THE SUBSURFACE COHERENT ROCK CONTENT OF THE MOON AS REVEALED BY COLD-SPOT CRATERS

Cold-spot craters are a class of lunar impact craters which were discovered by the Lunar Reconnaissance Orbiter (LRO) Diviner Lunar Radiometer Experiment (Diviner). They are characterized by low thermal inertia material (cold at night) which extends ~10-100 crater radii from the crater center [1, 2]. Cold spots appear to be common to all recent impacts and they fade within approximately 1 Myr, which presents a new opportunity to identify the youngest craters on the Moon [3].

Although surrounded by a large area of low thermal inertia material, the proximal ejecta of these craters are often very rocky. Here we capitalize on differences in the rock abundance in the proximal ejecta blankets of cold-spot craters to map the subsurface rock content on the Moon. Inferring regolith thickness from the rockiness of crater ejecta is a method that dates back to the Apollo era [e.g. 4]. However, once on the surface, rocks break down to a level of non-detection within 1 Gyr [5]. Therefore the rock abundance in crater ejecta depends on both the subsurface rock content and the age of the crater. By considering only cold-spot craters, we remove age as a variable.

We measure the rockiness of ejecta blankets of cold-spot craters larger than 250 m in diameter using Diviner rock abundance [1]. This reveals that cold-spot craters in the maria have rockier ejecta blankets than those in the highlands, which is consistent with a more recent resurfacing of the maria. However, large craters are able to excavate deeper into the subsurface than small craters, so individual craters cannot be directly compared based on their rock abundance alone. To compare all cold-spot craters, we assume that the volume fraction of coherent rock (in contrast to fine-grained regolith) increases exponentially with depth and calculate the “e-folding” depth over which this increase occurs for each cold-spot crater based on its observed rock abundance and expected excavation depth. With this model, we are able to map relative variations in the subsurface rock content on the Moon.

[1] Bandfield, J. L. et al. (2011), JGR: Planets, 116(E12).

[2] Bandfield, J. L. et al. (2014), Icarus, 231, 221–231.

[3] Williams, J.-P. et al. (2018), JGR: Planets, 123(9), 2380–2392.

[4] Rennilson, J. J. et al. (1966), NASA JPL Tech. Report #32-1023, 7–44.

[5] Ghent, R. R. et al. (2014), Geology, 42(12), 1059–1062.