Determining Boulder Size, Frequency, and Range Distributions Around Craters at Spacecraft Landing Sites

NOTE: The original PI of this project was Ryan Watkins. Upon accepting a position at NASA Headquarters in 2020, she arranged for Jamie Molaro to become PI.

Boulders on the lunar surface primarily occur around young or fresh craters that are large enough to excavate bedrock. Their size and frequency distributions as a function of time on the surface and distance from their source crater are important for understanding the rate at which rocks become regolith. Such craters are present at or near legacy landing sites (Apollo, Luna, Surveyor, and Chang’e-3), and we have absolute formation ages or absolute model ages for many of these craters. Using orbital data to characterize boulder populations at landing sites is therefore useful for understanding how the distribution of boulders varies as a function of crater age, crater size, and regolith properties. Counting boulders at locations where landers have safely touched down is also important for informing landing hazard criteria and site selection for future missions. This work will use high-resolution (~0.5-2 m/pixel) Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) images to:

1) Determine how far craters distribute boulders and how this distribution varies as a function of crater size and age.
2) Assess how variations in boulder distributions as a function of time can inform the rate at which rocks become regolith. 3) Establish whether systematic trends exist between regolith properties, crater size, and boulder distributions.
4) Quantify boulder populations at the landing sites to inform safety hazards for future missions.
5) Provide a robust comparison of LROC boulder counts with Diviner Rock Abundance (DRA) data.

Few studies have used orbital images to measure boulder distributions at landing sites, and even fewer have conducted quantitative analyses of boulder populations using LROC NAC images. We will count boulders >1 m around craters of varying sizes and ages, and in various terrain types, near landing sites using NAC images and CraterTools in ArcGIS. We will use these counts to determine boulder size-frequency, size-range, and range-frequency distributions to address the tasks above. Repeat NAC coverage of these areas exists at a range of viewing geometries, so we will count on images at different illumination conditions and use these to compare and verify boulder counts.

We will also count boulders within a 1 km2 area of each lander to quantify boulder populations at landing sites. We will couple these measurements with Apollo surface photography to verify the LROC counts and size measurements, and with DRA to extrapolate our counts to sub-meter boulder populations that may also pose a landing risk. Combining these data sets will inform boulder populations at varying distances from craters and aid in establishing safe landing zones for future missions. We will present boulder distributions around Cone Crater as proof-of-concept.

Significance: The results of this work will provide a robust set of information regarding boulder populations around craters of
varying sizes and ages, informing the distances at which craters ejecta boulders, the rate at which boulders become regolith, and how regolith properties (thickness and terrain type) affect boulder populations. We will provide the first quantitative analyses of boulder populations at areas where landers have safely touched down on the surface, informing landing hazards for future missions. We do not propose to identify or characterize specific future landing sites, but instead will provide criteria for safe boulder populations for future missions to consider.