Sublimation of Water Ice from a Population of Large, Long-Lasting Grains Near the Nucleus of 2I/Borisov?

Abstract

Potential effects of sublimation of water ice from very slowly moving millimeter-sized and larger grains, a product of activity at ~10 AU or farther from the Sun driven presumably by annealing of amorphous water ice, are investigated by comparing 2I/Borisov with a nominal Oort Cloud comet of equal perihelion distance of 2 AU. This comparison suggests that the strongly hyperbolic motion of 2I mitigates the integrated sublimation effect. The population of these grains near the nucleus of 2I is likely to have been responsible for the comet appearing excessively bright in pre-discovery images at 5-6 AU from the Sun, when the sublimation rate was exceedingly low, as well as for the prominent nuclear condensation more recently. All grains smaller then 2-3 cm across had been devolatilized by mid-October 2019 and some subjected to rapid disintegration. This left only larger chunks of the initial icy-dust halo contributing to the comet's suspected hyperactivity. Sublimation of water ice from the nucleus has been increasing since the time of discovery, but the rate has not been high enough to exert a measurable nongravitational acceleration on the orbital motion of 2I/Borisov.