  Hubble Space Telescope observations of interstellar comet 2I/Borisov near
perihelion show the ejection of large (>~100 um) particles at <~9 m/s speeds,
with estimated mass-loss rates of ~35 kg/s. The total mass loss from comet
Borisov corresponds to loss of a surface shell on the nucleus only ~0.4 m
thick. This shell is thin enough to be susceptible to past chemical processing
in the interstellar medium by cosmic rays, meaning that the ejected materials
cannot necessarily be considered as pristine. Our high-resolution images reveal
persistent asymmetry in the dust coma, best explained by a thermal lag on the
rotating nucleus causing peak mass loss to occur in the comet nucleus
afternoon. In this interpretation, the nucleus rotates with an obliquity of 30
deg (pole direction RA = 205 deg and Dec. = 52 deg). The subsolar latitude
varied from -35 deg (southern solstice) at the time of discovery to 0 deg
(equinox) in 2020 January, suggesting the importance of seasonal effects.
Subsequent activity likely results from regions freshly activated as the
northern hemisphere is illuminated for the first time.
