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Potential effects of sublimation of water ice from very slowly movingmillimeter-sized and larger grains, a product of activity at ~10 AU or fartherfrom the Sun driven presumably by annealing of amorphous water ice, areinvestigated by comparing 2I/Borisov with a nominal Oort Cloud comet of equalperihelion distance of 2 AU. This comparison suggests that the stronglyhyperbolic motion of 2I mitigates the integrated sublimation effect. Thepopulation of these grains near the nucleus of 2I is likely to have beenresponsible for the comet appearing excessively bright in pre-discovery imagesat 5-6 AU from the Sun, when the sublimation rate was exceedingly low, as wellas for the prominent nuclear condensation more recently. All grains smallerthen 2-3 cm across had been devolatilized by mid-October 2019 and somesubjected to rapid disintegration. This left only larger chunks of the initialicy-dust halo contributing to the comet's suspected hyperactivity. Sublimationof 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 nongravitationalacceleration on the orbital motion of 2I/Borisov.

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