Mon Jul 18 09:42:18 1994
After the early evening (and impact F) were obscured by thick clouds, we were able to open up and observe Jupiter from 3:15 till it set at 7:00 UT, July 18. Imaging at 2.30 microns clearly showed the E, A and C impact sites, with the E site having a compound appearance perhaps due to merger with the F site, as discussed by Spencer. Estimated central meridian crossing times are as follows, based on visual inspection of the images, and accurate to 5 minutes or so:
site Obs. UT Obs. long. Pred. long. (Sys III) ---------------------------------------- E/F 3:51 150 149/130 A 4:38 180 177 C 5:40 215 217 ----------------------------------------
Curiously, the derived Sys III longitudes agree almost perfectly with Paul Chodas' predictions of July 11, despite the apparent 10-20 min delay in most of the reported impact times. If the reported times are used, the best fit to the observed central meridian times requires a period more like Sys I. We thus confirm the Lick results that the 'aftermaths' move at very close to System III (certainly not drifting westward by 35 deg/day as the French reported). The E/F location (by far the brightest spot) fits much better with the prediction for E than for F.
A more tentative (and interesting) conclusion is that Chodas' times and longitudes were correct and most of the observed delays in the plume times are due to the time it takes the plumes to rise and become visible.
Imaging at 2.0, 2.05, 2.10, 2.15, 2.20, and 2.35 microns shows the relative brightness of the impact sites to increase steadily relative to other features on Jupiter around 20 deg S as one descends into the H2/CH4 band. This is consistent with the visible sites being located in Jupiter's stratosphere, rather than near the visible cloud deck. The images are clearly resolved, although the seeing was poor, with the E/F site being comparable in dimension to the GRS.
In parallel with the near-IR observations, we also observed with a mid-IR imaging spectrometer. We observed what we presume to be the recent impact site F (2.5 hours old) with SpectroCam-10 in Camera, LoRes, and HiRes spectral modes. At 7.9 um in camera mode the spot was quite prominent and about 3 arcsec in diameter, which made placement of the slit a straightforward task. At 8.8 the contrast was much lower, and at 5 um there was no obvious new feature among the usual 5 um structure. The other spots on the planet, A and C, were NOT detected in the mid-IR. LoRes spectra showed spot 'F' from about 8 to 11 um, beyond which the contrast with adjacent regions diminished appreciably. It looked like a continuum hot spot rather than bright lines. However, in HiRes mode we think we saw evidence for different spectral structure on the spot in the ammonia band at 10.5 um, an H2S line near 8.15 um, and possibly the acetylene band at 13.3 um. However, these impressions are gathered from looking at the raw data as it was acquired, and for the last two bands in particular we need to calibrate and flat field carefully before we can be sure.
CCD observations of Jupiter were also obtained at the 60 inch telescope, at regular intervals. The impact sites E/F, A and C were detected in both an H-alpha filter and in U band images, but not at 7300 A or in Gunn 'z' (8800-10,000 A). The sites exhibited a distinctly wispy structure at U.
Phil Nicholson, Gerry Neugebauer, Keith Matthews, Tom Hayward, Jeff Van Cleve, John Miles, Dave Shupe, Alicia Weinberger and Colleen McGhee.