Kuiper Belt objects, such as Arrokoth, which was visited by the New Horizons space mission in 2019, may have formed and evolved at large heliocentric distances, where the ambient temperatures are sufficiently low for preserving volatile ices, if not at the surface, then perhaps in the interior. The purpose of the present study is to follow the long-term evolution of small bodies, composed of amorphous water ice, dust grains and ices of other volatile species that are commonly observed in comets: CO, CH4, C2H6, CO2, NH3, HCN, CH3OH, etc. The heat sources are solar radiation and the decay of short-lived radionuclides, 26Al and 60Fe. The bodies are highly porous and gases released in the interior flow to the surface through the porous medium.
Under the assumed conditions – a circular orbit at distance of 44 AU, a fast-rotating body (namely, a uniform surface temperature), and a density of 0.5g/cm3 – the most volatile ices, CO and CH4, are found to be depleted even at the center of the body over a time scale on the order of 100 Myr. Sublimation fronts advance from the surface inward, and when the temperature in the inner part rises sufficiently, bulk sublimation throughout the interior reduces gradually the volatile ices content until these ices are completely lost. All the other ices survive, so it is not surprising that water and methanol were detected on the surface of Arrokoth (Stern et al. 2019, Science 364, 9771).
The effect of radionuclides is small, since their characteristic decay times are much shorter than the thermal and the gas diffusion time scales. Most of the radioactive energy is radiated away, which only requires a slightly higher surface temperature than the equilibrium value at the corresponding distance. At a distance of 100 AU, CO is depleted, but CH4 survives to present time, except for a few meters thick outer layer. At a distance of 200 AU, even CO survives at a depth of 10 m.
Since Kuiper Belt objects are considered the progenies of short-period comets, and since CO is abundantly detected in cometary comae, the conclusion of this study is that the source of highly volatile species in active comets is gas trapped in amorphous ice.