1992),10.1029/92JE01508 [12]McGlaun et al.(1990),10.1016/0734-743X(90)90071-3 

Acknowledgements.A.S. is funded by a UKRI Fellowship EP/Z003180/1.

How to cite: Sokolowska, A., Daubar, I., Ramanathan, L., Boslough, M., and Gao, A.: The origin of asymmetric "blast zone streaks" formed by small impacts on Mars, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-1114, https://doi.org/10.5194/epsc2026-1114, 2026.

Introduction

Ganymede, Jupiter’s largest moon and a primary target of the ESA’s JUICE mission [1], shows a distinct surface dichotomy between older, low-albedo (dark ice) terrains and comparatively younger, high-albedo (light ice) regions [2]. The surface preserves a complex record of impact cratering, resurfacing, and shallow crustal heterogeneity. Ray and halo craters across these terrains display distinct ejecta patterns and crater floor characteristics, providing important constraints on subsurface stratigraphy and material properties; however, the layer thicknesses and mechanical contrasts responsible for these morphologies remain poorly constrained. Here, we use iSALE2D numerical simulations [3, 4, 5] (strength input parameters in Table 1), informed by global photomosaics of Ganymede [6], to investigate how subsurface layering controls the formation of representative ray and halo craters [7].

Results

Our models reproduce the observed ejecta patterns of Antum, Kittu, and Nergal (Fig 1) through distinct near-surface stratigraphies. Antum (Fig 1a) is best explained by a 1.0 km thick dark ice layer overlying a light ice substrate (Fig 2). Kittu (Fig 1b) requires a more complex sequence, with 0.8 km of light ice above a 0.4 km thick dark ice layer and a deeper dark basement (Fig 3). Nergal (Fig 1c) exhibits the most elaborate structure, consisting of alternating layers of light and dark ice above a deeper light ice unit (Fig 4). These configurations indicate that Ganymede’s upper crust is vertically heterogeneous and likely shaped by a combination of tectonic resurfacing, cryovolcanic activity, and the episodic deposition of dust and organic-rich exogenic material.

Fig 1. a. Antum, a 15 km crater with dark ejecta on dark terrain with a bright crater floor; b. Kittu (indicated by the white arrow), a 15 km crater with dark ejecta on light terrain with a bright crater floor and c. Nergal, a 9 km crater with dark halo encircled by bright ejecta on light terrain with a dark crater floor.

Table 1. Summary of the input parameters for the strength model of target (dark ice and light ice)

Comparisons between the ANEOS [8] and Tillotson [9] equations of state further show that the thermodynamic treatment of ice strongly influences excavation flow and final crater morphology. ANEOS produces nested cavities, double ejecta curtains, and relatively shallow craters with dense (potentially liquefied) crater floor material, whereas Tillotson yields deeper craters and more uniform excavation. The nested cavity formation in ANEOS is associated with vaporization and phase changes in ice, suggesting a more physically realistic representation of impacts into icy targets, albeit at higher computational cost.

Fig 2. Simulation results for Antum using Tillotson (a, c, e) and ANEOS (b, d, f). Dark ice indicated by dark grey and light ice with light grey.

Fig 3. Simulation results for Kittu using Tillotson (a, c, e) and ANEOS (b, d, f).

Fig 4. Simulation results for Nergal using Tillotson (a, c, e) and ANEOS (b, d, f).

Discussion

The results from numerical simulations align well with previous studies [7] which use the Z-model approach [10]. Despite simplifying assumptions (like homogeneous material properties within layers and vertical impact geometry), the simulations provide quantitative constraints on Ganymede’s near-surface stratigraphy and ice rheology. The inferred variations in layer thickness and strength contrast generate testable predictions for JUICE observations and contribute to geophysical models of Ganymede’s thermal and geological evolution. Future three-dimensional simulations incorporating oblique impacts and heterogeneous target properties will further refine these interpretations and improve our understanding of impact processes on icy satellites.

References: [1] Grasset et al., Planet. Space Sci., 78, 1-21, 2013. [2] Pappalardo et al., in Jupiter, 363-396, Cambridge Univ. Press, 2004. [3] Amsden et al., Los Alamos Nat. Lab. Rep., LA-8095, 1980. [4] Collins et al., Meteorit. Planet. Sci., 39, 217-231, 2004. [5] Wünnemann et al., Icarus, 180, 514-527, 2006. [6] Kersten et al., EPSC2022-450, 2022. [7] Baby et al., Earth Space Sci., 11, e2024EA003541, 2024. [8] Thompson & Lauson, Sandia Nat. Lab. Tech. Rep., 1972. [9] Tillotson, Gen. Atomic Rep. GA-3216, 1962. [10] Maxwell, D. E. Impact and explosion cratering, 1003–1008, Pergamon Press, 1977.

How to cite: Das, R., Baby, N. R., Karagoz, O., Kenkmann, T., Stephan, K., and Wagner, R.: Numerical Modeling of Ray and Halo Craters on Ganymede: Insights into Impact Cratering on Icy Targets, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-457, https://doi.org/10.5194/epsc2026-457, 2026.

The kinetic impact of NASA’s Double Asteroid Redirection Test (DART) on Dimorphos generated a massive ejecta cloud, including material with velocities comparable to or lower than the escape speed of the satellite. Most of the low-velocity ejecta is expected to remain gravitationally bound to the Didymos-Dimorphos system and to re-impact Dimorphos within the first 10 hours after the impact. Understanding the fate of this material is essential for interpreting the surface state of Dimorphos at the time of the ESA Hera encounter.

In this work, we introduce RAVEL (Regolith Astrodynamics in Variable Effective Low-Gravity Environments), a model particularly well suited for investigating the early post-impact evolution of low-velocity ejecta produced by the DART impact. Here, we apply RAVEL to ejecta with initial velocities of 1-9 cm/s, using a realistic shape model of Dimorphos derived from DART/DRACO observations. We couple three-dimensional orbital dynamics in the binary system with surface transport on shape models of Dimorphos. The model accounts for self-gravity, centrifugal acceleration, Coriolis acceleration, tidal forcing by Didymos, rebounds after surface contact, and friction-controlled sliding. The particles are treated as Lagrangian tracers, allowing us to map the main transport pathways from launch to final deposition without assuming detailed grain-scale properties. We assume a crater radius of 35 m, consistent with current estimates, although this value will be refined after Hera’s arrival. The initial tracers are launched with angles between 24° and 43°, measured with respect to the plane perpendicular to the ejection-cone axis, following DART observations, and with velocity distribution derived from a Housen-Holsapple-type scaling law. Surface motion is computed on the Digital Terrain Model (DTM) derived from DART/DRACO images (Fig.1). Since the DART/DRACO-based DTM only covers the leading hemisphere imaged during the DART encounter, the non-imaged part of Dimorphos is therefore represented as a smooth surface in the model.

_{Fig.1: DART/DRACO-based DTM. The yellow diamond marks the DART impact site.}

Our simulations show that the dynamics of low-velocity ejecta is highly asymmetric and strongly controlled by the binary environment. Ejecta with the lowest velocities (<5 cm/s) re-impact close to the DART impact site, whereas faster particles can follow partial orbits around Dimorphos or even circulate around Didymos before returning to the surface. More than 90% of the ejecta is reaccreted within the first 5 hours, and most of the ejecta returns to Dimorphos within the first 20 hours (Fig. 2). After re-impact, the material does not remain at the first contact point: rebounding and frictional sliding can redistribute ejecta over distances comparable to a significant fraction of the body radius.

_{Fig.2: Re-impacting low-velocity ejecta on Dimorphos' surface.}

The final deposits show clear velocity-dependent sorting. Low-velocity ejecta preferentially accumulates around the DART impact region, intermediate velocities tend to produce antipodal accumulation, and the fastest reaccreted material generates a broader and more asymmetric surface pattern, with enhanced deposition on the trailing hemisphere and in polar regions. When a rough DART/DRACO-based terrain model is used, the interaction between ejecta and local topography can produce ray-like deposits around the impact site, mainly associated with the lowest-velocity material. These structures may extend farther than shown in our simulations, but their full extent cannot yet be assessed because the non-imaged part of the DART/DRACO-based DTM is represented as a smooth surface.

Our model, RAVEL, provides direct predictions for Hera observations. Ray-like structures, smooth mantling deposits, preferential resurfacing terrains, or antipodal accumulations would provide constraints on the low-velocity ejecta population, the post-impact redistribution of material, and the mobility of regolith under extremely low gravity. Our results therefore link DART impact physics, orbital dynamics, and surface transport, and provide a framework for interpreting the geomorphological state of Dimorphos during the Hera encounter.

_{Fig.3: Distribution of low-velocity ejecta after re-impacting on Dimorphos’ surface.}

How to cite: Herreros, I. and Charnoz, S.: The morphological signature of DART ejecta distribution on Dimorphos: Predictions for the Hera encounter, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-345, https://doi.org/10.5194/epsc2026-345, 2026.

Bolide impact events represent an important connection between solar system dynamics and the evolution of Earth’s early atmosphere and biosphere. Preserved carbonate platforms offer important records of these processes, as these successions have the ability to capture changes in sea water chemistry and biological processes. The Archean represents a critical time interval in Earth’s geobiological evolution when large scale bolide impacts occurred frequently. The Neoarchean in particular covers a time period in which major changes occurred in ocean and atmospheric composition, crustal evolution and the biosphere, potentially contributing to the rise in atmospheric oxygen during the great oxidation event (GOE).

The ~2.63 Ga Carawine Dolomite (Hamersley Group, Pilbara Craton, Western Australia) preserves an extensive carbonate platform succession, including a bolide impact event expressed as an impact spherule bearing megabreccia (SBMB)¹. Notably, stratigraphically above the impact layer, we find the shallow-water expression of the first banded iron formation (BIF) of the Hamersley Group (recorded in the Marra Mamba Iron Formation) within the Carawine Dolomite. In this study, we aim to reconstruct a detailed stratigraphic framework representing a conformable transect from deep to shallow-water facies, including the SBMB and BIF, and a chemostratigraphy using major-, trace- and rare earth element (+ yttrium, REY) concentrations and inorganic/organic stable carbon isotope pairs.  

Seven stratigraphic sections were measured in the field, covering >400 meters of stratigraphy. One shallow and one deep-water section were sampled in high resolution (every 0.5 to 1 m) for geochemistry. The shallow-water section comprises a shallow subtidal facies containing tented microbialites and aragonite (pseudomorph) crystal fans; a storm facies consisting of oncolytic peloidal grainstone; and sub- to peritidal facies assemblage consisting of shallow water sedimentary features including thrombolites, ooids and exposure surfaces. REY-patterns are consistent with a shallow marine environment with episodic mixing of river water. The absence of negative Eu-anomalies indicates only minor contribution of hydrothermal fluids to the REY-budget. The storm facies shows elevated lithophile element concentrations and flat, “shale-like” REY-patterns, which probably reflects contamination with siliciclastic material during storm events. Additionally, the presence of local true negative Ce-anomalies may suggest local oxygen accumulation sufficient to oxidize Ce.  δ¹³C_org values in the shallow facies are typical for photosynthetic biomass (-30.4 to -21.8 ‰), possibly oxygenic photosynthesis given the negative Ce-anomalies. REY-patterns of the deep-water facies are consistent with typical Archean marine patterns and possess a positive La- and Eu-anomaly, superchondritic Y/Ho ratio and HREE/LREE enrichment.

Ce-anomalies are absent below the impact layer, but a notable shift to positive anomalies is observed stratigraphically above and persists after BIF deposition. We suggest the onset of a redox stratified ocean state post impact event, which initiated a manganese and iron shuttle. This oxygenation of shallow water settings may have been driven by an increase in nutrient availability (for example via upwelling due ocean mixing or enhanced continental erosion of volcanic products due to post impact uplift/volcanism) and increased productivity².

Manganese and iron were then oxidized, and elements including Ce were scavenged by Mn-oxide minerals, in the upper water column. After sinking, and transport across the redoxcline, these elements were released and consequently enriched in the anoxic water column, where Mn-oxides were reductively dissolved. Positive Ce-anomalies consequently imparted to deeper water carbonate sediments, recording the operation of this redox-driven Mn shuttle³.

In our partially stratified ocean model, BIF deposition was driven by a biological Fe pump⁴. In this model, Fe(II) to Fe(III) oxidation occurs above the redoxcline, while Fe(III) is subsequently microbially reduced to Fe(II) below the redoxcline via dissimilatory iron reduction (DIR). This Fe(II) is readily bioavailable for photoferrotrophy, which re-oxidizes iron to Fe(III). We propose that impact related Fe(II) oversupply driven by increased terrestrial and/or hydrothermal input⁵, and Fe-oxidation rates, exceeded the rates of DIR, resulting in BIF precipitation.

Collectively, our results suggest that bolide impact related perturbations may have been sufficient to drive changes in ocean chemistry (increased iron concentrations and other nutrients) and in microbial activity (increased Fe cycling), and potentially led to ocean redox restructuring and served as a trigger for BIF deposition in the Neoarchean.  Further research focussing on Nd and Fe isotopes in the BIF interval may reveal the source of iron and the role of photoferrotrophy. Additionally, expanded geochemical analysis of stratigraphic sections, including carbonate and shale units associated with the impact layer, could further resolve the extent, duration, and intensity of redox stratification.

1. M. Simonson, K. A. Schubel and S. W. Hassler, Precambrian Research 1993 Vol. 60 Issue 1-4 Pages 287-335

2. Drabon, A. H. Knoll, D. R. Lowe, S. M. Bernasconi, A. R. Brenner and D. A. Mucciarone, Proceedings of the National Academy of Sciences 2024 Vol. 121 Issue 44

3. R. Warke, H. Strauss and S. Schröder, Precambrian Research 2020 Vol. 344 Pages 105767

4. O. Konhauser, T. Hamade, R. Raiswell, R. C. Morris, F. G. Ferris, G. Southam, Geology 2002 Vol. 30 Issue 12 Pages 1079-1082

5. Glikson and J. Vickers, Earth and Planetary Science Letters 2007 Vol. 254 Issue 1 Pages 214-226

How to cite: Hoogland, V., Flannery, D., Ghidan, O., Murphy, D., and Nothdurft, L.: Bolide impact–driven ocean redox stratification and microbial Fe cycling as a trigger for Neoarchean BIF deposition, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-700, https://doi.org/10.5194/epsc2026-700, 2026.

Impact processes are generally regarded as destructive events in the history of our Solar System. However, previous experiments have shown that hypervelocity impacts into simple ice mixtures (e.g., H2O, CO2, NH4) can produce sufficient energy to synthesise more complex matter, such as amino acids [e.g., 1], as well as alter existing organic material, such as forming longer-chain organics when impacting with a polymer projectile [e.g., 2]. The energy produced on impact varies depending on the impactor and target compositions, as well as the impact velocity, which influence the peak temperatures and pressures achieved on impact. Currently, we rely on computational (‘hydrocode’) modelling to inform us on the conditions achieved during impact, which subsequently inform us on the conditions required for synthesis. However, water ice has a complex solid phase diagram, and consequently is a difficult material to model, therefore experimental data (specifically temperature and pressure) are sorely needed to test and validate such models. 

Previous work has demonstrated the ability to experimentally determine temperatures achieved on impact through studying impact flashes (‘self-luminous plumes’) produced during high velocity phenomena [3,4], with a preliminary concept for experimentally measuring impact pressures [5]. This work, developed by Spathis et al., used prototype strain gauges embedded in epoxy, which were subsequently implanted into ice targets. However, it was difficult to calibrate these sensors and, as they were bulky, that may have interfered with the shock propagation through the ice. In conjunction with Odin Space Ltd, we have now refined our pressure sensor system to ones which are calibrated to the 10s of GPa range. These more compact sensors are then implanted into the ice as before and connected to front-end electronics to read the sensors’ output. 

Initial impact experiments were performed using the All-Axis Light-Gas Gun at the Open University, where water ice targets (Fig. 1) were impacted using a 2 mm stainless steel projectile at 1.40 kms-1 and 5.14 kms-1. Results (e.g., Fig. 2) show the change in the speed of the shockwave for the two shot regimes as well as the decrease in the amplitude, although the wave seems to decelerate very quickly moving between the sensors. We have data to calibrate the pressure seen by the sensors to validate the readings for comparison with computational data, and hydrocode modelling is ongoing using the strength model described in Ma et al., 2023 [6]. Furthermore, studies into the structure of the ice will be performed using cryo-Raman spectroscopy, in collaboration with colleagues at the Open University, to investigate how the compression experienced during impact events influences the ice lattice. Finally, we hope to resurrect our homemade high-speed spectrometer constructed from an electric toothbrush, presented at EPSC 2025 [7], to investigate the spectra of the impact flashes. 

With ice being prevalent across bodies in our Solar System and studies into material and organic synthesis increasing, it is important to validate hydrocode modelling so we can confidently model impact events impossible to recreate in the lab (due to sizes/impact speeds). Additionally, with the search for life in our Solar System turning towards icy worlds, and missions such as NASA’s Europa Clipper and the European Space Agency’s first Large-class mission of Voyage 2050, ‘L4’, to Enceladus and the Saturn System aiming to search for habitability and signs of life on the icy moons, it is important to understand what detections (e.g., of organic molecules) have a biological origin, and which may be the result of environmental processes. Developing this system will allow us to experimentally constrain the conditions required for synthesis, or those where we expect none, which can provide valuable insight into sampling depth considerations, as well as useful references for contextualising mission data. 

Acknowledgements: The authors would like to thank Dr Z. Emerland and Dr M. Sylvest from the Open University Hypervelocity Impact Laboratory for operating the light-gas gun. 

References: [1] Z. Martins et al. (2013). Nat. Geoscience, 6, 1045.; [2] V. Spathis et al., 2021, LPSC Abstract # 1623; [3] V. Spathis et al., 2021, LPSC Abstract # 1625; [4] Spathis et al., 2021, IAC Abstract # 64949; [5] M. C. Price et al., 2021, LPSC, Abstract # 1328; [6] Ma et al., 2023, IJIE, 172, 104375; [7] M.C. Price & V. Spathis, 2025, EPSC Abstract # 200. 

Fig.1: Water ice target with embedded sensor system prior to impact 

Fig.2: Output from the pressure sensors embedded in an ice target impacted at 5.14 km/s with a 2-mm diameter stainless-steel projectile (blue: top sensor, red: middle sensor, black: bottom sensor). The decrease in the speed and amplitude of the shock wave can be clearly seen. 

How to cite: Spathis, V. and Price, M.:  Hypervelocity impacts into water ice: measuring shockwave propagation to validate hydrocodes , Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-1274, https://doi.org/10.5194/epsc2026-1274, 2026.

Introduction

The L type ordinary chondrites preserve a complex record of impact processing. Preservation of High pressure (HP)  minerals is not controlled solely by peak shock pressure, but also by the post-shock pressure–temperature–time (P–T–t) evolution during decompression and cooling [1]. To evaluate the factors controlling HP phase preservation and understand why HP phases are so common in L-chondrites, we studied three meteorites with contrasting shock records: Northwest Africa (NWA) 4137 (L6), Homestead (L5), and Kunashak (L6).

Materials and Methods

We investigated one polished thin section of each meteorite. The Kunashak and Homestead samples were provided by the Natural History Museum Vienna (NHMW). Optical microscopy, SEM, EPMA, and Raman spectroscopy were used to fully characterize the melt veins (MVs).

Results

All meteorites preserve shock-related features including irregular and planar fractures, undulatory extinction, mosaicism, and MVs, indicating heterogeneous shock conditions ranging from S2 to S4. NWA 4137 (Figure 1) is characterized by a coarse-grained groundmass crosscut by a network of MVs, including a prominent MV (MV1) up to ~300 μm wide and a few thinner MVs up to 100 μm wide. The groundmass consists mainly of olivine (Fo_69–74), orthopyroxene (En_73–76Fs_21–25Wo_1–4), clinopyroxene, plagioclase, Fe-Ni metal, troilite, chromite, and phosphates. Using EPMA and Raman spectroscopy, we identified within MV1 the HP phases wadsleyite, majorite, albitic jadeite, tuite, and xieite.

Irregular wadsleyite up to ~20 μm in size show characteristic Raman peaks near ~722 and ~918 cm^–1 (Figure 2). Majorite, confirmed by the characteristic ~927 cm^–1 peak (Figure 3), occurs as isolated euhedral crystals and as equigranular clusters with ~120° triple junctions within the MV groundmass, suggesting crystallization from melt. Majorite composition yields a ~23 GPa pressure estimate [2], consistent with experimentally produced majorite in the range of 17–23 GPa. Irregular albitic jadeite up to ~50 μm across show the characteristic jadeite-family Raman peak at ~700 cm^–1 (Figure 4) but have a composition by EPMA rich in vacancies, (Na_0.61-0.64Ca_0.08K_0.02-0.05□_0.25)^M2(Al_0.76-0.83Si_0.12-0.17Fe_0.02-0.08Mg_0-0.15)^M1Si₂O₆. Tuite and xieite were identified within MV1 through their characteristic Raman spectra: tuite has peaks at ~412, ~578, and 975 cm^–1 (Figure 5), while xieite has a peak at ~607 cm^–1 (Figure 6). Collectively, the observed HP assemblage defines shock pressures of approximately ~11–23 GPa.

In contrast, Homestead and Kunashak preserve abundant MVs and shock features, but no HP polymorphs. Homestead contains a complex MV network dominated by Fe-Ni metal and sulfides, with silicate minerals occurring as isolated grains or lithic clasts within the MVs. Kunashak hosts thick MVs up to ~900 μm wide composed of quenched silicate melt with olivine, pyroxene, plagioclase, Fe-Ni metal grains, and troilite droplets.

Discussion

The contrasting mineralogical records, despite similar shock stage and likely similar peak conditions, indicate that preservation of HP phases was controlled primarily by post-shock thermal evolution. In NWA 4137, the coexistence of majorite, wadsleyite, xieite, tuite, and albitic jadeite indicates crystallization under transient HP conditions followed by rapid quenching during decompression [3-5]. Experimental studies and thermal models demonstrate that MV cooling rates strongly depend on vein thickness, with thinner veins cooling on millisecond timescales. The ~100–300 μm wide MVs in NWA 4137 cooled rapidly enough to preserve metastable HP assemblages before complete pressure release [1, 6].

Conversely, the absence of HP phases in Homestead and Kunashak suggests decompression during a prolonged period at elevated post-shock temperature. Homestead preserves extensive metal–sulfide melting and pervasive thermal overprinting, indicating elevated post-shock temperatures and slow cooling. Such a path favors back-transformation of HP phases into low-pressure equivalents. Similar processes likely affected Kunashak, where shock deformation and MV formation occurred but thermal conditions during decompression inhibited preservation of HP minerals.

Conclusions

HP phases in the studied meteorites record local P–T–t paths within impact-related MVs. MVs are capable of recording extreme pressure conditions but may later be erased during thermal overprinting. Therefore, the absence of HP phases in shocked ordinary chondrites should not be interpreted as evidence of lower shock pressures. Instead, preservation depends strongly on the thermal trajectory during pressure release and cooling. The contrasting records preserved in NWA 4137, Homestead, and Kunashak confirm that impact processing within the L-chondrite parent body was highly heterogeneous with respect to shock duration, melt generation, and cooling history.

References

[1] Hu, J., Sharp, T. G. (2022). Prog. Earth Planet. Science, 9(1), 6.

[2] Collerson K. D. et al. (2010). GCA, 74: 5939–5957.

[3] Baziotis I., et al. (2025). Am. Mineral., 110(9):1472-1480.

[4] Tomioka N., et al. (2016). Science Adv. 2: E1501725.

[5] Chen M., et al. (2008). Chinese Sci. Bull. 53:3341–3345.

[6] Baziotis, I., et al. (2018). Scientific Reports, 8(1):9851.

[7] Rauch, M., et al. (1996). Am. Mineral.:81(9–10):1289–1292

Figures

Figure 1. (A) Optical microscope mosaic and (B) BSE of the NWA 4137 section. (C) Enlarged area of the prominent MV (MV1) showing the analyzed areas (MV1-1 to MV1-6). (D) Phase map of NWA 4137.

Figure 2. NWA 4137 meteorite. (A) BSE image of MV1-1. (B) Analyzed positions of the olivine and wadsleyite. (C) Raman spectra of wadsleyite compared with respective from the RRUFF database.

Figure 3. NWA 4137 meteorite. (A) BSE image of position MV1-2. (B) Analyzed positions of majorite. (C) Raman spectrum of majoritic garnet within MV1 compared to majorite from Handbook of Raman Spectra)[7] and enstatite (RRUFF).

Figure 4. NWA 4137 meteorite. (A) BSE image of position MV1-3. (B, C) Irregular albitic jadeite grains. (D) Raman spectrum of albitic jadeite compared with jadeite spectrum, and omphacite  (RRUFF).

Figure 5. NWA 4137 meteorite. (A) BSE image of position MV1-6. (B) Tuite annotated for the analysed Raman position. (C) Raman spectrum of a tuite grain within MV1-6 compared with respective from RRUFF database (merrillite R150063; fluorapatite R050617.

Figure 6. NWA 4137 meteorite. (A) BSE image within position MV1-5 showing the analyzed Raman spot. (B) Xieite within MV1-4 annotated for it’s Raman position . (C) Raman spectrum of xieite within MV1-4 and MV1-5 compared with Raman spectrum of xieite from [5] and chromite (RRUFF).

How to cite: Baziotis, I., Effraimidou, S., Simopoulou, M., Ferrière, L., Klemme, S., Berndt, J., and Asimow, P.: High-pressure phase preservation in L-chondrite melt veins: Evidence for control by post-shock thermal evolution, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-1127, https://doi.org/10.5194/epsc2026-1127, 2026.

Introduction: Since 1969, six Apollo missions, three Luna missions, and the recent Chang’E missions have returned ~385.6 kg of lunar samples, providing a robust dataset for reconstructing lunar surface processes and geologic history [1-2]. However, despite advances from Chang’e-5 and Chang’e-6 [e.g., 3-5], direct sampling remains geographically restricted to specific landing sites. Lunar meteorites- which likely originate from unsampled lunar regions-offer complementary records that advance our understanding of the Moon’s origin and global evolution [6-7].

Samples: Dhofar 1442 is the second most KREEP (potassium, rare earth elements, phosphorus)-rich lunar meteorites known (after SaU 169). Its geochemistry points to the Procellarum KREEP Terrane (PKT) on the nearside, which hosts the Moon’s primary KREEP reservoir and records prolonged magmatic activity. Unlike the limited sampling from Apollo 12, Apollo 14, and Chang’E-5, Dhofar 1442 offers complementary insights into KREEP magmatism and impact history.

Methods: The in-situ U-Pb dating of zircons were performed on a Cameca IMS-1280/1280HR at the Institute of Geology and Geophysics (IGG), Chinese Academy of Sciences (CAS). A 3-5 mm O₂^- primary beam at ~0.2 nA was employed in single-collector mode, with a mass resolving power of 7000 (50% peak height definition). Pb/U fractionation was calibrated against zircon standard M257 via the power relationship between ²⁰⁶Pb/²³⁸U and ²³⁸U¹⁶O₂/²³⁸U. Common Pb was corrected using modern terrestrial value.

Results: Dhofar 1442 is a glassy-matrix regolith breccia containing a variety of mineral, lithic and glass clasts. The most abundant lithic clasts associated with zircons are granulitic, noritic, and impact melt breccia (IMB) clasts.

Granulitic clasts are metamorphic rocks formed at depth during large impacts. They exhibit a well-equilibrated mineral assemblage dominated by pigeonite, Na-rich plagioclase (average An₇₃) and Si, K-rich glass, with minor ilmenite, Fe-Ni metal, troilite, phosphate and zircon. Zircons from these clasts yield an upper intercept age of 3848 ± 29 Ma (Fig. 1a). After excluding highly discordant points, the remaining analyses give a weighted average ²⁰⁷Pb/²⁰⁶Pb age of 3848 ± 22 Ma. This age records a metamorphic event at ~3.85 Ga and provides a lower limit for the consolidation age of Dhofar 1442.

Norite clasts comprise roughly equal proportions of low-Ca pyroxene and plagioclase (average An₈₅), with minor ilmenite, troilite, phosphate and zircon. They preserve primary igneous textures (subophitic to granoblastic) and lack shock features. Zircons in these clasts give an upper intercept age of 3957 ± 26 Ma (Fig. 1b). After excluding discordant points, the weighted average ²⁰⁷Pb/²⁰⁶Pb age is 3945 ± 11 Ma, which is interpreted as the crystallization age of the noritic clast. This younger age records a later episode of magnesian magmatism on the lunar nearside, possibly linked to thermal effects from the Imbrium impact at ~3.92 Ga.

IMB clasts contain mineral and lithic fragments set in a fine-grained crystalline matrix. Pyroxene compositions vary widely, encompassing enstatite, pigeonite and augite. Plagioclase spans a range from An₅₇to An₉₃ (labradorite to anorthite). Zircons in IMB clasts yield an upper intercept age of 4342 ± 20 Ma (Fig. 1c) and a weighted average ²⁰⁷Pb/²⁰⁶Pb age of 4337 ± 9 Ma. This well-constrained age provides evidence for significant impact activity on the Moon pre-3.9 Ga.

In addition to lithic clasts, zircons also occur as matrix fragments. Although direct petrographic links to specific lithic clasts are limited, their ²⁰⁷Pb-²⁰⁶Pb age distribution closely resembles that of the lithic clasts, with prominent peaks at ~ 3.82, ~3.95, and ~4.32 Ga (Fig. 1d). This striking similarity suggests that matrix zircons are largely derived from the granulitic, noritic, and IMB clasts.

Discussion: Early Apollo studies identified the “lunar cataclysm” or Late Heavy Bombardment (LHB) at ~3.92–3.85 Ga [8], but pre-3.9 Ga impacts are increasingly recognized [9-10]. In Dhofar 1442, the ~4.34 Ga age of IMB clasts suggest a major impact. A similar ~4.35 Ga in other lunar meteorites such as Kalahari 009 and NWA 2995, is attributed to basin-forming impacts [10-11], though Borg and Carlson (2023) [1] proposed it as late Moon formation. Given our IMB zircon ages, we prefer an impact origin rather than Moon formation. As the South Pole-Aitken (SPA) basin is estimated at 4.25- 4.33 Ga [9-10], the impact recorded by Dhofar 1442 may be associated with SPA basin formation.

As a polymict regolith breccia, Dhofar 1442 provides a more representative record of lunar impact history than monomict specimens. Zircon age distribution indicates that impact activity at ~4.34 Ga was at least as intense as that during the ~3.94 Ga, with peak intensities decreasing progressively from ~4.34 to ~3.95 Ga and then to ~3.82 Ga, which is consistent with a declining impact flux over time [12].

Pre-3.9 Ga impacts are also recorded on Vesta (Ar-Ar: ~3.4–4.1 Ga; U-Pb: ~4.2 Ga) [13-14] and Mars (large basin: 4.10–4.25 Ga; NWA 7034/7533: 4.44, 4.35, 1.44 and 1.35 Ga) [15]. The 4.35 Ga zircon age in NWA 7034 is close to the oldest Martian basin [16], indicating major impact disturbance of the U-Pb system. These independent records support that significant bombardment began earlier than ~3.9 Ga, though establishing a synchronous chronology requires integration of multiple sample suites and geochronological techniques.

References: [1] Borg and Carlson, 2023, Annual Review of Earth and Planetary Sciences, 51, 25-52. [2] Neal et al., 2023, Reviews in Mineralogy & Geochemistry. [3] Li et al., 2021, Nature, 600, 54-58. [4] Cui et al., 2024, Science, 386, 1395-1399. [5] Zhou et al., 2025, Nature, 643, 371-375. [6] Joy et al., 2023, Reviews in Mineralogy & Geochemistry, 89: 509–562. [7] Korotev et al., 2005, Geochemistry, 65, 297-346. [8] Cohen et al., 2000, Science, 290, 1754-1756. [9] Su et al., 2025, National Science Review, 12, nwaf103. [10] Joy et al., 2025, Nature Astronomy, 9, 55-65. [11] Terada et al., 2007, Nature, 450, 849-852. [12] Yue et al., 2026, Science Advances, 12, eady9265. [13] Bogard, 1995, Meteoritics, 30, 244-268. [14] Zhou et al., 2011, 42^nd LPSC, 2575. [15] Yin et al., 2014, 45^th LPSC, 1320. [16] Marchi et al., 2021, The Astronomical Journal, 161, 187.

Figure 1: U-Pb concordia diagrams (a-c) and ²⁰⁷Pb/²⁰⁶Pb age probability density distribution (d) for zircons from different lithic clasts in Dhofar 1442. All uncertainties are reported at the 2σ level.

How to cite: Zhou, Q., Yin, Q.-Z., Zeigler, R. A., Jolliff, B. L., Li, Q.-L., and Li, C.: Zircon U-Pb Geochronology of Lunar Meteorite Dhofar 1442: Implications for Lunar Impact History and the Late Heavy Bombardment, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-140, https://doi.org/10.5194/epsc2026-140, 2026.

Introduction: Since the initial finding of a large number of secondary craters in southeast Wyoming [1, 2], the search for a source of the secondaries has been a major goal. Two potential locations of the primary crater, in the Denver basin and near the Hartville uplift near the town of Guernsey were proposed [3].

Recent field work was carried out in April 2026 in SE Wyoming, USA to investigate these potential sites of the primary and to prove and sample more secondary crater sites. Generally, accessibility to outcrops is challenging due to the prevalence of privately owned land; permission from landowners is mandatory for access to sites. Here, we report on our findings near the Glendo Reservoir, where Pennsylvanian to Permian lithologies were mapped as the Hartville Formation. We targeted the upper portions of this formation (Hartville divisions 1-3) as they are correlated to the upper Casper Formation, which contains secondaries further to the North and West.

Results: 15 new potential secondary craters were found north of the Glendo Reservoir and are currently being evaluated for shock effects. These secondaries occur as morphologically raised features in the field. Rings of partly brecciated quartzitic sandstones with diameters up 100 m were found. The rings show a stronger quartzitic overprint than surrounding sandstone layers. These quartzitic sandstones commonly form meter-high walls that delineate the rims of the circular to elliptical craters. A tightly spaced cluster of four secondaries with diameters of ~50 m each was found, where crater rims partially overlap. Within the craters the sandstones show strong brecciation and occasional folding, and thus resemble the secondary fields found, e.g., near Douglas [1].

To the south of the secondary craters, zones of intense deformation are found in sedimentary layers of the Hartville Formation. Subvertical fault zones consist of mostly monomict (and rare polymict) breccias that can reach over 10 meters in width. Surprisingly, only minor displacement of the local stratigraphy is visible and is confined to at most a few meters of throw. The intensity and width of these breccia zones do not match typical tectonic fault zones with minor displacement but could be explained through repeated, oscillating movement, e.g., seismic activation during the cratering process. These breccia zones have spacings of several 100 m.

Alongside the subvertical breccia zones, a horizontal bed of ~10 m thick polymict breccias could be traced over a lateral extent of roughly 1 km. The breccias are subdivided into a lower red clay- and silt-dominated zone and an upper grey carbonate-dominated zone. The breccias are located roughly 50 m below the stratigraphic horizon containing the secondaries. The unbrecciated sediment layers immediately above the breccias show a shallow thrust ramp with 2-3 m throw, indicating that the breccia layer may have served as a detachment zone for lateral displacement where softer, unconsolidated clays and silts were more easily deformed.

Finally, polymict breccias are found at the top of the Hartville Formation at a stratigraphic level corresponding to that of the secondary craters. We interpret these breccias as a potential continuous ejecta blanket. At the reservoir, the breccias consist mainly of carbonate clasts in a red silty matrix. Clast sizes here are usually in the cm to dm range and show intense deformation, with breccia thicknesses reaching a few meters. These breccias could be laterally traced over 1 km. Further to the north, decameter thick exposures of polymict breccias are locally found, with one noteworthy outcrop containing block sizes of over 10 m. In all cases, polymict breccias contain only sedimentary clasts, while metamorphic and igneous rocks of the Precambrian basement were not found.

Discussion: The widespread occurrence of monomict and polymict breccias along with some strata displacements and local strata tilting suggests that a section of a larger impact structure and a continuous ejecta blanket is exposed at the Glendo Reservoir. We assume that the largest portion of the crater is buried beneath younger strata. As only sedimentary rocks and no crystalline basement clasts were found within the polymict breccias, this gives a constraint on the maximum possible crater size. Profiles in geological maps from the area show that the thickness of sedimentary layers from the top of Precambrian igneous and metamorphic lithologies to the Permian is between 300-450 m (1000-1500 feet). Using a maximum excavation depth of 1/10 of the transient crater diameter [4], the potential crater should have a transient crater diameter of 4.5 km or less. Applying a maximum complex to transient crater diameter ratio of 2 [5] indicates a primary crater of less than 9 km diameter.

In summary, field results suggest a potential primary crater in close proximity to the Glendo reservoir, with numerous monomict and polymict breccias that are difficult to explain through endogenic or tectonic processes. The primary crater is presumably partially or fully buried in this region. Shock metamorphism is required for the recognition of impact craters [6]. Therefore, thin sections of sampled rocks are currently being scanned for shock features.

Acknowledgments: We acknowledge funding by the German Research Foundation, grants KE 732/30-1 and 732/30-2. We express our gratitude to the Reese family and Glendo State Park for granting us access for field work.

References: [1] Kenkmann, T. et al. (2018) Scientific Reports 8:13246; [2] Kenkmann, T. et al. (2022) GSA Bulletin, 134(9-10):2469-2484. [3] Sturm, S. et al. (2026) Met. Planet. Sci, submitted. [4] Melosh, H. J. (1989) Impact cratering. A geologic process. [5] Collins G. S. et al. (2020) LPSC #2028. [6] Cavosie, A. J. et al. (2026) Met. Planet.Sci., accepted.

How to cite: Kenkmann, T., Poelchau, M. H., Sturm, S., Karagoz, O., Fraser, A., and Sundell, K.: The Wyoming Crater Field: more secondaries, a continuous ejecta blanket, and a possible source crater., Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-530, https://doi.org/10.5194/epsc2026-530, 2026.

Rubble-piles may constitute nearly all of the asteroids in the size range 0.2-10 km [1]. The reason for the high abundance has been suggested to be their high durability against disruption due to their shock-absorbent nature compared with monolithic bodies [2]. The Lockne crater (7–12 km) and its smaller companion Målingen formed simultaneously at 458 Ma in a shallow sea, resulting in exceptional preservation of crater fill and near-field ejecta [3]. The impact event is linked to the Middle Ordovician breakup in the Main Asteroid Belt (~470 Ma), implying that the impacting bodies were rubble-pile aggregates [4].

The marine setting at the Lockne impact event, with seawater and sedimentary strata overlying a flat crystalline basement, represents an extreme case of layering with strong property-contrasts, known to influence crater morphology and produce concentric structures. Such effects also have relevance for Mars, where concentric craters can indicate sedimentary rock and former habitable environments.

At Lockne, an inner 7.5 km wide basement crater is surrounded by a shallow ~12 km outer crater recorded in the sedimentary target rocks [5, 6]. It formed by a shallow excavation flow prior to deposition of basement crater ejecta, and is offset downrange due to oblique impact [5, 6]. At Målingen, the 0.7 km basement crater’s ejecta distribution also indicates a wider but poorly preserved outer crater.

Previous 3-D simulations of the Lockne impact used a monolithic impactor [5]. For an impact at 45° and 15 km/s, these models indicate a ~600 m projectile and target water depth slightly less than the projectile diameter, producing a ~5 km transient basement crater. Målingen was estimated at ~150 m if massive [4]. However, rubble-piles of this size may deform during atmospheric entry forming a “pancake-like” cluster significantly wider than the original body [7]. Such clustered impacts distribute more energy near the surface producing shallower, wider craters. Obliquity increases the flattening, enhances near-surface energy release, and intensifies downrange asymmetry. This seems indicated in the morphology of the concentric Lockne impact structure where the shallow outer crater appears wider than what can be obtained with numerical simulation using a massive impactor [5, 6].

To investigate crater formation mechanisms, we performed impact experiments and numerical simulations of clustered impactors. Experiments were carried out with the EPIC single stage gas gun at CAB CSIC-INTA, Spain, to launch Delrin projectiles up to ~400 m/s. Clustered projectiles were made from weakly bonded 3 mm spheres to obtain equal mass to 20 mm solid reference projectiles, and high-speed cameras recorded both half-space and quarter-space impacts. Numerical modeling in iSALE-2D [8, 9] is ongoing, testing several rubble-pile configurations. Preliminary results indicate a peak energy release at higher position in the target for oblate rubble-pile projectiles ("pancake") than for massive spherical projectiles leading to wider, shallower impact structures consistent with observation in nature.

Acknowledgements: This work was supported by grants PID2021-125883NB-C22 and PID2024-160976NB-I00 by the Spanish Ministry of Science and Innovation/State Agency of Research MCIN/AEI/10.13039/501100011033 and by ‘ERDF A way of making Europe’, and the Spanish Research Council (CSIC) support for international cooperation I-LINK (#ILINK22061).

References: [1] Walsh, K. (2018), Rubble pile asteroids. Annu. Rev. Astron. Astrophys. [2] Jourdan, F. et al. (2023), Rubble pile asteroids are forever. Proc. Natl. Acad. Sci. U.S.A. 120(5). [3] Ormö, J et al. (2014), The geology of the Målingen structure: A probable doublet to the Lockne marine-target impact crater, central Sweden. Meteorit. Planet. Sci. [4] Ormö et al. (2014), First known Terrestrial Impact of a Binary Asteroid from a Main Belt Breakup Event. Sci. Rep.  [5] Lindström et al. (2005), Lockne crater as a result of marine-target oblique impact. Planet. Space Sci. [6] Ormö, J., et al. (2013), A new method to determine the direction of impact: Asymmetry of concentric impact craters as observed in the field (Lockne), on Mars, in experiments, and simulations. Meteorit. Planet. Sci. [7] Shuvalov, V. V. (2002), In Impacts in Precambrian Shields (eds Plado, J. & Pesonen, L. J.),Springer, Berlin-Heidelberg.  [8] Amsden et al. (1980), LANL Rep. LA-8095. [9] Wünnemann et al. (2006) Icarus, 180, 514–527.

How to cite: Ormö, J., Sturkell, E., Solana Gonzalez, P., Herreros, M. I., Agrawal, V., and King, Jr., D. T.: Lockne: A layered-target impact structure from a rubble-pile impactor analysed in field observations, impact experiments, and numerical simulation., Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-1257, https://doi.org/10.5194/epsc2026-1257, 2026.