SUBMICRON DUST AND THE COLLISION OF COMET SL-9 WITH JUPITER ( Letter to the Editor) N.C. WICKRAMASINGHE and MAX K. WALLIS School of Mathematics, University of Wales College of Cardiff, Cardiff, U.K. (Received 27 June, 1994; accepted 4 July, 1994) Abstract. Sub-micron sized dust grains released from the several fragments of the comet Shoemaker- Levy 9 could produce significant observable effects on the colour of the Jovian surface. Analysis of photometric data could lead to important insights into the nature and quantity of the dust, as well as the release mechanisms involved. The forthcoming collision of comet Shoemaker-Levy 9 (SL-9) with Jupiter is awaited with eager anticipation by astronomers and the general public alike. If the major comet fragments, some 20 of them, maintain their integrity until they plunge into the denser regions of the planet's atmosphere, they would explode in the invisible far side of the planet, producing fireballs that are extinguished in a matter of minutes (Weaver et al., 1994; Newbum, 1994). Cloud pattern changes may last longer, however, far longer than the 20 minutes it takes for the impact area to rotate into view. There is also the particular possibility that a small mass fraction of the cometary fragments will be released in the form of sub-micron dust particles of a type that was found to be abundant in Halley's comet. We know that the comet underwent Roche fragmentation in July 1992, when it passed within Jupiter's Roche limit and split into some 20 major pieces and very many smaller ones. One infers that the comet and its fragments are made of low density material, held loosely together by little more than self-gravity. The multitude of subcomets and fragments of all sizes coming in at 60 km s -1 will thus be able to lose their fragile outer parts at high altitude. As in the case of impacts into the Earth's atmosphere, micron and sub-micron particles thus entering Jupiter would slow down way above the clouds, keep cool, and be added non-destructively to an upper atmospheric haze. Because of the dynamics of entry, the haze particles released in July 1994 in the high atmosphere will not envelope the entire planet uniformly but would tend to form a distinct band straddling the planet in a timescale of about a day. On the other hand loosely held clumps of submicron dust, released in July 1992 could attend the larger fragments orbiting within a low-density meteor stream, and this component of dust might well be added over a planetwide scale. In this context it should be noted that individual submicron dust grains and smaller Astrophysics and Space Science 219: 295-301, 1994. (~) 1994 Kluwer Academic Publishers. Printed in Belgium.