UCLA

Impact craters dominate the landscapes of many planetary bodies. Among their most striking characteristics are their rays: radial streaks formed by high velocity ejecta launched to great distances. This dissertation investigates the influence of distal ejecta on planetary surfaces by studying two classes of features: secondary impact craters and lunar cold spots.

Secondary impact craters form when rock fragments ejected from a primary crater re-impact the surface at high velocity. Individual primary craters have been shown to produce upwards of 10^6-10^9 secondary craters which form nearly instantaneously in geologic time. This has led many to question whether crater chronology models can be applied effectively. In chapter 2, we develop a model for the global accumulation of secondary craters with time for Mars, accounting for the spatial clustering of secondaries. We show that the number of km-scale secondaries produced on Mars may exceed primaries after only a few 100 Ma. However, most secondaries are clustered around their parent primaries, and regions far from large primaries have significantly fewer secondaries than the global average. The crossover diameter between primary and secondary crater production on a typical surface is estimated to exceed 1 km after ~1-2 Ga, though subsequent crater erasure has significantly influenced the number of secondaries visible today.

In chapter 3, we produce updated global maps of nighttime temperature for the Moon using data from the Diviner Lunar Radiometer Experiment on the Lunar Reconnaissance Orbiter (LRO). We implement several improvements, including a correction for errors in instrument pointing, which result in a substantial increase in effective resolution. In addition, we develop a model which mostly removes the effect of topography on nighttime temperature by accounting for scattering and emission from the surrounding terrain. These improvements allow smaller and fainter thermal features to be identified than was previously possible.

Lunar cold spots are extensive ray-like regions of reduced nighttime temperature surrounding young impact craters on the Moon. In chapter 4, we show that South Ray crater at the Apollo 16 landing site has a faint cold spot. Its temperature anomaly and ~2 Ma age are consistent with the fading rate of other large cold spots. Additionally, we show that the mean depth of astronaut footprints is greater at the Apollo 16 landing site than the other Apollo sites. This suggests that cold spots are caused by a decompaction of the upper regolith, consistent with estimates derived from thermal modeling. In chapter 5, we present the thermophysical properties of a global survey of cold spots and several new cold spots formed during the LRO mission lifetime. We show that the temperature anomaly of new cold spots scales with crater diameter, forming an upper envelope to the properties of pre-existing cold spots. This indicates a greater depth of regolith modification by larger cold spots. Using thermal modeling, we present bounds on the depth of regolith modification for new cold spots and estimate how this scales with crater size.