Characterization of space weathering from Lunar Reconnaissance Orbiter Camera ultraviolet observations of the Moon

Abstract

We investigate the effects of space weathering at ultraviolet wavelengths using a near-global seven-band (321-689 nm) mosaic from the Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC). We confirm that for moderate-to high-iron compositions (≳ 5 wt % FeO), the steeply positive UV slope at wavelengths <415 nm shallows with increasing exposure to space weathering. We measure these differences in LROC WAC data as variations in the 321/415 nm ratio, which has low values for fresh craters in the mare and moderate-iron highlands. For low-iron highland compositions, the break in slope occurs at shorter wavelengths, and it is instead the 321/360 nm ratio that increases with exposure to the space-weathering environment, whereas the 321/415 nm ratio appears to be largely controlled by the degree of shock experienced during the impact. The effects of shock may be more important at highland craters because modest shock pressures result in the solid-state transformation of plagioclase to its glass equivalent, maskelynite, and can help distinguish between primary shocked ejecta and locally exposed fresh material in rays. While all of the "fresh" craters we examined have UV spectral properties consistent with substantial alteration due to space weathering, the UV spectra of lunar swirls (magnetically shielded from the solar wind) are consistent with exposure of immature, crystalline material. Together these results suggest that lunar space weathering is dominated by the solar wind and "saturates" in the UV at I s /FeO values of 40 (submature).

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

“…Our Off/On ratio map of Tycho seems to share similarities with the 321‐/415‐nm ratio map in Denevi et al () (low ratio halo, high ratio rays). This may suggest that lunar impact melt glasses and/or maskelynite are similarly absorbent in the FUV (at Off‐band wavelengths).…”

“…In Figures and we see that the crater halos and cavities have higher FUV reflectance (in addition to having lower Off/On ratio) than the corresponding rays and local background regions. This is different from the NUV, where the reverse is true (i.e., crater halos have low reflectance, and crater rays have high reflectance; Denevi et al, ). This may suggest that in the FUV (where sensing depth decreases and nearly all minerals are opaque; Henry et al, ) differences in regolith physical properties (e.g., porosity/roughness) become more apparent than at longer wavelengths.…”

Formation of lunar swirls by magnetic field standoff of the solar wind

“…A relative lack of solar wind interaction with the lunar surface at swirl sites is supported by recent observations from the Moon Mineralogy Mapper (M 3 ) instrument that confirm the Clementine observations of a substantial reduction in optical maturity and also show a reduction of the presence of surface hydroxyls at the swirl sites 15 . Recent observations in the ultraviolet by the Lunar Reconnaissance Orbiter Camera Wide Angle Camera subsystem also suggest that micrometeoroid bombardment, and not solar wind interaction, plays the dominant role in space weathering at the lunar swirls 16 .…”

Petrogenesis of lunar impact melt rock meteorite Oued Awlitis 001

“…Ultraviolet spectral properties that record high reflectance commensurate with low degrees of space weathering (Denevi et al. ) furthermore confirm the relative freshness of Giordano Bruno ejecta. Further constraints for the age of Giordano Bruno to 5–10 Ma have been based on presumed secondary craters at the Luna 24 landing site (Basilevsky and Head ).…”