Fresh impact ejecta deposits on the lunar surface can be characterized as heterogeneous mixtures of boulders, cobbles, and fine-grained regolith that are deposited on the lunar surface during the impact crater formation process. Over time, the boulders associated with ejecta deposits break down into fine-grained regolith due to a combination of bombardment and thermal fatigue. Several qualitative observations of old (>2.0 Ga) kilometer-scale lunar impact ejecta deposits made here in high-resolution images reveal tens of large (>1 m) boulders associated with kilometer-scale crater rims and near-proximal ejecta deposits on the lunar maria. These observations went undescribed in prior measurements of lunar boulder breakdown which suggested that lunar boulders should be destroyed in <300 Myr due to micrometeoroid impacts and other processes (e.g., Basilevsky et al., 2015). Here, we use a combination of radar and thermal-infrared data from the Lunar Reconnaissance Orbiter spacecraft to show that kilometer-scale impact crater rims exhibit elevated rock abundances for the lifetime of the lunar maria. We interpret these results as indicating that boulders are continually being uncovered at crater rims due to downslope movement of the overlying regolith. Moreover, rocks found at crater rims that have been exhumed from depth in geologically recent times are locally derived and unlikely to have come from other areas of the Moon. Future collection of lunar samples at crater rims will serve to mitigate the potential for sample contamination from distal sources, helping to ensure accurate geologic interpretations from the collected samples.

A nearly pole-to-pole survey near 140°E longitude on Europa revealed many areas that exhibit past lateral surface motions, and these areas were examined to determine whether the motions can be described by systems of rigid plates moving across Europa’s surface. Three areas showing plate-like behavior were examined in detail to determine the sequence of events that deformed the surface. All three areas were reconstructed to reveal the original pre-plate motion surfaces by performing multi-stage rotations of plates in spherical coordinates. Several motions observed along single plate boundaries were also noted in previous works, but this work links together isolated observations of lateral offsets into integrated systems of moving plates. Not all of the surveyed surface could be described by systems of rigid plates. There is evidence that the plate motions did not all happen at the same time, and that they are not happening today. We conclude that plate tectonic-like behavior on Europa occurs episodically, in limited regions, with less than 100 km of lateral motion accommodated along any particular boundary before plate motions cease. Europa may represent a world perched on the theoretical boundary between stagnant and mobile lid convective behavior, or it may represent an additional example of the wide variations in possible planetary convective regimes. Differences in observed strike-slip sense and plate rotation directions between the northern and southern hemispheres indicate that tidal forces may influence plate motions.