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Hubble Space Telescope observations of interstellar comet 2I/Borisov nearperihelion 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 cometBorisov corresponds to loss of a surface shell on the nucleus only ~0.4 mthick. This shell is thin enough to be susceptible to past chemical processingin the interstellar medium by cosmic rays, meaning that the ejected materialscannot necessarily be considered as pristine. Our high-resolution images revealpersistent asymmetry in the dust coma, best explained by a thermal lag on therotating nucleus causing peak mass loss to occur in the comet nucleusafternoon. In this interpretation, the nucleus rotates with an obliquity of 30deg (pole direction RA = 205 deg and Dec. = 52 deg). The subsolar latitudevaried 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 thenorthern hemisphere is illuminated for the first time.

Coma Anisotropy and the Rotation Pole of Interstellar Comet 2I/Borisov