COMPREHENSIVE MODEL FOR THE NUCLEUS OF PERIODIC COMET TEMPEL-2 AND ITS ACTIVITY

Observational data obtained primarily during 1988 are analyzed and synthesized to develop a comprehensive physical model for the nucleus of Periodic Comet Tempel 2, one of the best studied members of Jupiter's family of short-period comets. It is confirmed that a previous investigation (Sekanina 1987b) provided reliable information on the comet's spin-axis orientation, which implies an obliquity of 54-degrees of the orbit plane to the equatorial plane and which appears to have varied little-if at all-with time. This conclusion is critical for fitting a triaxial ellipsoid to approximate the figure of the nucleus. Its dimensions are found to be 16.4 X 9.8 X 7.0 km, if A'Hearn et al.'s (1989) visual geometric albedo of 0.022 is accepted. While the nucleus of P/Temple 2 is similar to that of P/Halley in volume and surface area, it is distinctly more flattened in the polar direction. Forced precession, caused by torques from outgassing, is shown to be negligible, but there may be an effect on the nucleus spin rate. If this effect was insignificant in the period of February-June 1988, the remarkable invariability of the comet's sidereal rotation period, derived to be 8h55m55.2s with a formal uncertainty of only +/- 0.2s, would indicate general correctness of the employed spin-vector positional determination. Temporal variations in the production of water cannot be fitted on the assumption of outgassing from a flat region (or regions). An appreciable depth is implied, leading to the conclusion that the source is probably a vent-like depression (or depressions). Assuming that outgassing is possible only from the depression's floor when it is sunlit and that there is no outgassing from the walls, the best match is found for an effective diameter-to-depth ratio of 1.6, a total floor area of 14 km2, and a slope angle of the walls of 81-degrees. The total loss rate of water is estimated at 10(13) g per revolution about the Sun. Deconvolution of the observed optical and thermal infrared diurnal light curves consistently reveals the existence of three isolated vents on the nucleus within about 30-degrees of the northern rotation pole. Modeling of a collimated flow of dust ejecta from a vent indicates the presence of disintegrating particles in the coma, whose typical lifetimes are 25-30 hr. The differential size distribution for large grains varies inversely as the cube of particle size, with a poorly defined cutoff radius of 1 mm. There is also some evidence for a population of very short-lived, perhaps water ice, grains. The dust production rate on 22-30 June is found to have amounted to 10(5) g/s, implying a dust-to-water production rate ratio of 0.7-1.0 by mass. The proposed model indicates that the calculated nucleus figure corresponds to one of the hydrostatic equilibrium configurations known as Jacobi ellipsoids. The derived sideral rotation period suggests an "equilibrium" nucleus density of 0.54 g/cm3 and implies that the comet's nucleus cannot be subjected to significant hydrostatic forces. To facilitate future testing of the nucleus model, predictions are provided for expected temporal variations (i) in the axial orientation of the fan-shaped coma, (ii) in the parameters of the optical and thermal infrared diurnal light curves of the nucleus, and (iii) in the water production rate. It is noted that the comet's return of 2005 is so extremely unfavorable to ground-based optical monitoring for several months on either side of perihelion that the choice of P/Tempel 2 as the target for a dedicated rendezvous mission at this return can only be described as an unfortunate decision.