Europlanet Science Congress 2020
Virtual meeting
21 September – 9 October 2020
Europlanet Science Congress 2020
Virtual meeting
21 September – 9 October 2020

Oral presentations and abstracts

TP4

Impact processes shaped the solar system and modify planetary surfaces until today. This session aims at understanding planetary impact processes at all scales in terms of shock metamorphism, dynamical aspects, geochemical consequences, environmental effects and biotic response, and cratering chronology. Naturally, advancing our understanding of impact phenomena requires a multidisciplinary approach, which includes (but it is not limited to) observations of craters, strewn field or airbursts, numerical modelling, laboratory experiments, geologic and structural mapping, remote sensing, petrographic analysis of impact products, and isotopic and elemental geochemistry analysis.

We welcome presentations across this broad range of study and particularly encourage work that bridges the gap between the investigative methods employed in studying planetary impact processes at all scales.

Convener: Robert Luther | Co-conveners: Natalia Artemieva, Christopher Hamann, Isabel Herreros, Elena Martellato, Jens Ormö, Francisco Javier Rodriguez Tovar

Session assets

Session summary

Chairperson: Robert Luther, Elena Martellato, Jens Ormo, Christopher Hamann, Francisco Javier Rodríguez Tovar, Isabel Herreros
Giant impacts
EPSC2020-217ECP
Nicole Güldemeister, Lukas Manske, Miki Nakajima, and Kai Wünnemann

Planetary collisions play an important role in the compositional and thermal evolution of the planetary system. The Moon-forming impact event is thought to be Earth’s last giant collision event, marking the end of the main accretion phase of the Earth. This large event (re)set the conditions for the subsequent thermochemical evolution of both bodies, Earth and Moon. Large parts of proto-Earth are thought to melt as a consequence of the impact and the extent of melting affects the evolution of the Earth’s interior and atmosphere. It is critical to address the initial conditions of the proto-Earth and the volume and shape of a possible magma ocean after the impact to understand the Earth’s subsequent evolution. To address these questions, we consider two different impact scenarios and present a benchmark study by comparing two different numerical codes. Further, we investigate the effect of strength on melt production for the two different impact scenarios. We compared results using the shock physics code iSALE [1,2], an Eulerian code with a fixed grid in space and a Langrangian mesh-free, so-called smoothed particle hydrodynamics code, (SPH [3]).

Therefore, we consider two different impact scenarios that are described in Nakajima and Stevenson (2015); the canonical model (i) (e.g., [4]) and a fast-spinning Earth model (ii) [5]. The former (i) assumes a Mars-size impactor colliding with proto-Earth at an impact angle of 41° (90° corresponds to head-on collision) at a velocity corresponding to the escape velocity of Earth. The colliding bodies are differentiated into a mantle and core. The latter (ii) case is characterized by a smaller impactor colliding with a rapidly rotating Earth at a steeper impact angle of 73° and a much higher impact velocity of 20 km/s. In iSALE the impactor and target mantle consist of dunite, the core of iron; both materials are represented by an Analytical Equation of state (ANEOS [6]), the SPH code uses an MgSiO3 liquid and ANEOS for forsterite and iron. In iSALE we neglect the core of the impactor as well as the initial spin of Earth. The pre-impact temperature of the colliding bodies is 2000 K and near the solidus. Nakajima and Stevenson (2015) only consider hydrodynamic behavior of the colliding bodies. In the iSALE models a constitutive model accounting for strength is included. Thus, in addition to a comparison of the two different codes, we investigate the effect of strength on melt production. The simulations of the two codes are compared in terms of thermodynamic parameters (pressure evolution, Figure 1) as well as melt production (Figure 2) after the moon-forming impact.

In Figure 1, a comparison of pressure evolution of both impact scenarios and the two numerical approaches is shown. In general, a good agreement in terms of pressure ranges for the two codes is observed. In the fast-spinning model a large fraction of mantle undergoes an extensive expansion and becomes subject to low pressure before some part falls back towards the core where it gains high pressures (Nakajima and Stevenson 2015). This process is not as prominent in the standard model. In the iSALE simulation we do not observe the fall back of the material although the post-impact state of Earth looks similar in both simulations.

The SPH simulations show that the majority of mantle experiences melting (between 80 and 100% mantle melting) pointing to the existence of a global magma ocean with a base close to the core-mantle boundary in both impact scenarios (Nakajima and Stevenson 2015).

The melt production obtained by the iSALE simulation is at least for the standard impact scenario smaller. For the fast-spinning Earth model about 60% of mantle material are molten. Thus, for the standard model, we only observe partial mantle melting whereas for the fast-spinning model we can assume that a global magma ocean exists. However, we have to note that the iSALE simulations have not run as long as the SPH simulations yet and that with a longer run time melt production most likely increases. Neglecting an impactor core in iSALE will also underestimate melt production. The accretion of the core would increase melting.

Regarding the effect of strength, we observe a slight increase of melting when the colliding bodies exhibit some initial strength. However, the effect of the chosen impact scenario is much more significant as seen in Figure 2. The melt efficiency (melt volume normalized by the impactor volume) is over three times larger for the fast-spinning Earth model (2.66 corresponding to 60% of mantle volume) than for the standard model (0.73 corresponding to 20 % of mantle volume).

The presented benchmark study shows an overall good agreement of two numerical codes and indicates the significance of the chosen impact scenario with respect to melt production on Earth during the Moon-forming impact. However, the differences of the two numerical methods may also be caused by some differences in the setup (e.g. different EoS, spin of Earth, differentiated impactor).

In addition, in an ongoing study we systematically carry out simulations of the moon-forming impact event using iSALE. We vary the impact angle (15° to 60°) and impact velocities (12 to 20 km/s) and use different initial temperature profiles. First results show that with increasing impact angle as well as with increasing impact velocity, melt production increases. The effect of the pre-impact temperature becomes more prominent for increased impact angles.

How to cite: Güldemeister, N., Manske, L., Nakajima, M., and Wünnemann, K.: Numerical modelling of the thermal state of Earth after the Moon-forming impact event - A benchmark study, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-217, https://doi.org/10.5194/epsc2020-217, 2020.

EPSC2020-1033ECP
Yonatan Shimoni, Oded Aharonson, and Raluca Rufu

Introduction: A common outcome of a giant impact event is the formation of a circumplanetary debris disk, and in some cases, the capture of the surviving impactor, which results in a system composed of a primary, secondary, and a debris disk. The material in the debris disk may be accreted by the primary body (and secondary, if it exists), escape the system, or coalesce into larger clumps. A classic example of a system thought to originate by such an impact event is the Pluto-Charon binary system (Canup, 2005, 2011), in which all six bodies (including the other smaller satellites) lie approximately on the same plane and have nearly circular orbits. 

Such impacts are simulated using smoothed particle hydrodynamics (SPH) codes including self gravity. The equation of state (EOS) governs the relationship among the thermodynamic variables of the simulated material. The specific EOS used in the simulation may influence the final post-impact structure. In order to quantify the typical differences,  we compare two approaches, one simple analytic and one tabulated EOS. Tillotson (Tillotson, 1962) is a widely used analytic EOS and is computationally fast, but it lacks important details such as the treatment of phase changes. Sesame (Bennett et al., 1978) is a  commonly used tabulated EOS  and is more accurate, but is computationally slower and may be poorly sampled in the required thermodynamic phase-space. Here we show a set of SPH impact simulations that assume similar geometric and dynamic initial conditions but different  EOS. 

Methods: We performed ~100 SPH simulations using SWIFT code (Schaller et al., 2018), simulating Pluto-like impacts with 105-106 particles. The initial bodies are assumed to be differentiated, with target to impactor mass ratio of 1 or 7/3, impact angle, ξ, of 0, 30, 45, 60°. Impact velocity was chosen to be relatively small, 1-1.1 times the escape velocity. The impactor was either spin-less or rotating with a period of 5 or 10 hours. This parameter space was motivated by previous simulations (Canup, 2011) for the formation of the Pluto-Charon system. We compare results using the Tillotson and Sesame EOS. The simulations were stopped after 4 days, the typical time for the central body to relax to a stable spherical shape. We developed an algorithm to detect post-impact clumps. Two particles were considered in contact if their mutual distance was smaller than their combined smoothing lengths. The orbital elements of each clump (“satellitesimal”, defined as 100 particles in pairwise contact, equivalent to ~10-3Mpluto) were computed and studied.

Results: Disk systems were formed for impact angles >30°, in using both EOSs. Satellitesimals, when formed, showed different properties. Figure 1 panels (a,b) show the final snapshots of an impact that produced a debris disk and several satellitesimals highlighted in color corresponding to their masses. In this example, a more massive debris disk with a larger number of satellitesimals is obtained when using Tillotson EOS than using Sesame. Moreover, in addition to the target body, at least one large satellitesimal was formed in each of the simulations, but their composition, mass, and orbital elements differ between the EOSs. In comparing the Sesame run to Tillotson, the largest satellitesimal has a mass of 0.015Mpluto (with water fraction of 0.16) and 0.008Mpluto (with water fraction of 0.30) respectively. Its orbital elements are e=0.67, a=10.07Rpluto for Sesame runs, and e=0.15, a=3.2Rpluto for Tillotson. At smaller masses, using Tillotson EOS produces a greater number of clumps, as seen in Figure 1c. Three more satellitesimals were formed (some beyond the plot limits of 1b), with e=0.86, 1.35, 0.10. Note that the satellitesimal mass is  an order of magnitude  smaller than Charon, so alone they do not predict Charon’s formation (Canup, 2011). 

In the final snapshot, a greater fraction of debris disk particles lie within the Roche limit (computed using present-day Charon’s density (McKinnon et al., 2017)) in the Sesame simulation, whereas the Tillotson disk extends to a greater distance. In terms of mass, angular momentum, and composition, the debris disks are similar (0.025 versus 0.033 of the total mass; 0.19 versus 0.25 of the total angular momentum; water fraction of 0.44 versus 0.37 for Sesame and Tillotson respectively). We note the execution time was 2-3 times longer for the simulations using Sesame than Tillotson, a factor which may be considered in choosing EOS.

Exploring the parameter space, we note that head-on impacts (ξ~0°) produce a merged single body, with the vast majority of ejected particles accreted by the planet, and the rest ejected to space. Oblique, faster than escape velocity impacts (vimp/vesc=1.1 and ξ=60°) resulted in two unbound bodies, with little mass transfer between the two, consistent with previous studies of planetary impacts (Leinhardt & Stewart, 2012).

 

References:

Bennett, B. I., Johnson, J. D., Kerley, G. I., & Rood, G. T. (1978). Recent developments in the Sesame equation-of-state library. https://doi.org/10.2172/5150206

Canup, R. M. (2005). A Giant Impact Origin of Pluto-Charon. Science, 307(5709), 546–550.

Canup, R. M. (2011). On a Giant Impact Origin of Charon, Nix, and Hydra. The Astronomical Journal, 141(2), 35.

Leinhardt, Z. M., & Stewart, S. T. (2012). Collisions Between Gravity-Dominated Bodies. I. Outcome Regimes and Scaling Laws. The Astrophysical Journal, 745(1), 79.

McKinnon, W. B., Stern, S. A., Weaver, H. A., Nimmo, F., Bierson, C. J., Grundy, W. M., et al. (2017). Origin of the Pluto–Charon system: Constraints from the New Horizons flyby. Icarus. https://doi.org/10.1016/j.icarus.2016.11.019

Schaller, M., Gonnet, P., Chalk, A. B. G., & Draper, P. W. (2018, May 1). SWIFT: SPH With Inter-dependent Fine-grained Tasking. Astrophysics Source Code Library. Retrieved from https://ui.adsabs.harvard.edu/abs/2018ascl.soft05020S

Tillotson, J. H. (1962). Metallic equations of state for hypervelocity impact (No. Rep. GA-3216 ). General Dynamics San Diego CA.

 

How to cite: Shimoni, Y., Aharonson, O., and Rufu, R.: Properties of Debris Disk and Satellitesimals in Pluto-Like Impacts Under Different Equations of State, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-1033, https://doi.org/10.5194/epsc2020-1033, 2020.

EPSC2020-665ECP
Harry Ballantyne and Martin Jutzi
 

Introduction

Almost 50 years ago, NASA’s Mariner 9 space probe performed the first complete orbit of a planet other than Earth. This was around our smaller celestial neighbour – Mars. Along the way, this spacecraft obtained images of approximately 85% of the planet’s surface in unprecedented detail, revealing a stark contrast between the northern and southern hemispheres now known as the martian crustal dichotomy. This moniker predominantly refers to the 4-8 km difference in elevation between the southern hemisphere and an apparent basin covering roughly 42% of the north. Other associated features include a higher density of volcanoes and visible impact craters in the south relative to the north.

Some studies have attempted to explain these properties through endogenic means; namely via degree-1 mantle convection studied using large-scale thermochemical models (e.g. Keller and Tackley, 2009). Others have taken the exogenic route, proposing that a giant impact early in Mars’ history caused the excavation of a large mass of material from the northern hemisphere, thus giving rise to the observed dichotomy (e.g. Marinova et al., 2008). Given that such collisions are expected to be very common in the final stages of terrestrial planetary accretion, this approach is highly feasible. The latter studies have, however, generally ignored any long-term geodynamical consequences on the martian interior that such an event may cause.

Recent work has proved the importance of coupling these methods, introducing a hybrid exogenic-endogenic scenario whereby a giant impact triggered a localized magma ocean and subsequent superplume in the southern hemisphere (Golabek et al., 2018). This hypothesis has, however, only been investigated using a very limited range of initial parameters, all of which lead to significant heating deep into the mantle. This therefore motivates an interesting area of study – could there be a parameter space that leads to a hemispherically-thickened crust without significantly heating the mantle? We aim to answer this question using a suite of smoothed-particle hydrodynamics (SPH) simulations that explore a large parameter-space chosen with the intention of limited internal heating, allowing us to neglect any long-term geodynamical effects with reasonable confidence.

Method

The chosen initial parameters for the main suite of SPH simulations are as follows: impact angles of 0-90° in steps of 15°; impact velocities of 1.0, 1.2 and 1.4 times the mutual escape speed; impactor radii of 1000km, 1500km and 2000km; and relative core masses of 25% and 50%. All of these simulations use a resolution of 200,000 SPH particles, with near head-on collisions (0-30°) being modelled for 50 hours after impact and oblique collisions (45-90°) for 200 hours to allow for any secondary (or even tertiary) impacts. In addition, a smaller set of high resolution (1,000,000 SPH particle) simulations are being used to investigate extremely low-velocity (less than mutual escape speed) collisions in a bid to quantitively identify the transition from impacts of large-scale mantle heating to those of relatively cool accretionary piles similar to those described in Jutzi and Asphaug (2011). This then allows for discussion of the potential mechanisms that could lead to such events and their feasibility.

Each model includes the effects of shear strength and plasticity (via a Drucker-Prager-like yield criterion) as such effects have been shown to be significant on the scales concerned in this study (Emsenhuber et al., 2018). Moreover, the sophisticated equation of state ANEOS is being used along with a Mars-specific solidus (Duncan et al., 2018) to accurately calculate the physical environment in which such solid characteristics must be considered.

In all of these studies, both Mars and the impactor are treated as differentiated bodies composed of an iron core and a silicate mantle. An example result of one of these simulations can be seen in Figure 1.

Figure 1: The resulting temperature field of a Mars-like body from a 106 particle SPH simulation 12hr after an impact with a ≈1000km radius impactor at a 0° impact angle and a velocity of 80% of the mutual escape speed.

Preliminary Results

The initial results of this study have revealed promising hemispherical features in certain cases, particularly when examining the melt distribution in the upper mantle. Of notable interest are the results of the grazing impact angles, as some of the initial kinetic energy of the impactor is converted to rotational energy of Mars, allowing for subsequent merging events of decreased impact velocity. In addition, the effects of material strength have been found to be non-negligible, in contrast to previous beliefs that such aspects can be ignored on the length-scales involved in planetary collisions.

References

Canup, R. and Salmon, J. (2018). Origin of Phobos and Deimos by the impact of a Vesta-to-Ceres sized body with Mars. Science Advances, 4(4).

Duncan, M. S., Schmerr, N. C., Bertka, C. M., and Fei, Y. (2018). Extending the Solidus for a Model Iron-Rich Martian Mantle Composition to 25 GPa. Geophysical Research Letters, pages 211–220.

Emsenhuber, A., Jutzi, M., and Benz, W. (2018). SPH calculations of Mars-scale collisions: The role of the equation of state, material rheologies, and numerical effects. Icarus, 301:247–257.

Golabek, G. J., Emsenhuber, A., Jutzi, M., Asphaug, E., I., and Gerya, T. V. (2018). Coupling SPH and thermochemical models of planets: Methodology and example of a Mars-sized body. Icarus, 301:235–246.

Jutzi, M. and Asphaug, E. (2011). Forming the lunar farside highlands by accretion of a companion moon. Nature, 476(7358), 69-72.

Keller, T. and Tackley, P. J. (2009). Towards selfconsistent modeling of the martian dichotomy: The influence of one-ridge convection on crustal thickness distribution. Icarus, 202(2):429–443.

Marinova, M. M., Aharonson, O., and Asphaug, E. (2008). Mega-impact formation of the Mars hemispheric dichotomy. Nature, 453(7199):1216–1219.

Murchie, S. L., Thomas, P. C., Rivkin, A. S., and Chabot, N. L. (2015). Phobos and Deimos.

How to cite: Ballantyne, H. and Jutzi, M.: Forming the Martian Crustal Dichotomy Without Significant Mantle Heating, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-665, https://doi.org/10.5194/epsc2020-665, 2020.

Processes and products of impact cratering
EPSC2020-1095
Elliot Sefton-Nash, Olivier Witasse, and Zoe Faes

1. Introduction

Moonlets from an equatorial debris disk caused by a giant impact (a mechanism by which Phobos and Deimos may have formed (Craddock 2011; Rosenblatt 2011), may have had slowly decaying orbits leading to craters formed at low impact angles (< 5°). The absence of crater morphologies consistent with very low impact angles has been used to argue against the spiralling moonlet hypothesis for the formation of elongated craters (Bottke et al. 2000). The absence of comprehensive atmospheric entry and impact modelling for decaying moonlets leaves open the possibility that some only moderately elongated craters on Mars were formed by decaying moonlets in a thicker atmosphere. Indeed, the 12.5km spatial separation and apparent 3-3.7Ga age and cogenesis of double-oblique impact craters observed by (Chappelow and Herrick 2008) could be inconsistent with formation by a fast (i.e. non-moonlet) meteorite impactor unless a thicker atmosphere provided drag to increase impact angle during spiralling.

2. Method

To explore the effect of ancient atmospheric conditions on the impact angle of potential spiralling moonlets, we construct a simple 2D model to spiral down a moonlet from the top of a hypothetical Mars atmosphere. A body that is at mean circular orbital velocity at the top of the atmosphere,

experiences gravitational pull

and atmospheric drag,

Where G is the gravitational constant, Mm is the mass of Mars, r is the distance between the centres of mass of the impactor and Mars’, CD is the coefficient of drag, 𝜌 is the atmospheric density, and A is the cross-sectional area of the impactor (with respect to the velocity direction).

The model proceeds at each time step (1s) by updating the impactor velocity with a vector of the resultant acceleration, calculated using instantaneous gravitational and atmospheric drag forces. We simulate impact angle, retrieved at the point of contact between spherical impactors and the Mars surface for plausible parameter ranges for impactors and ancient Mars atmosphere.

CD is typically determined experimentally, and is a function of many parameters. At low speeds and for a small spherical projectiles, CD  ~ 0.5 (Miller and Bailey 1979). For large projectiles and mach numbers > 1, behaviour of CD becomes complex and may reach values > 1. To account for this we run simulations for a range of CD up to 1.1.

Impactors 10m to 10km in diameter, surface atmospheric pressures 5 to 500 mbar, coefficients of drag 0.5 to 1.1, and densities covering the range between Phobos and Deimos (1400-1900 kg m-3). An atmospheric temperature profile from  (Schofield et al. 1997) was used, and linearly interpolated as a function of altitude.

We assumed a present day atmospheric composition, with component fractions of 0.7532, 0.227, 0.016 and 0.08 of CO2, N2, Ar and CO, respectively. It is highly likely that an ancient Mars atmosphere had a different composition. As a cursory assessment of the potential impact of different atmospheric composition, we note that substituting 20% of Mars CO2 fraction for N2, the mean molecular mass, 𝜇, would change from 0.0455 to 0.0415, a shift of ~9%. Given that 𝜇 appears only a first order denominator in the calculation of scale height (H = RT/𝜇g) we do not expect uncertainty on past atmospheric composition to have a major impact on our conclusions.

A summary of the parameters used is shown in Table 1. We ran the model for all permutations of the parameter grid, yielding ~2.6x105, model runs.

Table 1: Domain and sampling intervals for parameter space explored for spiralling impactors.

Figure 1: Impact angle as a function of surface atmospheric pressure (mbar) and ballistic coefficient (BC). Surface is a bilinear interpolation of BC run over the parameters listed in Table 1.

3. Discussion

With a drag coefficient of 0.5, Phobos has a ballistic coefficient ~ 8x107 kg m-2, while Deimos’ is ~2.4 x 106 kg m-2. Both moons are therefore sufficiently large and dense that their impact angle would not be substantially modified by atmospheric drag, even for a 0.5 bar atmosphere. However, moonlets decaying from a quasi-stable equatorial debris disk has been proposed to be in the range 1-4km (Rosenblatt et al. 2016). A 1km diameter object with Deimos density would have a BC ~1.8x106. Such an object would experience a shift in impact angle of several 10s of degrees in atmospheres > 100 mbar.

Importantly, we note that the model does not account for non-spherical shapes, heterogeneous density distribution in the impactor, or fragmentation of impactors. Fragmentation would produce a group of impactors, each with lower ballistic coefficients than the parent impactor, and therefore whose impact angle may be steeper than if fragmentation did not occur.

4. Conclusions

We find that apparent lack of very oblique impacts that should be expected from decaying moonlet hypothesis could be due to increased atmospheric drag in an ancient, thicker atmosphere. Based on a simplistic model, impact angle deviations > 10° (compared to present day) are possible in atmospheres of few 100 mbars, for objects with low ballistic coefficients in the ~sub-km diameter range.

5. References

Bottke, W. F., Love, S. G., Tytell, D., and  Glotch, T. 2000. “Interpreting the Elliptical Crater Populations on Mars, Venus, and the Moon.” Icarus 145(1): 108–21.

Chappelow, J. E., and R. R. Herrick. 2008. “On the Origin of a Double, Oblique Impact on Mars.” Icarus 197(2): 452–57.

Craddock, R. A. 2011. “Are Phobos and Deimos the Result of a Giant Impact?” Icarus 211(2): 1150–61.

Miller, D. G., and Bailey, A. B. 1979. “Sphere Drag at Mach Numbers from 0·3 to 2·0 at Reynolds Numbers Approaching 107.” Journal of Fluid Mechanics 93(3): 449–64.

Rosenblatt, P. 2011. “The Origin of the Martian Moons Revisited.” Astronomy and Astrophysics Review 19(1).

Rosenblatt, P. et al., 2016. “Accretion of Phobos and Deimos in an Extended Debris Disc Stirred by Transient Moons.” Nature Geoscience 9(8): 581–83.

Schofield, J. T. et al. 1997. “The Mars Pathfinder Atmospheric Structure Investigation/Meteorology (ASI/MET) Experiment.” Science 278(5344): 1752–58.

How to cite: Sefton-Nash, E., Witasse, O., and Faes, Z.: Impact angle of impactors decaying from circular orbits in an ancient Mars atmosphere, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-1095, https://doi.org/10.5194/epsc2020-1095, 2020.

EPSC2020-708ECP
Andrea Rajsic, Katarina Miljković, Gareth Collins, Kai Wünnemann, Mark Wieczorek, Natalia Wojcicka, and Ingrid Daubar

Introduction: Meteoroid bombardment is one of the sources of seismic activity on planetary bodies. The very first seismometer operating on the surface of another planet was successfully deployed by the NASA InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission to Mars. It gives us an opportunity to investigate the seismicity of Mars, including impact-induced seismic activity. This work investigated the seismic efficiency associated with small meteorite impacts on Mars, using numerical methods in targets analogue to the Martian surface. The Martian crust was simulated as non-porous bedrock (0% porosity) or regolith with different porosities (25%, 44% and 65%)

The seismic efficiency, k, is presented as a portion of impact energy that is transferred into seismic energy. It has been suspected that consolidated (bedrock) and non-consolidated (regolith) materials will have different values of seismic efficiency. Estimates of seismic efficiency range from k=10-2 to 10-6 (Schultz and Gault, 1975; Daubar et al., 2018; McGarr et al., 1969; Hoerth et al., 2014; Richardson & Kedar, 2013; Güldemeister & Wünnemann, 2017). High seismic efficiency is typical in bedrock or highly consolidated materials (k>10-3). Low seismic efficiency is typical for sediments or unconsolidated sands and soils (k<10-5) (e.g., Patton and Walter 1993). In this work, we used a simplified approach (e.g. Güldemeister & Wünnemann, 2017) that defines the seismic efficiency as: ; where x represents distance from the impact point, P is the amplitude of the pressure pulse, t is the duration of the pressure pulse, ρ is the density of the target, Cp is the speed of sound in the target and Ek is the kinetic energy of the impactor.

Numerical impact modelling: All simulations were performed with the iSALE-2D shock physics hydrocode (Collins, et al., 2004; Wünnemann et al., 2006). The impact conditions were modelled to replicate recent fresh meter size impact that occurred on Mars since the landing of InSight (Daubar et al., 2020). Impact crater was estimated to be ~1.5 m in diameter. Impactor radius was 4.4 cm and kinetic energy of 1.8x106 J.

To simulate bedrock and fractured bedrock (25% porosity) we used the ROCK strength model (Collins et al., 2004). To simulate the regolith (44% and 65% porosity) we used Lundborg strength model (Lundborg, 1968) (Table 1). We used the Tillotsen equation of state for basalt (Tillotson, 1962; Wójcicka et al., 2020). For porous cases, we used the ε-α porosity model (Wünnemann et al., 2006;) (Table 2).

 

Table 1. Strength model parameters for targets with different porosity

Parameter 0% 25% 44%  65% 
Strength model ROCK ROCK LUNDD LUNDD
Strength (damaged) (kPa) 10 0 10 0.3
Friction (damaged) 0.6 0.67 0.7 0.7
Limiting strength (damaged) (GPa) 3.5 0.17 0.25 0.25
Strength (intact) (MPa) 10 0.2    
Friction (intact) 1.2 1.8    
Limiting strength (intact) (GPa) 3.5 0.17    

 

Table 2. ε-α porosity model parameters (Borg et al., 2005; Wünnemann et al., 2006; Wójcicka et al, 2020).

Parameter 25% 44% 65%
Initial distension, α 1.33 1.8 2.8
Elastic threshold, ε0 -4x10-4 10-4 10-5
Distension at transition αx 1.1 1.15 1.0
The rate change of distension with respect to volumetric strain, k 0.98 0.98 0.98
Ratio of speed of sound in porous over non-porous medium, χ 0.6 0.33 0.21

 

All variables in the equation for the seismic efficiency were calculated from iSALE outputs. The pressure wave was observed via gauges cells, placed at 45° equidistantly throughout the target. The pressure wave amplitude and pulse duration were calculated at full width half maximum. The sound speed was calculated from assumed bulk modulus of basalt (Wójcicka et al, 2020).

Results: Seismic efficiency was calculated for the same impact conditions in all four material models, representing the reference 1.5 m crater recently observed on Mars. There is a clear decrease in seismic efficiency with increasing porosity. It is of the order of 10-5 for porous and highly porous regolith and 10-4 for fractured bedrock. Estimates for the non-porous basalt bedrock are in the order of  10-3 (Figure 1).

Porosity of the target affected pressure wave amplitudes, duration of the pressure pulse and speed of sound in the target. These are all parameters used in calculation of seismic efficiency. This implies that if impact occurs on very dusty parts of Mars with thick regolith cover, efficiency would be smaller than efficiency of the impact that occurred on the bedrock, or area with thinner regolith cover.

Figure 1. Seismic efficiency calculated in targets with different porosity

Conclusions. Impact cratering represents one of the most important geological processes in the Solar System. Defining relationship between target’s properties and seismic efficiency is of interest to the NASA InSight science, since it helps in understanding the properties of the uppermost crust on Mars. Previous approximations of seismic efficiency were of 2x10-5 with an order of magnitude uncertainty for seismic efficiency on Mars (Teanby and Wookey, 2011) and Daubar et al. (2018) adopted the seismic efficiency of 5x10-4 calculated from the seismic moment (Gudkova et al., 2011; 2015; Teanby 2015). In this work, we calculated the seismic efficiency in meter-size impacts on Mars to be different for bedrock (order of 10-3) and porous materials (order of 10-5).

How to cite: Rajsic, A., Miljković, K., Collins, G., Wünnemann, K., Wieczorek, M., Wojcicka, N., and Daubar, I.: Seismic efficiency of Martian upper crust simulant., Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-708, https://doi.org/10.5194/epsc2020-708, 2020.

EPSC2020-869ECP
Christopher Hamann and Lutz Hecht

Introduction

Understanding the shock behavior of the calcium sulfates gypsum (CaSO4·2H2O), bassanite (CaSO4·0.5H2O), and anhydrite (CaSO4) is essential for understanding hypervelocity impacts into evaporite sediments that are widespread on Earth and which also occur on Mars [1,2]. Most interest focuses on quantification of the impact-induced release of volatiles (H2O and SO2/SO3) to assess its role in the generation or modification of planetary atmospheres [3–11]. Estimates of the amount of gas released from volatile-bearing materials in hypervelocity impact scenarios rely on assignment of the shock-pressure thresholds for incipient and complete vaporization [10]. However, these thresholds are poorly constrained for calcium sulfates, and many studies produced inconsistent to even contrasting results (cf. [5,7,9]). In addition, reports of sulfate impact melts [6] suggest that the shock behavior of the calcium sulfates is more complex than traditionally thought and possibly involves interaction with coexisting silicate impact melts.

Based on our previous study on the interaction between carbonates and silicates in laser-generated melts [12], we report here on the fate of gypsum and anhydrite in laser-irradiation experiments. Specifically, we laser-irradiated gypsum, gypsum–granite, and anhydrite–granite targets to investigate dehydration, decomposition, and melting of the calcium sulfates as well as interaction between calcium sulfates and silicates in an impact-related context.

Materials and methods

We used a continuous-wave fiber laser of 1.07 µm wavelength at Fraunhofer Institut für Kurzzeitdynamik, Freiburg, Germany, to irradiate a gypsum plate (experiments L1130 and L1131) and pulverized gypsum–granite (experiment L1133) and anhydrite–granite (experiment L1132) mixtures in air at 1 bar and room temperature (Fig. 1). The laser emitted a power of 2 or 5 kW for 2 or 5 s, and 1/e2 beam diameters of 6 or 17 mm were used. The laser intensity varied between 8.8 × 102 and 1.8 × 104 W/cm2 and the laser–matter interaction zone was observed using the setup described in [13]. Temperature measurements (Fig. 2) show that peak temperatures were in the range of 1200 K for the gypsum plate and 2200 K for the composite targets. Textural and compositional characterization of the recovered samples employed transmitted-light microscopy, Raman spectroscopy, and scanning electron microscopy.