Mini-RF S-band Radar Characterization of a Lunar South Pole–crossing Tycho Ray: Implications for Sampling Strategies

Abstract

One of the youngest features on the Moon is Tycho, an 85 km diameter impact crater with a vast ray system that spans much of the lunar nearside. As such, it serves as an important stratigraphic marker for the Moon. One of Tycho’s longest rays crosses the South Pole, where it intersects several candidate landing sites for NASA’s Artemis III mission, which intends to return new lunar samples. Identification of ray-related effects are thus important to understand the provenance of collected material. To help contextualize sampling strategies, here we characterize the South Pole–crossing Tycho ray using monostatic S-band radar observations from the Lunar Reconnaissance Orbiter’s Miniature Radio Frequency instrument. We found that the ray is a ∼15 km wide radar-bright feature extending at least ∼1600 km from Tycho. Polarimetric analysis revealed that the measured radar backscatter is consistent with a terrain enhanced in centimeter-to-decimeter-scale scatterers. Moreover, we found that the abundance of these scatterers likely decreases with distance from the primary crater, suggesting there may be less Tycho-disturbed material, in particular, poleward of 85°S, where the candidate landing sites are located. Nevertheless, we identified craters along the ray and, importantly, within the Haworth candidate landing site that exhibit secondary crater characteristics, such as radar-bright, asymmetric ejecta deposits. We showed, based on solar illumination and topographic slopes, that the likely Tycho-related secondaries within Haworth are accessible by landed missions. Exploration of this site may thus directly sample Tycho-disturbed material, including a nearby permanently shadowed region, providing new insights into lunar surface processes.