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Abstract

Hubble Space Telescope observations of interstellar comet 2I/Borisov near perihelion show the ejection of large (gsim; 100 μm) particles at ≲9 m s-1 speeds, with estimated mass-loss rates of ∼35 kg s-1. 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°(pole direction R.A. = 205° and decl. = 52°. The subsolar latitude varied from -35° (southern solstice) at the time of discovery to 0° (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.