A Comet Active Beyond the Crystallization Zone

TL;DR

This paper reports the discovery of a long-period comet active at record distances from the Sun, likely driven by supervolatile ices, challenging existing models of comet activity.

Contribution

It provides observational evidence of distant comet activity beyond the crystallization zone, suggesting supervolatile sublimation as the driving mechanism.

Findings

Comet active at 23.7 AU and 8.4 AU from the Sun.

Dust coma composed of large particles, >0.1 mm.

Activity likely driven by supervolatile ices, not water ice.

Abstract

We present observations showing in-bound long-period comet C/2017 K2 (PANSTARRS) to be active at record heliocentric distance. Nucleus temperatures are too low (60 K to 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 10$^5$ km in radius, with a total scattering cross section of $\sim$10$^5$ km$^2$. 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 $\gtrsim$ 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 $<$ 9 km. Pre-discovery observations from 2013 show that the comet was also active at 23.7 AU heliocentric distance. While neither water ice sublimation nor exothermic crystallization can account for the observed distant activity, the measured properties are consistent with activity driven by sublimating supervolatile ices such as CO$_2$, CO, O$_2$ and N$_2$. Survival of supervolatiles at the nucleus surface is likely a result of the comet's recent arrival from the frigid Oort cloud.