Comet lightcurves: Effects of active regions and topography

The lightcurves and derived production rates of volatiles such as water for short-period comets show asymmetries around perihelion. These asymmetries do not follow a specific trend, and short-period comets are as likely to be brighter preperihelion as postperihelion. This and images of the nucleus of Comet P/Halley suggest that the activity of comet nuclei is driven by a small number of active regions. Lightcurve asymmetries may then be understood by changing geometry of the active regions with respect to the Sun. Solutions of the heat diffusion equation for a short-period comet with active regions covering the full range of spin axis orientations show that active regions can produce asymmetries of several orders of magnitude in the sublimation rate of water from the nucleus. These large and sudden swings in activity are reduced if the active zone lies within a crater or topographical depression on the nucleus. Simulation of the sublimation rate of H2O from craters on a comet nucleus show that topography can qualitatively change the dependence of water production on heliocentric distance. The effect of negative relief topography is to slow the loss of heat from the nucleus and maintain higher sublimation rates than from a flat surface active region experiencing winter. In the absence of topography, a single active region can lead to sublimation rates that vary over several orders of magnitude on short time scales. Negative relief topography reduces the magnitude and speed of these seasonal changes in comet activity, with rougher surfaces having smaller seasonal variations. The magnitude of seasonal changes in comet activity can be used with the results of this model to place upper limits on the amount of surface roughness at known active regions. (C) 1997 Academic Press.