Is the nucleus of comet Halley a low density body?

Cometary Activity, Discrete Outgassing Areas, and Dust-Jet Formation

Major progress has recently been achieved in the understanding of the coma morphology of dust comets. The conceptual models for the various types of feature observed in the coma (jets, spirals, halos, fans, etc.), their computer simulation, and the hydrodynamic models for jet formation are critically reviewed, and evidence for anisotropic, strongly collimated flows of ejecta emanating from discrete active regions (vents) on the rotating cometary nuclei is presented. The techniques employed to generate synthetic comet images that simulate the observed features are described and their relevance to the primary objectives of coma-morphology studies is exemplified. Modelling of temporal variations in the water emission from discrete active regions suggests that production curves asymmetric with respect to perihelion should be commonplace. Critical comparisons are offered with the activity profiles of Encke’s comet and with light curves of disappearing comets and comets that undergo outbursts. Discussed next is the potential evolutionary significance of nuclear shape, the fractional area of the surface that is active, the vents’ erosion rate, and available information on their activation, aging, dormancy, migration, rejuvenation, and extinction. Recent developments in the understanding of the processes that cause the nongravitational perturbations of cometary motions are reviewed and the observed discontinuities are identified with birth of new sources and/or deactivation of old vents.

Cometary delivery of organic molecules to the early Earth

It has long been speculated that Earth accreted prebiotic organic molecules important for the origins of life from impacts of carbonaceous asteroids and comets during the period of heavy bombardment 4.5 x 10(9) to 3.8 x 10(9) years ago. A comprehensive treatment of comet-asteroid interaction with the atmosphere, surface impact, and resulting organic pyrolysis demonstrates that organics will not survive impacts at velocities greater than about 10 kilometers per second and that even comets and asteroids as small as 100 meters in radius cannot be aerobraked to below this velocity in 1-bar atmospheres. However, for plausible dense (10-bar carbon dioxide) early atmospheres, we find that 4.5 x 10(9) years ago Earth was accreting intact cometary organics at a rate of at least approximately 10(6) to 10(7) kilograms per year, a flux that thereafter declined with a half-life of approximately 10(8) years. These results may be put in context by comparison with terrestrial oceanic and total biomasses, approximately 3 x 10(12) kilograms and approximately 6 x 10(14) kilograms, respectively.

Chemical evolution of interstellar dust, comets and the origins of life

It now appears that the chemical evolution of the pre-solar system interstellar dust ensures that a major fraction of comets is in the form of complex organic molecules at least partially of a prebiotic nature and that the submicron interstellar dust preserves its chemical integrity as result of forming a very tenuous low density comet structure whose solid matter occupies approximately 1/5 of the total volume. This low density micro structure further provides a physical basis for comets bringing a significant fraction of the original interstellar organic molecules to the earth unmodified by the impact event. Finally, the evidence for a large number of comet collisions with the early earth ensured that the major organic molecular budget on the earth's surface was "continuously" supplied along with water well before 3.8 billion years ago which is the earliest date for life. The chemistry and morphological structure of a comet nucleus as an aggregate of interstellar dust is used to provide comparisons with a variety of comet Halley results: the density of the nucleus and of the dust; the dust cloud model and its consequences on the production of C+ and CN in the coma by small organic grains; the surface albedo and the low nucleus heat conductivity and high surface temperature; the appearance of 10(-14) g and 10(-17) g dust particles along with higher masses; the mass spectra of dust and infrared spectroscopy as evidence for complex organic grain mantles and of very small (approximately 0.01 micrometer) carbonaceous and silicate grains; the appearance of small grains resulting from breakup of larger grains. The cosmic ray dosage of a comet nucleus during its 4.5 billion years in the Oort cloud appears to be many orders of magnitude less than the dosage of the preaggregated interstellar dust by ultraviolet photons except perhaps in the outer few meters of the nucleus of a new comet. The heat conductivity calculated for aggregated dust is certainly less than 10(-4) that of crystalline ice. This, in combination with the interstellar dust microstructure, provide a basis for showing that solar heating of the interior of a nucleus is lower than previously estimated.

Thermal evolution of Comet P/Tempel 1--representing the group of targets for the CRAF and CNSR missions

The properties of the outer layers of comets considered for the future Comet Rendezvous and Asteroid Flyby and Comet Nucleus Sample Return missions are studied, by following numerically the thermal evolution of spherically symmetric models of the nucleus, in the orbit of Comet P/Tempel-1. The evolution starts from isothermal (10 degrees K) and homogeneous nuclei, composed of amorphous ice and dust. The crystallization of amorphous ice at 137 degrees K is taken into account. As the ice sublimates, a permanent dust mantle is allowed to accumulate, at a rate which is proportional to the sublimation rate. Evolutionary sequences are computed for different values of the density, the dust/ice mass ratio, and the (constant) fraction of the dust which is not carried away with the sublimating ice. The main conclusions are (a) the temperatures at the outer and inner surfaces of the dust mantle are not very sensitive to changes in the parameters; (b) although the dust is assumed permeable to water vapor the rate of erosion of the nucleus slows down as the dust mantle grows and its insulating effect increases; (c) the temperature at a depth of 10 m is approximately 160 degrees K for all models considered and hence, the ice at this depth is crystalline; (d) the total thickness of the crystalline ice layer, between the dust mantle and the amorphous ice core, varies from 40 to 240 m, depending on the parameters assumed. Consequently, it should be difficult for the probes of the two comet missions to sample pristine amorphous ice, unless they are aimed at the bottom of an active crater.