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Optical Maturity on the Walls of Lunar Craters

Chae Kyung SIM
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Abstract Text: 

Recent studies have found that the optical maturity (OMAT) and the mean grain size of the lunar regolith have latitude dependences (Hemingway et al. 2015, Icarus; Jeong et al. 2015, ApJS), probably because of the reduced flux of space-weathering agents at high latitudes. Here we extend these works to the inner walls of lunar impact craters, dividing a wall into four quadrants, the North, South, East, and West. We consider 3,746 craters whose latitudes are between −50° and +50° and whose diameters are between 5 km and 120 km in the Lunar Impact Crater Database 2015 from the LPI. We adopt the topography-corrected OMAT data from the Kaguya/MI observations (Lemelin, personal communication). The OMAT differences between the N and S walls in a crater are insignificant near the equator, but at high latitudes, the equator-facing walls have generally smaller (more mature) OMAT values than the pole-facing walls. This trend confirms that the incoming direction of the space-weathering agents is predominantly along the ecliptic plane. The longitudinal curve of the mean [OMAT(E) − OMAT(W)] value is sinusoidal and has a minimum and maximum near longitudes −60° and +60°, respectively. We assert that this is due to the shielding of solar wind particles during the Moon’s passage through the Earth’s magnetotail. We present a simple toy model to show that the shielding by the Earth’s magnetotail is indeed responsible for the sinusoidal behavior of the [OMAT(E) − OMAT(W)] curve. This implies that the dominant contributor to the OMAT evolution of lunar regolith is the solar wind particles.

Jack O. Burns (Center of Astrophysics and Space Astronomy, University of Colorado at Boulder)"...
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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."