Cratering Experiments on the Self Armoring of Coarse-Grained Granular Targets

Recently published crater statistics on the small asteroids 25143 Itokawa and 433 Eros show a significant depletion of craters below approx. 100 m in diameter. Possible mechanisms that were brought up to explain this lack of craters were seismic crater erasure and self armoring of a coarse, boulder covered asteroid surface. While seismic shaking has been studied in this context, the concept of armoring lacks a deeper inspection and an experimental ground truth. We therefore present cratering experiments of glass bead projectiles impacting into granular glass bead targets, where the grain sizes of projectile and target are in a similar range. The impact velocities are in the range of 200 to 300 m/s. We find that craters become fainter and irregular shaped as soon as the target grains are larger than the projectile sizes and that granular craters rarely form when the size ratio between projectile and target grain is around 1:10 or smaller. In that case, we observe a formation of a strength determined crater in the first struck target grain instead. We present a simple model based on the transfer of momentum from the projectile to this first target grain, which is capable to explain our results with only a single free parameter, which is moreover well determined by previous experiments. Based on estimates of typical projectile size and boulder size on Itokawa and Eros, given that our results are representative also for km/s impact velocities, armoring should play an important role for their evolution.Comment: accepted for publication in Icaur

The Role of Ejecta in the Small Crater Populations on the Mid-Sized Saturnian Satellites

We find evidence that crater ejecta play an important role in the small crater populations on the Saturnian satellites, and more broadly, on cratered surfaces throughout the Solar System. We measure crater populations in Cassini images of Enceladus, Rhea, and Mimas, focusing on image data with scales less than 500 m/pixel. We use recent updates to crater scaling laws and their constants to estimate the amount of mass ejected in three different velocity ranges: (i) greater than escape velocity, (ii) less than escape velocity and faster than the minimum velocity required to make a secondary crater (v_min), and (iii) velocities less than v_min. Although the vast majority of mass on each satellite is ejected at speeds less than v_min, our calculations demonstrate that the differences in mass available in the other two categories should lead to observable differences in the small crater populations; the predictions are borne out by the measurements we have made to date. Rhea, Tethys, and Dione have sufficient surface gravities to retain ejecta moving fast enough to make secondary crater populations. The smaller satellites, such as Enceladus but especially Mimas, are expected to have little or no traditional secondary populations because their escape velocities are near the threshold velocity necessary to make a secondary crater. Our work clarifies why the Galilean satellites have extensive secondary crater populations relative to the Saturnian satellites. The presence, extent, and sizes of sesquinary craters (craters formed by ejecta that escape into temporary orbits around Saturn before re-impacting the surface) is not yet well understood. Finally, our work provides further evidence for a "shallow" size-frequency distribution (slope index of ~2 for a differential power-law) for comets a few km diameter and smaller. [slightly abbreviated]Comment: Submitted to Icarus. 77 double-spaced pages, including 25 figures and 5 table

Reexamination of Early Lunar Chronology With GRAIL Data: Terranes, Basins, and Impact Fluxes

Flooding of the lunar surface by ancient mare basalts has rendered uncertain the ages of lunar geochemical terranes and several impact basins. Here we combine craters having recognizable surface expressions with craters identified only by their gravitational signatures in Gravity Recovery and Interior Laboratory data to reassess the chronological sequence of lunar impact basins and the ages of major lunar geochemical terranes. Our results indicate that although volcanically flooded regions are deficient in craters with diameters greater than 20 km by more than 50% relative to unflooded regions, craters with diameters greater than 90 km can be readily recognized either by topography or by gravity anomaly. On the basis of the areal density of craters with diameters greater than 90 km we conclude that (1) the Serenitatis basin could be as young as the Imbrium basin; (2) the areal density of craters within the Procellarum KREEP Terrane is significantly lower than that for the South Pole‐Aitken basin and the Feldspathic Highlands Terrane; (3) if the youngest age of final crystallization of the lunar magma ocean is adopted as a lower bound on the age of the Procellarum KREEP Terrane, a minimum age of approximately 4.3 Ga is inferred for ~40% of lunar impact basins, including South Pole‐Aitken; and (4) the flux of impactors capable of forming craters with diameters of at least 90 km decreased substantially through the Nectarian and Imbrian periods