Potential effects of sublimation of water ice from very slowly moving millimeter-sized and larger grains .
a product of activity at \sim 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 than 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 strongly 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 .