Mathematics – Logic
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p12d0527g&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #P12D-0527
Mathematics
Logic
5410 Composition, 5480 Volcanism (8450), 6250 Moon (1221)
Scientific paper
As an incompatible and radioactive element, thorium can be used to characterize the terranes resulting from surface processing of the Moon. Lunar Prospector measurements revealed a thorium distribution that is not uniform, with concentrations of high-thorium abundances on the nearside in close association with mare terranes. Although high-thorium mare materials are quite rare in the returned samples, Lunar Prospector data show numerous pixels with high-thorium abundances that overlay mare terranes. In this work, we show large variations of the thorium content within mare basalt units. We use this result to propose a classification of some lunar mare basalts based on their thorium content. Thorium data were projected on 0.5x0.5 degree map, and smoothed with a 64~km FWHM gaussian. Only pixels that overlay 100% of mare units in geological maps were considered. To minimize the effects due to instrument spatial response, we discarded all the pixels that were closer than 90~km (3σ of the instrumental spatial resolution) from any non-mare pixel, i.e. pixels that overlay only non-mare materials. These criteria result in selecting only nearside mare basalts. Note that other parameters, such as iron abundance, will be used in the future as additional mare basalt selection criteria. The histogram of the thorium abundance in selected mare basalt regions reveals strong patterns. At least five peaks can be clearly distinguished, and interpreted as five different distributions of thorium. Thorium abundances range from 1.7 to about 8~ppm in the selected mare basalt regions, which were subsequently divided in five populations according to their thorium content. The chosen limits in thorium are 3.1, 3.7, 4.5, and 5.7~ppm. The associated map shows continuous units, which supports the present analysis. Basically, low-Th basalts are found primarily in the eastern maria (Crisium, Fecunditatis, Tranquillitatis, Serenitatis) and also in Humorum; mid-Th basalts (3.7-5.7~ppm) are found in Imbrium and Oceanus Procellarum; and high-Th basalts are close to non-mare thorium ``hot spots'' (regions of Aristillus, Mairan and Kepler craters, Aristarchus plateau, and Fra Mauro formation). >http://nis-www.lanl.gov/ěrb"~"gasnault/agu/
Elphic Richard C.
Feldman William C.
Gasnault Olivier
Lawrence D. Jr. J.
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