Interpreting Ultraviolet LAMP & LROC Observations of the Moon with the Aid of Laboratory Measurements of Fe-Impregnated Silica Gel Analogues

Airless bodies such as the lunar surface undergo space weathering by micrometeorite bombardment and charged particle irradiation. This produces submicroscopic Fe particles that are deposited as either fine-grained rims on mineral grains or as larger particles within agglutinates. As a result, it is well known that VNIR spectra display an increase in the continuum slope (redden), a reduction in albedo (darken), and an attenuation of absorption features. Additional complexities are apparent for larger particles (<100-3000 nm) which only darken, but do not redden the VNIR spectra. Modeling techniques in the VNIR have also advanced with knowledge of these systematics by applying Mie theory to account for the variance in particle size. This information is also critical toward interpreting the lunar surface maturity and composition in the UV as well but has yet to be fully explored like the VNIR.

The LRO features two UV instruments: the Lyman Alpha Mapping Project (LAMP) and the LRO Camera (LROC) Wide-Angle Camera (WAC) which allow us to examine space weathering with two new sets of UV eyes. LAMP has already revealed a variety of intrinsically interesting and informative observations regarding lunar space weathering and swirls, photometric anomalies, impact crating, and surface hydration. These observations are complicated by minimal spectral contrast between the highland and mare in the mid-UV ( 180 nm), that transitions to a full inversion towards Ly-a ( 120 nm) with the relative brightness of mare and highlands regions reversing. While this is informative, it is urgent additional UV laboratory analyses of well characterized samples be collected, particularly the FUV. The FUV is a region of the spectrum we have little laboratory data to contextualize and interpret these observations.

Here, we are performing complementary spectroscopy, Mössbauer, and TEM microscopy of lunar sample analogs to provide guidance for interpretation of FUV, NUV, and VNIR spectra. These analogs consist of silica gels impregnated with Fe particles ranging from 2-3000 nm from Noble et al. [2007]. We will present UV spectra of these samples to show the spectral variations present in the UV with varying systematically with SMFe particle size and abundance. We then leverage this information toward understanding UV observations of the lunar surface.