  Recent observations show activity in long-period comet C/2017 K2 at
heliocentric distances beyond the orbit of Uranus. With this as motivation, we
constructed a simple model that takes a detailed account of gas transport modes
and simulates the time-dependent sublimation of super-volatile ice from beneath
a porous mantle on an incoming cometary nucleus. The model reveals a localized
increase in carbon monoxide (CO) sublimation close to heliocentric distance rH
= 150 AU (local blackbody temperature around 23 K), followed by a plateau and
then a slow increase in activity towards smaller distances. This localized
increase occurs as heat transport in the nucleus transitions between two
regimes characterized by the rising temperature of the CO front at larger
distances and nearly isothermal CO at smaller distances. As this transition is
a general property of sublimation through a porous mantle, we predict that
future observations of sufficient sensitivity will show that inbound comets
(and interstellar interlopers) will exhibit activity at distances far beyond
the planetary region of the solar system.
