Thermal infrared lightcurves of the impact of comet Shoemaker‐Levy 9 fragment R

Abstract

The impact of fragment R was observed at thermal infrared wavelengths of 7.85, 10.3 and 12.2 µm from the NASA/Infrared Telescope Facility on July 21 UT, using the MIRAC2 mid‐infrared array camera. Thermal emission at the three wavelengths was sampled sequentially using a 1.8% circular variable filter, with an average time interval of 17 seconds between observations at different wavelengths. Continuous imaging of Jupiter in this mode began at 5:08 UT and extended to 5:55 UT. We present calibrated lightcurves for the three wavelengths. Clear evidence for enhanced emission from the impact region first appears at 5:41 UT, with the peak in emission at all three wavelengths occurring ∼3.5 minutes later. The information content of the data is presented in terms of plots of the product of emissivity times angular size versus source temperature for each wavelength. Assuming that the peak in the lightcurves is due to rotation of the hot impact site into view from Earth, we estimate a diameter of ∼1900 km for the source emitting area at 5:44:30 UT and estimate a lower limit on the source temperature at this time of ∼1350 K. This lower limit drops to 800 K if the diameter of the emitting region was actually a factor of two larger.