David Crisp, Vikki Meadows, Stuart Lumsden, Jonathan Pogson, and Steven Lee
The InfraRed Imaging Spectrometer (IRIS) is being used on the 3.9 m Anglo-Australian Telescope (AAT) at Siding Spring Observatory (near Coonabarabran, Australia) to monitor the impacts of the fragments of Comet Shoemaker-Levy 9 with Jupiter. On 18 July, observations of Jupiter were initiated at 6:30 UT, about one hour before sunset. The skies were clear, and the seeing was better than 1 arc-second. Near-infrared images and spectra at wavelengths between 2 and 2.4 microns taken between 6:30 and 7:30 UT on 18 July 1994 revealed two bright features near the evening terminator at about 45 South latitude. These features were identified as the impact sites of fragments A and C, which had been monitored on the previous day. Bright flashes associated with the impacts of fragments G2 and G1 were first detected on the morning limb of Jupiter at 7:33 and 7:40 UT, respectively. The impact flashes were tens to hundreds of times brighter than those seen previously. These sites were monitored until 13:30 UT.
Instrumentation and Observing Technique:
IRIS is a near-infrared camera/spectrometer with a 128 by 128 element Mercury-Cadmium-Telluride (NICMOS2) detector. This instrument can be used for direct imaging at wavelengths between 0.9 and 2.5 microns, or for the acquisition of spatially-resolved near-infrared spectra of extended objects. To monitor the G fragment impact events, IRIS was used primarily as an imaging spectrometer. Images of the Jovian disk were obtained simultaneously at 128 wavelengths (or colors) within H-band (1.4 to 1.8 micron) and K-band (2.0 to 2.4 microns). These "spectral image cubes" were collected using the IRIS H and K grisms which provide a spectral resolution of 300, and a spatial resolution of 0.6 arc-seconds per picture-element along a 1 by 60 arc-second slit. Full-disk images of Jupiter were compiled by allowing the planet to drift perpendicular to the slit while recording spectra (which are dispersed perpendicular to the slit) at slit positions offset by 0.6 arcsec. With a 1.5 second sampling time at each slit position, a complete image of the Jovian disk can be produced in about two minutes.
The Evolution of the Fragment G Impact Sites:
IRIS K-grism cubes revealed a faint flash on the morning limb of Jupiter at 7:33 UT. This feature was initially much dimmer than the C-impact site, which was still on the evening limb of Jupiter. The brightness of this feature increased by about a factor of 10 by 7:35 UT and remained stable until about 7:40 UT. At that time, the morning limb suddenly brightened dramatically, saturating the detector, producing a brilliant star-like image with diffraction spikes. To bring the detector back on scale, the 3.9 meter AAT mirror was partially closed, reducing the effective aperture of the telescope to less than 1 meter (less than one tenth of its full size).
After 8:10 UT, the G impact site had rotated onto the Earth-facing hemisphere of Jupiter, and its surface brightness at wavelengths near 2.34-microns decreased to about 12 times that of the south polar hood. The size of the feature was comparable to that of the Great Red Spot. Unlike the impact sites of the earlier fragments, the G impact site was visible at all wavelengths where observations were collected. It was even visible in the TV camera that was being used to guide the telescope. This TV camera employed a spectral filter centered at 0.55 microns, and revealed the impact site as low-contrast dark spot with dimensions comparable to those seen at 2.34 microns. A similar feature was seen in IRIS images taken at 1.083 microns. IRIS H-grism images taken within the 1.7 micron methane band showed a 15000 km diameter bright feature surrounded by a dark apron, which had about twice that diameter. IRIS K-grism images taken within the strong methane bands near 2.34 microns resolved the site into a double-lobed feature. Hubble Space Telescope observations suggest that the second smaller component of this feature may have been associated with the impact site of fragment D, which was monitored on the previous evening (Heidi Hammell, personal communication, 1994). Peter McGregor and Mark Allen used the CASPIR infrared camera at the 2.3 m Australian National University Telescope at Siding Spring Observatory to acquire observations at wavelengths between 3 and 4 microns. At these wavelengths, they detected a bright spot 2000 to 3000 km in diameter, surrounded by distinct 30,000 km diameter bright ring.
The impact of the first of the G fragments was detected at about 45 S latitude on the morning (left-hand) limb. This impact event is most visible at the longer wavelengths, where the methane bands are strongest. Images 3 and 4 were taken at 7:35 and 7:39 UT, as the peak brightness of this feature increased to more than 10 times the surface brightness of the C impact site and the south polar cap. The 5th image shows the initial stages of the rapid brightening which started just after 7:41 UT. The peak brightness of this feature increased to roughly 400 times that of the south polar hood at about 7:47 UT. The last image shows this feature at 7:58 UT, after its surface brightness decreased to about 12 times that of the south polar hood.
The long-term evolution of the G-impact site is shown in this image. The three columns of images in the image show the appearance of the Jovian disk at 8:45, 9:30, and 12:05 UT. the Jovian disk at 7:55, 9:30, and 12:05 UT.
In H-band, the evolution of the G impact site is most obvious in the
bottom row of three images, which were created by summing H-band images at
wavelengths between 1.67 and 1.75 microns, within a strong methane band. The
images in the top two panels were extracted at wavelengths spanning 1.48 to
1.61 microns, and 1.56 to 1.61 microns, where the contrast of the impact site
is much lower.
Images, Images, Images