NONGRAVITATIONAL EFFECTS AND THE AGING OF PERIODIC COMETS

We present a statistical analysis of nongravitational parameters (A2) looking for evidence relating to the evolution of cometary nuclei. Our sample is restricted to short-period comets but is otherwise essentially complete up to 1990. Alternative representations of the advance or delay of the time of perihelion passage (DELTA-P', DELTA-P", DELTA-P(e) are introduced on the basis of simple theory. A correlation with the light curve perihelion asymmetry previously shown to exist for DELTA-P' [Festou et al., Nat, 345, 235 (1990)] is interpreted as evidence for a statistical increase of the free-sublimating fraction of the nuclear surface and/or a decrease of the nuclear radius with perihelion distance. This trend is found to be due to an increasing dynamical instability in the sense that recent large perturbations of the cometary orbits become more common with increasing perihelion distance q. Comparison with data on the dynamical histories is done by assigning "dynamical ages" (i.e., number of revolutions since the last major decrease of the perihelion distance) to the nongravitational parameters. The largest values of \DELTA-P"\ are found to be uniquely associated with comets having experienced recent perturbations DELTA-q < - 0.5 AU, and a rapid statistical decrease of \DELTA-P"\ with dynamical age is found for all comets with DELTA-q less-than-or-similar-to - 0.1 AU. This is interpreted as evidence for rapid buildup of dust mantles on the nuclei. A picture of the long-term evolution of Jupiter-family comets is tentatively suggested, whereby sizeable nuclei become dust mantled in successive spurts associated with the settling into new orbits with smaller perihelion distances following blow-offs of the earlier mantles. Their active lifetimes could hence be estimated as the time required to reach the overall minimum of the perihelion distance, i.e., as roughly half the dynamical lifetimes.