  It has been suggested that the comet-like activity of Main Belt Comets is due
to the sublimation of sub-surface water-ice that is exposed when these objects
are impacted by meter-sized bodies. We recently examined this scenario and
showed that such impacts can in fact excavate ice and present a plausible
mechanism for triggering the activation of MBCs (Haghighipour et al. 2016).
However, because the purpose of that study was to prove the concept and
identify the most viable ice-longevity model, the porosity of the object and
the loss of ice due to the heat of impact were ignored. In this paper, we
extend our impact simulations to porous materials and account for the loss of
ice due to an impact. We show that for a porous MBC, impact craters are deeper,
reaching to approximately 15 m implying that if the activation of MBCs is due
to the sublimation of sub-surface ice, this ice has to be within the top 15 m
of the object. Results also indicate that the loss of ice due to the heat of
impact is negligible, and the re-accretion of ejected ice is small. The latter
suggests that the activities of current MBCs are most probably from multiple
impact sites. Our study also indicates that in order for sublimation from
multiple sites to account for the observed activity of the currently known
MBCs, the water content of MBCs (and their parent asteroids) needs to be larger
than the values traditionally considered in models of terrestrial planet
formation.
