Other
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999nvm..conf...46p&link_type=abstract
Workshop on New Views of the Moon 2: Understanding the Moon Through the Integration of Diverse Datasets, p. 46
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1
Anorthosite, Iron Oxides, Lunar Crust, Lunar Maria, Lunar Rocks, Lunar Soil, Mineralogy, Moon, Lunar Evolution, Selenology, Calibrating, Clementine Spacecraft, Galileo Spacecraft, Gamma Ray Spectrometers, Lunar Prospector, Remote Sensing, Spectrometers, Spectroscopy
Scientific paper
There is much evidence to support the hypothesis that a giant impact on early Earth created the Moon and that a magma ocean was present on the young Moon. As the magma cooled and crystallized, plagioclase flotation could have produced the upper part of the Moon's original crust. But how much of this original crust has survived to the present? Has it been entirely disrupted, or do portions remain relatively unchanged? Remote sensing studies of the lunar highlands, combined with analysis of lunar materials returned from known locations on the surface of the Moon, have allowed the determination of the lithologies present in many locations on the Moon. Our study of the distribution of the various lunar-highland rock types has revealed large-scale patterns that suggest the broad outlines of the evolution of portions of the lunar crust. Our previous efforts have used Earth-based spectra and Galileo SSI to study the lunar nearside. We have used Clementine UV-VIS data to extend our studies of the lunar highlands to the farside. Calibration of the Clementine UV-VIS data is essentially complete, and FeO and Ti02 values derived from the Clementine data have been derived from this well-calibrated data. In addition, Lunar Prospector data are now available in preliminary form and can add to our understanding of the composition of highlands units on the farside. We can use this combined dataset to (1) study the composition of farside highlands units; (2) identify and determine the distribution of anorthosite on the lunar farside; and (3) investigate the stratigraphy of the farside crust. The great majority of the Moon's highlands surface is composed of only a few minerals, and these are easily distinguishable using reflection spectroscopy at wavelengths from the UV through visible light and into the near infrared. The mafic minerals pyroxene and olivine contain Fe that causes the minerals to absorb light with a wavelength near 1 micron In contrast, plagioclase feldspar does not absorb light near 1 micron, although plagioclase can show absorption of light near 1.25 micron if it has not been highly shocked by impacts. Through the use of Earth-based telescopic reflectance spectra, it is possible to determine the lithologies present in the area observed, typically from 2 to 6 km in diameter. The Galileo and Clementine spacecraft returned multispectral images of the Moon that, while of lower spectral resolution than Earth-based spot 'spectra, covered large areas of the Moon and used filters at wavelengths useful for determining the lithologies present. These spacecraft data have also been used to determine the abundance of FeO and Ti02 present in lunar surface materials. Other products, such as band-ratio maps, have been produced, and spectra have been extracted from coregistered image cubes. Lunar Prospector has collected a large quantity of gamma-ray and neutron spectrometer data. While much of the data will require further processing before reliable quantitative interpretations can be made, some data from that mission have already been made available. In particular, the gamma-ray spectrometer counting data for Th, K, and Fe can be used to confirm and extend our knowledge of the composition of the lunar farside crust. A preliminary Th distribution map has been produced from the raw data by utilizing ground truth from the lunar landing sites. Noritic anorthosite and anorthositic norite are the predominant rock types at the surface of the nearside lunar highlands. Lesser amounts of anorthosite, norite, troctolite, and gabbroic rocks are also present. Studies of Earth-based reflectance spectra initially revealed the presence of anorthosite in isolated outcrops extending in a narrow band from the Inner Rook mountains in the west to the crater Petavius in the east. More recently, additional outcrops of anorthosite have been identified in the central peaks of some craters, such as Aristarchus, and in the northern and northeastern nearside . In most cases, these anorthosite deposits have been exposed by impacts that removed a more mafic overburden and raised them to the surface from deeper in the crust, for example in the peak rings of the Orientale, Humorurn, and Grimaldi Basins. Much of the nearside lunar highlands likely shares this stratigraphic sequence: a layer of pure anorthosite overlies a more mafic lower crust and is in turn overlain by a somewhat more mafic layer. Much anorthosite likely remains hidden by this surface layer on the nearside today. On the lunar farside, the giant South Pole Aitken (SPA) Basin shows a mafic anomaly that is the dominant compositional feature on the farside. At 2500 km in diameter, SPA (centered at 55S, 180E) is the largest unambiguously identified impact basin in the solar system. It is also the oldest identified lunar basin except possibly for Procellarum. There is a 13-km difference in elevation from the interior of the basin to the surrounding highlands. The interior exhibits a 7-10% FeO enrichment relative to the surrounding highlands, and portions of the interior exhibit enhanced Ti02 values [131. Portions of the interior also display elevated Th and K abundances. The high-FeO values found inside SPA drop off with increasing distance to the north of the basin. The region between 100E and 100W and between40N and 70N exhibits very low values of FeO and Ti02. The new Lunar Prospector data show very low-Th and-K abundances there as well. FeO maps produced from high spatial resolution Clementine data reveal extremely low FeO values near the crater Fowler (43N, 145W) in the vicinity of the Coulomb-Sarton Basin. We interpret these data as indicating a region in which pure anorthosite is dominant. Many areas in this region appear to contain nothing but anorthosite. Between SPA and the far north, the data generally indicate intermediate FeO values and low Th values. Additional information is contained in original.
Blewett Dave T.
Hawke Bernard Ray
Jeffrey Taylor G.
Lucey Paul G.
Peterson Charlotte A.
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