Preimpact porosity controls the gravity signature of lunar craters

We studied a data set of 28 well‐preserved lunar craters in the transitional (simple‐to‐complex) regime with the aim of investigating the underlying cause(s) for morphological differences of these craters in mare versus highland terrains. These transitional craters range from 15 to 42 km in diameter, demonstrating that the transition from simple to complex craters is not abrupt and occurs over a broad diameter range. We examined and measured the following crater attributes: depth (d), diameter (D), floor diameter (Df), rim height (h), and wall width (w), as well as the number and onset of terraces and rock slides. The number of terraces increases with increasing crater size and, in general, mare craters possess more terraces than highland craters of the same diameter. There are also clear differences in the d/D ratio of mare versus highland craters, with transitional craters in mare targets being noticeably shallower than similarly sized highland craters. We propose that layering in mare targets is a major driver for these differences. Layering provides pre‐existing planes of weakness that facilitate crater collapse, thus explaining the overall shallower depths of mare craters and the onset of crater collapse (i.e., the transition from simple to complex crater morphology) at smaller diameters as compared to highland craters. This suggests that layering and its interplay with target strength and porosity may play a more significant role than previously considered.

show abstract

“…; Milbury et al. ). The GRAIL data indicate that the Moon has average crustal porosity of ~12% (Wieczorek et al.…”

Section: Discussionmentioning

confidence: 99%

Transitional impact craters on the Moon: Insight into the effect of target lithology on the impact cratering process

Osinski

Silber

Clayton

et al. 2018

Meteorit & Planetary Scien

View full text Add to dashboard Cite

show abstract

“…The inclusion of preimpact porosity, however, would affect the results of our simulations by causing the fragmentation field to be smaller. Impacts into the porous upper crust of the Moon have been implicated in the finding of positive free air gravity anomalies in midsized lunar craters (Milbury et al, ). However, we leave the implementation of porosity for a future study.…”

Section: Discussionmentioning

confidence: 99%

Impact Fragmentation and the Development of the Deep Lunar Megaregolith

et al. 2019

View full text Add to dashboard Cite

The megaregolith of the Moon is the upper region of the crust, which has been extensively fractured by intense impact bombardment. Little is known about the formation and evolution of the lunar megaregolith. Here we implement the Grady‐Kipp model for dynamic fragmentation into the iSALE shock physics code. This implementation allows us to directly simulate tensile in situ impact fragmentation of the lunar crust. We find that fragment sizes are weakly dependent on impactor size and impact velocity. For impactors 1 km in diameter or smaller, a hemispherical zone centered on the point of impact contains meter‐scale fragments. For an impactor 1 km in diameter this zone extends to depths of 20 km. At larger impactor sizes, overburden pressure inhibits fragmentation and only a near‐surface zone is fragmented. For a 10‐km‐diameter impactor, this surface zone extends to a depth of ~20 km and lateral distances ~300 km from the point of impact. This suggests that impactors from 1 to 10 km in diameter can efficiently fragment the entire lunar crust to depths of ~20 km, implying that much of the modern day megaregolith can be created by single impacts rather than by multiple large impact events.

show abstract

“…Additional challenges arise from target layering due to volcanism, erosion, the presence of volatiles [e.g., Kieffer and Simonds , ], thermal evolution [e.g., Miljković et al ., ], and the differences in mechanical and physical properties at laboratory and planetary scales [e.g., Grady and Kipp , ]. Further complexity is induced by the temporal and spatial variations associated with some of these factors, e.g., the observed density and porosity variations across the lunar surface [ Wieczorek et al ., ] and the decrease of porosity with depth due to the increasing lithostatic pressure [e.g., Han et al ., ; Milbury et al ., ; Soderblom et al ., ]. Moreover, target heterogeneities may have strong effects on small scales, but not on larger scales, or vice versa [Cooper, ].…”

Section: Discussionmentioning

confidence: 99%

The effect of target properties on transient crater scaling for simple craters

et al. 2017

View full text Add to dashboard Cite

The effects of the coefficient of friction and porosity on impact cratering are not sufficiently considered in scaling laws that predict the crater size from a known impactor size, velocity, and mass. We carried out a systematic numerical study employing more than 1000 two‐dimensional models of simple crater formation under lunar conditions in targets with varying properties. A simple numerical setup is used where targets are approximated as granular or brecciated materials, and any compression of porous materials results in permanent compaction. The results are found to be consistent with impact laboratory experiments for water, low‐strength and low‐porosity materials (e.g., wet sand), and sands. Using this assumption, we found that both the friction coefficient and porosity are important for estimating transient crater diameters as is the strength term in crater scaling laws, i.e., the effective strength. The effects of porosity and friction coefficient on impact cratering were parameterized and incorporated into π group scaling laws, and predict transient crater diameters within an accuracy of ±5% for targets with friction coefficients f ≥ 0.4 and porosities Φ = 0–30%. Moreover, 90 crater scaling relationships are made available and can be used to estimate transient crater diameters on various terrains and geological units with different coefficient of friction, porosity, and cohesion. The derived relationships are most robust for targets with Φ > 10–15%, applicable for a lunar environment, and could therefore yield significant insights into the influence of target properties on cratering statistics.

show abstract

Preimpact porosity controls the gravity signature of lunar craters

Cited by 59 publications

References 38 publications

Transitional impact craters on the Moon: Insight into the effect of target lithology on the impact cratering process

Transitional impact craters on the Moon: Insight into the effect of target lithology on the impact cratering process

Impact Fragmentation and the Development of the Deep Lunar Megaregolith

The effect of target properties on transient crater scaling for simple craters

Contact Info

Product

Resources

About