The Galileo Near Infrared Mapping Spectrometer (NIMS) has detected heating by fallback of ejecta onto the Jovian upper atmosphere, in spectra of the G impact event. The spectra were recorded onboard Galileo and are being played back this month.
Integrated spectra of Jupiter show a warming in the normally dark 3.0-4.4 micron methane band. The warming was first seen in a spectrum taken at 07:39:41 on July 18, 1994 (time corrected to Earth-observed time), 6 minutes and 9 seconds after the initial G impact as detected by the Galileo PPR and UVS experiments at 07:33:32 UT (the NIMS detection of the G fireball was 5.33 seconds later at 07:33:37). The heating is seen in three subsequent spectra taken at 10.67 second intervals, and continues to get warmer and brighter. Ten additional spectra of the G impact at similar temporal resolution will be returned between now and December 22.
Initial analyses of the spectra are underway. The spectra are indicative of temperatures of several hundred to 1,000 K. The time between impact and ejecta fallback implies a vertical velocity of about 4.1 km/sec.
The fallback of impact ejecta from the initial impact of the cometary fragments is believed to be what generates the immense infrared brightening seen by Earth-based observers as the impact sites rotated into view. At the same time as the NIMS detection, an HST green image shows the impact plume (or cloud) from the G impact still rising above the Jupiter limb and getting brighter. Thus, the NIMS detection is likely of the earliest re-impacting of ejecta debris. This data provides a valuable link between the Galileo, HST, and ground-based observations.
Playback of eight minutes of data from the R impact as observed by NIMS, PPR, and UVS will be conducted during January 1995.
Robert Carlson, Paul Weissman, William Smythe, Marcia Segura, John Hui,
Robert Mehlman, and Frank Leader
Galileo NIMS Team, Jet Propulsion Laboratory and UCLA