Impact Flashes: exploring the far side of the Lunar surface

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.

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[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.