Observations of comet 19P/Borrelly from the Miniature Integrated Camera and Spectrometer (MICAS) aboard Deep Space 1

Abstract

Images from the DS1 MICAS CCD camera reveal in three dimensions, the complex characteristics of Borrelly's nucleus, coma, and jets. The images acquired during the last 2 hours of the approach, as the nucleus became resolved and grew to roughly 150 pixels in length, provide stereo coverage of both the nucleus and inner coma over a wide range of phase angle and exposure time. The principal structure in the coma is a sunward-pointed collimated jet that is also visible in ground-based images. This jet is canted about 30 degrees off the sun line and appears to be roughly aligned with the local vertical at the surface from where it originates. Long-exposure images reveal details of the structure of the inner coma. They show the jet, visible at long range, to be composed of at least three discrete components whose locations evidently correspond to specific surface features. The elongated nucleus exhibits topographically distinct terrains and strong albedo variegations (of at least a factor of 2). The jets emanate from within the brighter smoother rolling plains. A consistent model is that the main jets are co-aligned with the rotation axis of the nucleus and issue from regions on the plains that are currently in constant sunlight. The other major terrain is a rough unit that is darker than the average, includes even darker isolated spots, and appears as a jumbled topography. Other surface features include parallel ridges, crater-like depressions, numerous narrow dark fracture-like features, and areas of mottled albedo. However no small fresh impact craters are evident attesting to a geologically young, actively evolving surface.