Comet Shoemaker–Levy 9 Dust

Abstract

Comet Shoemaker–Levy 9 (S–L 9) was imaged with the Hubble Space Telescope from January 1994, through solar opposition on April 29, and until impact in July 1994. As noted by several observers, no anti-sunward dust tails were detected east of the fragments after opposition (D. Jewitt and J. Chen 1994,Periodic Comet Shoemaker–Levy 9(1993e), IAU Circular 5924; Z. Sekanina, P. W. Chodas, and D. K. Yeomans 1994,Astron. Astrophys.289, 607–636; H. A. Weaveret al.1994,Science263, 787–791; G. P. Chernova, N. N. Kiselev, and K. Jockers 1995, inEuropean SL-9/Jupiter Workshop(R. West and H. Böhnhardt, Eds.), pp. 11–16, ESO Conference and Workshop Proceedings No. 52), a fact from which some have concluded that the S–L 9 fragments were inactive (e.g., Sekaninaet al.1994, Weaveret al.1994, Chernovaet al.1995). However surface brightness profiles of the brighter fragments G1, H, K, L, and S comae suggest the opposite. Images acquired during January and March 1994 show the brighter S–L 9 fragments' azimuthally averaged brightness profiles varied as ρ−1.1±0.1in the inner ρ ≲ 1″ comae and ρ−1.5±0.2in the outer ρ ≳ 1″ comae, where ρ is the projected distance from a coma photocenter. These profiles are consistent with active comet fragments surrounded by dust comae disturbed by radiation pressure. However inactive comet models, which assume the dust was created during a burst of activity immediately following breakup, produce comae that remain highly elongated along the fragment axis throughout their orbit. This type of coma morphology was not observed.Simulations of an active, dust-producing comet in S–L 9's orbit are presented. Comparisons of models with observations show that the dominant light-reflecting grains in the S–L 9 comae and tails had radii ranging between about 5 and 500 μm emitted from the fragments at outflow velocities of order 1 m/sec. The models show radiation pressure swept the dust into tails which appeared west of the fragments both before and after solar opposition. As the fragments neared Jupiter in late June 1994, the combined effects of the jovian tide and radiation pressure distorted the dust tails into broad fans that consisted of grains larger thanR≳ 100 μm and were oriented along the Jupiter–comet direction. The smallerR≲ 100 μm grains were confined to narrower tails whose projected orientation near the nucleus were approximately in the direction of Jupiter. The additional light contributed by the smaller grains caused the S–L 9 dust to appear brighter on the jovian side than the trailing side of the dust fans.