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A Global Far-Ultraviolet Perspective of Lunar Swirls and Regolith Maturation

Joshua Cahill
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Introduction: The Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP) is providing insights into the upper ~100 nm of the regolith, specifically detecting surface frost and estimating porosity of lunar polar regions in the far-ultraviolet (FUV) [1-3]. LAMP also routinely collects both day and nighttime data of polar and equatorial regions of the Moon. Efforts to examine these non-polar data have studied latitudinal variations in hydration, the examination of swirl and swirl-like photometric anomalies, and cratering deposits [4-6]. These studies are providing a unique new view of the Moon.
Data Set: LAMP is a FUV push-broom photon-counting imaging spectrograph collecting data in the 57-196 nm spectral range [1]. Here, global nighttime Lyman-α (Ly-α; 121.6 nm) normal albedo data are examined for low-albedo features as they are related to lunar regolith maturity. This data set is unique in that it collects naturally reflected light at night from surfaces theoretically diffusely lit by solar Ly-α scattered off of interplanetary H atoms from all directions. This is a simplification, of course, as the Ly-α sky glow intensity varies with respect to the motion of the solar system and point sources from UV-bright stars, which are more plentiful in the southern hemisphere owing to the Galactic plane [1, 8]. Thus, the signal-to-noise of the LAMP nighttime data varies with latitude, increasing from north to south.
A New FUV View of Surface Maturation: Many of the interesting new perspectives in the FUV include crater rays, pyroclastic deposits, and swirls, all of which have a low Ly-α albedo relative to their surroundings, contrasting with high NUV and VIS albedos of these deposits. This is because regolith particles are not transparent in the FUV and particle reflections dominate [9, 10]. Particularly near 120 nm where transition metals no longer dominate the reflectance properties. This provides a unique view of maturity nearly devoid of compositional effects that make quantifying maturation difficult in the VIS and NIR [11, 12]. In stark contrast, young craters show high Ly-α albedo relative to their rays and surroundings.
Two examinations of swirls have been performed in the FUV [5, 6] and provide insight regarding lunar surface maturation. Hendrix et al. [5] detailed examinations of the Reiner Gamma and Gerasimovich swirls using LAMP wavelengths >130 nm noting swirls to be characterized by reddened FUV albedos and noting that immature regolith becomes brighter (i.e., bluer) and flattened with exposure. Cahill et al. [6] concentrated their examination on Lyman-α signatures of more enigmatic lunar features including swirls, normally associated with magnetic anomalies.
References: [1] Gladstone et al. (2012) JGR, 117, 10.1029/2011JE003913. [2] Hayne et al. (2015) Icarus, 255, 68. [3] Mandt et al. (2015) Icarus, 10.1016/j.icarus.2015.07.031. [4] Hendrix et al. (2012) JGR, 117. [5] Hendrix et al. (2016) Icarus, accepted. [6] Cahill (2016) LPSC, XXXXII. [7] Stickle et al. (2016) LPSC. [8] Pryor et al. (1992) AJ, 394, 363. [9] Shkuratov et al. (2011) PSS, 59, 1326. [10] Henry et al. (1976) Moon, 15, 51. [11] Lucey et al. (2000) JGR, 105, 20377. [12] Cheek et al. (2011) JGR, 116, 10.1029/2010JE003702.

"Maria T. Zuber...
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Recognizing that science and human exploration are mutually enabling, NASA created the Solar System Exploration Research Virtual Institute (SSERVI) to address basic and applied scientific questions fundamental to understanding the Moon, Near Earth Asteroids, the Martian moons Phobos and Deimos, and the near space environments of these target bodies. As a virtual institute, SSERVI funds investigators at a broad range of domestic institutions, bringing them together along with international partners via virtual technology to enable new scientific efforts."