Comet Impacts Still Visible

From John Rogers
Date: Wed, 24 May 1995 11:29:08 +0059 (BST)
Subject: Impact belt still visible

Greetings again. Here is an excerpt from an interim report on Jupiter in 1995 which I have just written for the BAA, giving visual observers' view of the present status of the dark belt in impact latitudes.

IMPACT BELT

The first observations in the new apparition were made in mid-December, 1994, as previously reported, and it was immediately apparent that the impact belt remained very prominent, at least half way round the planet. All observers reported a broad patchy dark belt, second only to the NEB in intensity, in spite of poor seeing. These results were confirmed in January. In 1995 Jan.-Feb., the impact belt seemed to be fading; Devadas & Murugesh sometimes did not see it at all. However, it is still visible at least half way round the planet in April-May. Recent observations by Gray, Devadas & Murugesh, Rogers, and David Levy, give the following picture:

The impact belt is strongest in the hemisphere centred around l2 200; similar to STB in darkness, but broader and diffuse; the most conspicuous belt after the NEB and SEB; certainly unusually prominent for a belt in these latitudes. This sector presumably derived from the great sites K,L, and G.
In the opposite hemisphere (centred around the Great Red Spot), it is less evident; P. Devadas and Mrs. Murugesh draw only a narrow band which may be normal (S)SSTB, whereas Gray draws a very broad diffuse belt; it is possible that these are no longer impact-related features.

It is interesting to compare these observations with the Hubble S.T. map, made on 1995 Feb. 13 at 410 nm (violet), which Amy Simon, Reta Beebe and Nancy Chanover presented at the STScI/IAU Colloquium in 1995 May. The impact belt is not obvious, in contrast to visual observations! However, in the hemisphere where we see a strong belt, the HST map shows dark material over much of the S.S.S. and S.S. Temperate latitudes, but it has sharp irregular boundaries and internal cyclonic circulations that look just like the normal belt features. I suspect this belt is at the normal cloud-top level, comprising an unusually dark SSSTB, but either it is overlaid by a very diffuse dark haze from the impacts, or (more interesting and perhaps more probable) the dark impact material has mostly settled into the troposphere, and has been removed from anticyclonic circulations by convection in the clouds, but remains as a dark stain in the belt regions.

Addendum: On May 19, I viewed the GRS side of the planet and there did appear to be a distinct impact belt there, which was darker following the GRS.

John Rogers
British Astronomical Association.

Comet Shoemaker-Levy Home Page