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

Abstract

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 (~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.

New Insights into Aristarchus Crater with LRO LAMP Far-ultraviolet Observations

“…Given that both sites have very little difference in the LAMP spectra, this suggests that the LAMP FUV spectra are sensitive to an electronic transition occurring around 170-190 nm in plagioclase feldspar and may not be sensitive to mafic components mixed into the surface material. This result is consistent with other LAMP studies, which showed that feldspathic-rich highlands had higher Off/On-band ratios, as well as high Off-band spectral slopes compared to the neighboring maria (Hendrix et al 2016;Byron et al 2020Byron et al , 2021). LAMP's Off/ On-band ratio maps and FUV spectra of the crater central peak and southwest crater rim regions are consistent with a plagioclase feldspar-rich composition and are corroborated by M 3 spectra.…”

“…Our analyses show that LAMP-measured Off/On-band albedo ratios of the Aristarchus central peak are consistent with laboratory-measured Off/On-band ratios for lunar anorthite ([CaAl 2 Si 2 O 8 ]; Table 1). This confirms previous LAMP results that attributed LAMP's high Off/On-band ratio values to plagioclase feldspar-rich compositions (Hendrix et al 2016;Byron et al 2020).…”

“…The edge of an absorption band seen in LAMP spectra and Off/On-band ratio maps is probably caused by the interaction of the silicate tetrahedra with alkaline atoms. Feldspathic lithologies typically are characterized by the amounts of sodium (Na), potassium (K), or calcium (Ca) present, and the presence of these lithologies explains the previously reported Off/On-band values from Byron et al (2020) around bright rayed craters. The Off/On-band values at Aristarchus crater (1.15) are higher than what was previously reported for other plagioclase-rich craters.…”

“…Weight percentages for each sample were taken from the following studies. ( Hendrix et al 2016;Cahill et al 2019;Byron et al 2020Byron et al , 2021. Previous FUV spectra of returned Apollo samples and terrestrial analogs show that plagioclase feldspars have a higher albedo longward of ∼170 nm (Figure 5; Hapke et al 1978;Wagner et al 1987).…”

“…The origin and composition of Aristarchus's ejecta blanket have been studied since the 1960s and are high-priority science targets for current and previous lunar missions, including the Lunar Reconnaissance Orbiter (LRO; McEwen et al 1994;Chevrel et al 2009;Glotch et al 2010Glotch et al , 2021. The Aristarchus plateau is detectable and observed in LRO Lyman Alpha Mapping Project (LAMP) global maps as an area of low Lyα albedo (Cahill et al 2019), high .57 nm)/On-band (129.57-155.57 nm) albedo ratio (Figure 1), and positive Off-band spectral slope (Figure 1; Byron et al 2020). In previous studies farultraviolet (FUV) spectral bands were chosen to investigate water ice abundance at the lunar poles (Gladstone et al 2012).…”