Far-UV Observations of Lunar Rayed Craters with LRO-LAMP

Using data from the Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP), we investigate the spectral properties of rayed craters in the far-ultraviolet (FUV). Because LAMP is sensitive to the uppermost layer of the lunar surface and regolith grains, it is ideal for characterizing regolith maturity and space weathering products such as submicroscopic iron. We find that crater rays from a survey of the largest Copernican-age craters have high Off-band (155-190 nm)/On-band (130-155 nm) albedo (Off/On) LAMP product ratios, consistent with immature regolith and low amounts of submicroscopic iron. The Off/On ratio of the highlands crater rays decreases linearly over time (0.095 per 100 My), and we use this trend to estimate the age of Jackson crater (similar to 152 My). Some large young highlands craters (e.g., Tycho, Jackson, Giordano Bruno, and Necho) display lower ratio halos around the crater cavity, at regions where previous studies have suggested abundant impact melt exists. The lower Off/On ratio is likely due to the increased glass component of the regolith at these highlands regions, which would act to increase absorption at Off-band wavelengths. We also find that ejecta blankets from large maria craters (e.g., Copernicus and Aristillus) have a similar Off/On ratio to the mature background highlands. This supports previous findings that determined that the rays from these craters are composed of highlands material excavated from beneath the maria and subsequently weathered to maturity. Plain Language Summary Using data from the Lyman Alpha Mapping Project (LAMP) instrument onboard the Lunar Reconnaissance Orbiter (LRO), we analyze young rayed craters on the Moon. The LAMP instrument is useful in studying young craters because its wavelength range (the far-ultraviolet [FUV]; 100-200 nm) is sensitive to space weathering products that darken the lunar surface over time. We find that rays from young lunar craters have properties consistent with a lack of these space weathering products and then use these properties to estimate the age of Jackson crater. Some of the young craters are surrounded by dark halos, which are interpreted to be caused by a greater modal abundance of impact melted glassy material. We hypothesize that the glassy material is more absorbent in the FUV than the nonmelted material and that this increase in absorption causes the darkening observed by LAMP. We also find that two large rayed craters in the lunar maria, Copernicus and Aristillus, have similar properties to mature highlands material. This supports previous findings that suggested that their rays are not immature but are visible because they contain higher-reflectance highlands material that stands out against the lower-reflectance maria.