A Comet Active Beyond the Crystallization Zone

We present observations showing inbound long-period comet C/2017 K2 (PANSTARRS) to be active at a record heliocentric distance. Nucleus temperatures are too low (60–70 K) either for water ice to sublimate or for amorphous ice to crystallize, requiring another source for the observed activity. Using the Hubble Space Telescope we find a sharply bounded, circularly symmetric dust coma 105 km in radius, with a total scattering cross-section of ~105 km2. The coma has a logarithmic surface brightness gradient −1 over much of its surface, indicating sustained, steady-state dust production. A lack of clear evidence for the action of solar radiation pressure suggests that the dust particles are large, with a mean size greater than or similar to 0.1 mm. Using a coma convolution model, we find a limit to the apparent magnitude of the nucleus V > 25.2 (absolute magnitude H > 12.9). With assumed geometric albedo p(V) = 0.04, the limit to the nucleus circular equivalent radius is 2, CO, O2, and N2. Survival of supervolatiles at the nucleus surface is likely a result of the comet's recent arrival from the frigid Oort Cloud