Other
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p52b..03o&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P52B-03
Other
[5410] Planetary Sciences: Solid Surface Planets / Composition, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [5470] Planetary Sciences: Solid Surface Planets / Surface Materials And Properties, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
Different remote sensing techniques measure different depths within the lunar regolith, and measurements of the optical surface may not accurately represent the regolith mixture beneath the surface, even at a few centimeters depth. For example, ilmenite (FeTiO3) is a high-density oxide that is sometimes abundant in lunar samples [1]. Compositional information derived from Clementine UVVIS spectral reflectance, which assumes that ilmenite is the dominant opaque in mare basalts and the primary carrier of titanium, uses ilmenite absorptions in the UV as a proxy for titanium abundance [e.g., 2]. The UVVIS titanium estimates do not match well with those measured by the Lunar Prospector neutron/gamma-ray spectrometers [3]; UVVIS reflectance is controlled by only the top few microns while the neutron spectrometer senses to a depth greater than 10 cm. The goal of this investigation is to determine whether the lunar regolith may exhibit compositional sorting at the optical surface (few 100 μm) compared to the subsurface (10 cm). Regolith samples from the Apollo missions reveal that relative concentrations of ferromagnesian minerals, such as ilmenite, decrease with decreasing grain size and different minerals are thought to comminute at different rates [4]. These observations suggest that ilmenite may persist in larger grain size fractions than other regolith components. We investigate the possibility of mechanical sorting of the topmost layer of regolith such that denser particles preferentially sink. Mechanical sorting of the regolith may occur in two different regimes: 1) during ejecta emplacement following an impact event of any size, and 2) as a result of seismic disturbance created by nearby impacts or moonquakes. We created an experimental procedure to mimic seismic shaking, which may be a primary process promoting density-driven mechanical sorting of particles within the regolith. We tested a vertical shaking regime and horizontal shaking tests are in progress. During vertical shaking, the vibrations were constant due to the set frequency of the electromechanical shaking table, whereas the horizontal vibrations were created manually and were thus intermittent. Results from the vertical shaking scenario suggest that grain-size sorting is influenced by density and these results do not preclude compositional vertical stratification within the lunar regolith. Horizontal shaking tests are still in progress to determine if horizontal perturbation also promotes density-driven sorting in our sample regolith mixture. The results from these experiments may have important implications with respect to the differences in titanium concentration estimates derived from Clementine UVVIS spectral reflectance and Lunar Prospector neutron/gamma-ray measurements within the maria and may help determine whether the differences in these estimates represents true compositional vertical stratification of the lunar regolith. References: [1] Papike J. J. et al. (1991) in The Lunar Sourcebook. [2] Lucey P. G. et al. (1998) JGR, 103(E2); Lucey P. G. et al. (2000) JGR, 105(E8). [3] Elphic R. C. et al. (2000) JGR, 105(E8). [4] Papike J. J. et al. (1982) Rev. Geophys. & Space Phys., 20(4).
Ostrach Lillian R.
Robinson Mark S.
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