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The outgassing observed in the form of comet tails is evidence for the existence of present-day ice within the asteroid belt. A study published in the Astrophys. J. 682, 697 (2008) investigates whether it is physically plausible that ice has survived in the main asteroid belt over the age of the solar system. It explores the loss rate as a function of about a dozen parameters and introduces the concept of a “buried snow line”. For a dusty surface layer, ice can persist within the top few meters of the surface over billions of years, if the mean surface temperature is less than about 145 Kelvin. Variations in surface layer properties within a plausible range are unlikely to change this threshold temperature by more than 10 K. Longevity of ice in the shallow subsurface of asteroid 7968 Elst-Pizarro, also known as Main Belt Comet 133/P Elst-Pizarro, is plausible. Parameter regions for ice to survive over the age of the solar system exist for all of the main asteroid belt, but preferentially for large distances from the Sun and slowly rotating bodies with surfaces consisting of small particles. Rocky surfaces, in contrast to dusty surfaces, are rarely able to retain ice in the shallow subsurface.

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