  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.
