Physics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p42a..08s&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #P42A-08
Physics
5420 Impact Phenomena, Cratering (6022, 8136), 6020 Ices, 6022 Impact Phenomena (5420, 8136), 6210 Comets (6023), 6213 Dust
Scientific paper
The Deep Impact experiment is providing fundamental new insights into the nature of the upper 20m of a middle-age comet. In this experiment the speed and mass of the impacting probe was known. But the nature of the surface (composition, structure, roughness) and the final impact angle could not be predicted. We now know that the impact occurred at an angle near 30° in a region that appears to have a thick veneer filling old craters. The cratering event produced successive stages of the cratering process that were very similar to experimental results for gravity-controlled craters: (a) impact flash, downrange expanding plume composed of heated organics and H2O; (b) distinctive conical ejecta curtain that grew without detachment from the surface of the comet; (c) zone of ejecta avoidance uprange. There were also distinctive differences: a very faint initial flash, uprange-directed plume, high-angle column of ejecta, curving ejecta rays, and an evolving composition of the ejecta. These differences can be explained in terms of a highly porous upper surface. With assumed values for ejection angles and gravity-scaled velocity distribution, a bulk density of ~0.6 g/cc has been derived for the comet. Very likely the excavated material, however, was much lower. Based on estimates of the mass ejected from the crater (ground based observers) and the evolution of the ejecta (from the spacecraft), the size of the crater can be estimated to range from 130m to 200m in diameter. The actual crater, however, could not be seen during the flyby due to dust above the crater. This dust can be interpreted as high-angle, low velocity ejecta typical for impacts into high-porosity targets.
DI Team
Schultz Peter H.
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