Lunar Space Weathering in the Ultraviolet: What Laboratory Analysis of Analogs Can Tell Us

Abstract:

The lunar surface undergoes space weathering by micrometeorite bombardment and charged particle irradiation. Space weathering produces submicroscopic iron (SMFe⁰) particles that are deposited as either fine-grained rims on mineral grains or as larger particles within agglutinates. As a result of space weathering, visible-near infrared (Vis-NIR) spectra display an increase in the continuum slope (redden), a reduction in albedo (darken), and an attenuation of absorption features. There are also complexities in the manifestation of weathering in lunar spectra that are imposed as a result of Fe particle size; larger Fe particle sizes (<100-3000 nm) only darken, but do not redden the Vis-NIR spectra.

All of this information is relevant and critical towards interpreting lunar surface maturity and composition in the ultraviolet (UV) as well, but has yet to be fully explored in a similar manner to the Vis-NIR. The Lunar Reconnaissance Orbiter (LRO) features two UV instruments: the Lyman Alpha Mapping Project (LAMP) and the Lunar Reconnaissance Orbiter Camera Wide-Angle Camera (WAC). The lunar ultraviolet albedo is very different than the visible albedo. In fact, the relative brightness between mare and highlands reverses moving to shorter wavelengths in the far-UV.

We collected UV spectra (150-570 nm) in the Laboratory for Spectroscopy under Planetary Environmental Conditions (LabSPEC) under high vacuum conditions using MgF₂ as the standard. This study uses silica gels of Noble et al. [2007] which are impregnated with Fe particles that vary in size from 5–200 nm, with four separate series representing variable size ranges. For example the SG2 series contains 5-15 nm Fe⁰ particles whereas the SG50 series contains 20-200 nm Fe⁰ particles. Each series contains 6-10 samples with variable Fe⁰ content (0-2%). Our work reveals that the largest Fe⁰ particles (SG50 series) darken the spectra with increasing Fe⁰ abundance, as observed in the Vis-NIR. However, the smallest Fe⁰ particles (SG2 series) actually show brightening in the far UV (<260 nm) with increase Fe⁰ abundance. This suggests that the UV may be sensitive to space weathering in a way that could provide greater constraints the grain size fraction of space weathering products.