Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p21a1192b&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P21A-1192
Physics
[5410] Planetary Sciences: Solid Surface Planets / Composition, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [6235] Planetary Sciences: Solar System Objects / Mercury
Scientific paper
Two-color images returned by Mariner 10 revealed impact craters on Mercury that contain material with high reflectance and blue color compared with the hemispherical average. MESSENGER's first two flybys of Mercury have provided multispectral images and higher-spatial-resolution monochrome images of many additional examples of these "bright crater-floor deposits" (BCFDs). Notable examples include deposits on the floors of two large dark-rimmed craters inside the Caloris basin, Sander and Kertesz. Material with similar albedo and color properties is associated with the peak-ring mountains of Eminescu and Raditladi. Plots of key color ratios against reflectance constructed from multispectral imaging demonstrate that the BCFDs are global spectral outliers that do not fall along the space-weathering maturation trends of the main crustal color units on Mercury. This finding strongly suggests that BCFD material has major differences in composition or physical state from most of the planet's surface. Thus, these enigmatic features are not analogous to the lunar swirls, which owe their high reflectance to the presence of immature (unweathered) material. Previous speculation on the origin of bright crater material on Mercury involved the suggestion that fumarolic activity along impact-generated fractures might have produced physical or chemical alteration leading to the characteristic color and albedo. The association with crater interiors and the flow-like form of some BCFDs could indicate that the deposits are related to impact melts. Meteoroid impact speeds at Mercury are considerably higher than at the Moon. High-velocity impacts should produce higher proportions of melting. Craters formed in especially energetic events could have thick deposits of impact-generated melt on their floors. The occurrence of BCFD in many central peak mountains implies that central-peak BCFD material originally resided at depth and was uplifted during the rebound phase of crater formation. A few smaller occurrences of BCFDs are within the area targeted for high-resolution imaging and spectral observations during MESSENGER’s third flyby, and data from larger BCFDs away from the targeted area will also be useful. Ultimately, the mission's orbital phase will allow ultraviolet-visible-near-infrared spectra obtained by the Mercury Atmospheric and Surface Composition Spectrometer and high-resolution images from the wide- and narrow-angle cameras to better characterize the composition and morphology of the BCFDs.
Blewett Dave T.
Coman Ecaterina I.
Denevi Brett Wilcox
Ernst Carolyn M.
Gillis-Davis Jeffery J.
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