Smaller comets appear CO₂-rich while large ones are CO-dominated.
Small comets lose all of their CO ice early in their lives, while they are orbiting near Neptune. From the model, a comet smaller than 4 km in diameter completely loses its CO.
Larger comets don’t lose all their CO, they become CO-poor only in their upper layers, CO ice survives deep below the surface around 500 m during the some million years near Neptune.
For comets that have CO buried deep inside, when they move outward into the cold Oort Cloud their surface cools, and CO gas from the interior travels outward and refreezes near the surface. When they return toward the Sun, this refrozen CO layer becomes the main source of activity.
CO sublimation (turning directly from solid to gas) begins around 50 AU for a 10 km comet and even 150 AU for a 50 km comet. CO₂ activity starts closer to the Sun around 13 AU. Crystallization of amorphous water ice (which releases trapped gases) begins around 7 AU, causing strong outgassing bursts.
They calculated Solar heating (via boundary conditions at the surface), Internal heat transport, Sublimation and condensation, Crystallization of amorphous ice, radioactive energy release.
The second term on the left-hand side represents the transfer of heat by conduction and advection by flowing volatiles, if present, and the terms on the right-hand side include the rates of absorption/release of latent heat by sublimation/condensation in pores and radioactive energy release.
Source: https://arxiv.org/abs/2510.26549
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