干旱区生态环境知识资源中心

Three distinct hashes were observed at near-infrared wavelengths during the impacts of larger fragments of Comet Shoemaker-levy 9. The first flash and the subsequent one are thought to be due to the entry flash and plume development, respectively, The third event, which began to brighten at 6-7 min after entry, was the brightest and lasted about 10 min, We propose that this early stage of the main event could be caused by optical depth enhancement due to nucleation of some sort of grains. We estimated the emissivity variation of the K-impact plume from the observed Aux (OAO/OASIS) and the color temperature (AAT/IRIS) variation assuming a simple plume geometric model, The emissivity variation indicates a sudden increase which would be a consequence of grain formation in the plume. The later part of the third brightening was caused by shock heating due to splash back of the plume material, One possible composition of the grains is a magnesium silicate which condensed from previously vaporized cometary dust. The reported color temperature variation of the K impact is consistent with an experimental nucleation curve of silicate grains. (C) 1996 Academic Press, Inc.