Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p51c1442c&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P51C-1442
Astronomy and Astrophysics
Astronomy
[5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [6250] Planetary Sciences: Solar System Objects / Moon, [6949] Radio Science / Radar Astronomy
Scientific paper
Flow-like deposits of impact melt have been previously recognized on the Moon, and they are often associated with fresh craters and oblique impact sites [Howard and Wilshire, 1975; Hawke and Head, 1977]. These flows are thought to be mixtures of clasts and melted material that are emplaced during the late stages of impact crater formation [Hawke and Head, 1977; Chadwick and Schaber, 1993]. The Mini-RF instrument on the Lunar Reconnaissance Orbiter has acquired S-band (13 cm wavelength) radar imagery of numerous impact craters at a resolution of 15x30 m. The radar measures two orthogonal received polarizations, and the data can therefore be used to generate the four Stokes polarization parameters as well as daughter products such as the circular polarization ratio. Mini-RF data can be used to assess melt flow roughness, to determine whether there is structure internal to the flow, and to map flow boundaries, all of which can lead to a better understanding of how impact melt flows are emplaced. The flows observed to date display a wide variety of behaviors. The 43 km diameter crater Glushko (8° N, 282.4° E) has a complex melt flow pattern with channeling and ponding that appears similar to many impact melt flows observed on Venus. The Glushko flows have a complex texture; radar bright areas have high circular polarization ratios similar to rough lava flows on Earth. Radar-dark flow patterns are visible between bright areas and may be smooth surfaces that were emplaced with a slower ground velocity than the radar-bright areas. In contrast, the 26 km diameter crater Gerasimovich D (22.3° S, 238.4° E) has a radar-bright impact melt flow extending ~2 crater radii from the rim that shows no clear evidence of channels or ponding. There is little variation in backscatter cross-section or circular polarization ratio internal to the flow. In some cases, impact melt appears to have been ejected with significant velocity parallel to the surface. For example, north of the 76 km diameter King crater (5° N, 120.5° E), melt from the King impact ponded in a previously existing depression. Mini-RF images show that some of the melt from this impact moved with a velocity that carried it past the melt pond, across the elevated rim of the pond, and away from the impact site towards the north. Finally, the 55 km diameter crater Aristillus (33.9° N, 1.2° E) has long and particularly thin impact melt flows that extend over 2 crater radii from the rim [Campbell et al. 2010]. In Mini-RF data, the Aristillus flow margins have a complex shape that was likely generated by the effects of topography on the moving melt. There is little change in backscatter cross-section and no evidence of channeling within the flow. Additional analysis of the radar polarimetric properties and comparisons with optical imagery will be useful in determining the surface textures and emplacement process for these and other impact melt flows.
Bussey Ben
Cahill Jameson
Carter Lynn Marie
Heggy Essam
Mini-Rf Team
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