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D. T. Britt (University of Tennessee)
Introduction: The Deep Space One fly-by of the Comet 19P/Borrelly provided imagery of the object's surface at up to 47 meters per pixel. The surface of this active comet as reveled in the imagery, is fundamentally different from the surfaces of imaged asteroidal small bodies. Asteroidal surfaces are dominated by cratering processes, crater formation, and ballistically-emplaced ejecta. The surface of comet Borrelly, and probably most active comets, are dominated by features directly or indirectly formed by sublimation of volatiles. Ballistic morphologies are either absent or rare. The study, characterization, and modeling of Borrelly's sublimation morphologies can provide insight into cometary processes, and testable predictions for surface features to be seen in future comet encounters. Two particularly interesting morphological units on Borrelly are the Mesas and the Mottled terrain. Mesas: Are characterized by areas of bright slopes surrounding darker, flat-appearing tops. These features are primarily in the central portion of the comet and appear, along with the Smooth Terrain, to be associated with the active jets. On Borrelly, darker areas are stratigraphically higher and, by implication, have had longer surface exposure. The tops of the Mesas are probably older, darker lag deposits that insulate its substrate against outgassing. In this model the Mesa slopes are freshly exposed areas that are subject to gas/dust loss. Outgassing occurs at the Mesa slopes, eroding the volatile-rich substrate under the cap of lag material. As the Mesa slope outgases, it undermines the cap lag causing slope failure. The lag material avalanches down onto the Smooth Terrain unit. While the Smooth Terrain was named because of its smooth appearance at pixel scales (i.e. 50 meters), photometric data show that it is rougher than average at sub-pixel scales, perhaps suggesting a more tumbled surface from Mesa slope failure. Mottled Terrain: Are characterized by areas rough at pixel resolutions with pits, troughs, aligned hills, and ridges. Textures suggest some degree of desiccation and terrain softening. This unit is generally darker than average, but shows strong albedo variations. The morphology and albedo suggest that the Mottled Terrain represents older surface lag deposits that have been subjected to reworking and terrain collapse. These morphologies are probably driven by the slow sublimation of volatiles that are insulated by thick lag deposits.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.