Disintegration of Dust Aggregates as Origin of the Boundaries in Halley's Coma: Derivation of the Sublimation Parameters

Abstract This paper is focused on the pair of best pronounced boundaries (enhanced gradients of dust flux) which have been observed by the SP-1 and -2 detectors, one inbound and the other outbound, during both Vega encounters of Comet Halley. First, it is shown that the V-detector of the DUCMA instrument, also aboard the Vega spacecraft, served as an additional dust sensor, recording large particle impacts on the spacecraft and its$jytarget area (0.2–0.1 m 2 ), as well as its two mass thresholds, VL ∼5 × 10 −8 g, and VH ∼10 −6 g, are derived. An extensive analysis of the boundaries based on the SP-1, SP-2, and V-detector data has led to the conclusion that the two dust boundaries: (1) are common for a very wide mass range, from ∼10 −17 g up to ∼10 −7 g; (2) are observed at nearly equal distances, inbound and outbound; (3) during the Vega-1 flyby were closer to the nucleus (∼60,000 km) than during the Vega-2 flyby (∼84,000 km). It is shown that none of these properties can be interpreted in terms of the fountain model. On the other hand, the first two can be easily explained as a consequence of disintegration of dust aggregates if only the aggregates were confined to a limited region around the nucleus. Assuming that the disintegration was due to sublimation of the volatile component gluing together dust particles, the condition for the aggregate confinement is formulated and a set of equations describing the formation and expansion of a dust boundary is derived. The boundary expansion speed drawn from the observations allows estimates for two sublimation parameters. In particular, the latent heat is determined to be L ≈ 16 ± 1.5 kcal/mole. Using the derived parameters, some model calculations are made. They show that the dust boundary was first formed about 20 days after the comet perihelion at a distance of ∼45,000 km from the nucleus and expanded with a speed increasing from ∼35 m/sec to ∼140 m/sec during the next 20 days. The size of aggregates (at the time of emission from the nucleus) reaching the boundary was decreasing rapidly, by an order of magnitude in a 10-day interval, but the aggregate lifetime varied weakly. The expansion of the aggregate domain combined with the decrease of disintegration rate resulted in a rather fast decline of the boundary. The DID4 data from the Giotto encounter of Comet Halley give evidence in support of the model predictions.