GAS AND DUST EMISSION FROM COMETS AND LIFE SPANS OF ACTIVE AREAS ON THEIR ROTATING NUCLEI

Various aspects of outgassing and dust emission from discrete regions on the nuclei of comets are investigated with the aim of learning about the processes of activation, dormancy, reactivation, and extinction. The issues discussed include variations in the total erosion rate per orbital period as a function of the surface location; the duration and other characteristics of an outburst of P/Schwassmann-Wachmann 1; and the enormous diversity of the light curves and patterns of coma morphology among comets. The objects of particular interest are P/Halley, P/Encke, P/Tempel 2, P/Machholz, P/Takamizawa, Bowell (1982 I) and some other new comets, and the comets that disappear before the eyes of observers. Also studied is the extent to which the birth and extinction of discrete vents can be inferred from temporal variations in the nongravitational perturbations of the orbital motions of the short-period comets. It is concluded that the relationship between the rate of insolation of the nucleus surface and the observed activity characteristics is extremely complex and that strong evidence points to other conditions that also must be satisfied for the sources to be activated and deactivated. The evolution of an active region at one location on the nucleus surface appears to be largely independent of the evolution of another active region located elsewhere on the nucleus. If so, the aging of a comet cannot be described in terms of its global characteristics, and the traditional perception of cometary deactivation as a slow and monotonic process that concludes with the object's irreversible extinction needs to be replaced with a more dynamic concept of intermittent periods of dormancy and reactivation of individual vents on highly variable timescales. Indications are that the size of the outgassing area of active regions can also vary greatly, increasing generally during the "polar summer." Typical life spans of discrete sources of activity are relatively short, being estimated at not more than a few hundred revolutions about the Sun for comets with perihelia at heliocentric distances of <2 AU, if the bulk density is very low. Cometary nuclei appear to be morphologically the more heterogeneous the smaller is the scale investigated.