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Regional Variations in Lunar Far-Ultraviolet Characteristics

Amanda Hendrix
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Abstract Text: 

The Lyman Alpha Mapping Project (LAMP) onboard Lunar Reconnaissance Orbiter (LRO) has been making far-UV (FUV) spectral measurements of the lunar dayside since September 2009. We report here on recent work analyzing LAMP data, focusing on weathering and hydration effects in different regions across the moon.
The LAMP instrument [1] is a photon-counting imaging spectrograph. The entire passband is 57–196 nm, in the far-UV (FUV) spectral region. Approximately once per month LRO flies over any particular region; and although LAMP halts acquistion of dayside data when at high phase angles, there are numerous sets of spectra of each region at differing geometries; the emission angle is small while the incidence angle is larger and varies depending on the beta angle of the orbit. To determine the lunar FUV reflectance, we divide the LAMP data from each latitude bin by the full-disk solar spectrum from SORCE SOLSTICE [2], taken for the day of each observation and convolved to agree with the LAMP resolution and line spread function. In our analyses we utilize a derived photometric correction to generate regional images as well as reflectance spectra.
The FUV hosts a strong H2O absorption edge near 165 nm, allowing LAMP to study hydration on the Moon [3]. Past analyses of LAMP dayside data (e.g. [3]) have shown that the measurement of spectral slopes in the 164-173 nm range is an indicator of hydration, while spectral slopes in the 175-190 nm region are insensitive to hydration but good indicators of weathering and composition. We use this spectral slope information to study hydration and weathering effects in different regions across the moon, including mature and immature regions (e.g. after [4]), swirls, and volcanic regions. In the ultraviolet, lunar space weathering effects are the reverse of those at VNIR wavelengths: more weathered regions are bluer than less weathered regions [5], due to a weathering-related degradation of the UV absorption edge in iron-bearing silicates. For instance, we find that swirls are spectrally red compared to surrounding terrain, and appear to be less mature than “immature” regions, consistent with a lack of weathering in these zones. We have shown that a hydration signature is apparent on the dayside of the Moon early and late in the day and at higher latitudes [3][7]; here we investigate hydration signatures in specific regions.

References: [1] Gladstone, G. R. et al. (2010) SSR, 150, 161-181. [2] McClintock et al. (2000) Proc. SPIE Earth Obs. Syst., 4135, 225–234. [3] Hendrix et al. (2012) JGR, 117, E12001, doi:10.1029/2012JE004252. [4] Lucey, P. G. et al. (2000) JGR, 105, 20377. [5] Hendrix. A. R. & F. Vilas (2006) A.J., 132, 1396-1404. [6] Hendrix, A. R. et al. (2016) Icarus, in press [7] Hendrix, A. R. et al. (2016) LPSC abstract #2857.

Anthony Colaprete, Ted Roush, Joshua E. Benton, Joshua B. Forgione, Richard Bielawski, Stephen Battazzo, Erin Fritzler, Robert McMurray
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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."