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Title:
Earth-based observations of radar-dark crater haloes on the Moon: Implications for regolith properties
Authors:
Ghent, R. R.; Leverington, D. W.; Campbell, B. A.; Hawke, B. R.; Campbell, D. B.
Affiliation:
AA(Center for Earth and Planetary Studies, Smithsonian Institution, PO Box 37012, Washington, DC 20013-7012 United States ; ), AB(Center for Earth and Planetary Studies, Smithsonian Institution, PO Box 37012, Washington, DC 20013-7012 United States ; ), AC(Center for Earth and Planetary Studies, Smithsonian Institution, PO Box 37012, Washington, DC 20013-7012 United States ; ), AD(HIGP, University of Hawaii, Honolulu, HI 96822 United States ; ), AE(Department of Astronomy, Cornell University, Ithaca, NY 14853 United States ; )
Publication:
American Geophysical Union, Fall Meeting 2004, abstract #P23A-0231
Publication Date:
12/2004
Origin:
AGU
AGU Keywords:
5420 Impact phenomena (includes cratering), 5460 Physical properties of materials, 5464 Remote sensing, 5470 Surface materials and properties
Bibliographic Code:
2004AGUFM.P23A0231G

Abstract

Earth-based radar has been used for over 30 years to image the nearside of the Moon and to investigate the physical properties of the lunar regolith. The physical characteristics of the regolith are of great importance to fundamental questions in lunar geology, such as the spatial limits and relative timing of flow units or of materials associated with impacts. In contrast with spectral reflectance methods that are sensitive to chemical variations in the upper several microns of surface materials, radar signals penetrate to greater depths and thus return information representing a larger integrated volume of regolith. Improved resolution in both Earth-based radar and orbital multispectral data provide a new opportunity to explore the physical and chemical properties of the lunar regolith at a variety of scales. In this work, we focus on distinctive radar-dark haloes surrounding nearside impact craters. These haloes show characteristically low radar returns in Earth-based observations at 70 cm wavelength and have not been examined systematically to date, though they were noted in previous work. Using previously existing and newly acquired Earth-based radar observations at 70 cm wavelength in conjunction with UV/VIS multispectral data from the Clementine mission, we investigate variations in physical and/or chemical properties that may be responsible for the presence of radar-dark haloes. The radar-dark haloes examined in the present study are different from the so-called ?dark-haloed cratersO that result from impacts into buried mare material or from volcanism, and which appear dark in optical datasets; the features examined in our study specifically show low radar reflectivity at 70 cm wavelength. This characteristically low radar return could result from (1) the presence of a regolith component with higher loss tangent than the surrounding terrain; or (2) the presence of a relatively block-poor mantling deposit produced by the impact process. In this paper, we use 70-cm radar observations in conjunction with multispectral data to distinguish between these two possible mechanisms. We present results for 39 nearside highlands and mare craters indicating that 70-cm radar-dark haloes are closely tied to the physical properties of impact crater ejecta, and do not result from loss tangent variations in the ejecta or target material. In addition, we investigate the implications of radar-dark crater haloes for impact-related processes and for the longevity of the material comprising the haloes in the face of ongoing bombardment by impactors of various sizes, and whether these haloes can provide clues for inferring the relative ages of impact craters.
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