The Journal of Geophysical Research: Planets doesn’t often handle pristine relics from the dawn of the Solar System- ‘pristine’ implies a distinct lack of geophysics. Yet in Vol. 128, #12:
Hartzell, C. M. Zhang, Y. Characterizing the Modulation and Activation-Triggering Mechanisms of Main-Belt Comets via 3D Thermophysical Modeling of an Ellipsoidal Body 2023JE008047
‘Main-Belt Comets’ are exactly what they sound like. Objects that show cometary activity (mass loss of some sort), despite having the orbit of an asteroid, within the chief reservoir, the Belt between Mars and Jupiter. So, how is a Main-Belt body somehow a comet, and not ‘just’ an asteroid? The distinction is ‘of some sort’. The most straightforward way to cause activity is to impact a target; the spray of ejecta will do a good impression of a comet tail… temporarily. (Just look at Dimorphos, after DART.) Other possibilities include YORP spinup, causing rocks to fly off from the centrifugal force. And yet another option is, in fact, sublimation of ices, as in a ‘real’ comet. We now know at least one Main-Belt Comet is active because of ice loss, when the James Webb Space Telescope measured the spectra of its emitted vapors.
Hartzell et al. continue this line of inquiry. Given multiple possibilities for activity, and what we know and do not know about the conditions and populations in the Main Belt, what can we say about more likely or less likely causes of activity? Multiple, previous authors had made clear ices can survive within the outer Belt- they just have to be buried under the surface. The issue is how deep they need to be, which would make them more or less likely to ‘activate’ by a given triggering process. Hartzell et al. thus take it from ‘can there be ice’ to ‘can there be ice loss.’
Asteroids: there can be more than you thought there was.