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°K) and homogeneous nuclei, composed of amorphous ice and dust. The crystallization of amorphous ice at 137°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 ∼160°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.