  Rosetta observations of 67P/Churyumov-Gerasimenko (67P) reveal that most
changes occur in the fallback-generated smooth terrains, vast deposits of
granular material blanketing the comet's northern hemisphere. These changes
express themselves both morphologically and spectrally across the nucleus, yet
we lack a model that describes their formation and evolution. Here we present a
self-consistent model that thoroughly explains the activity and mass loss from
Hapi's smooth terrains. Our model predicts the removal of dust via re-radiated
solar insolation localized within depression scarps that are substantially more
ice-rich than previously expected. We couple our model with numerous Rosetta
observations to thoroughly capture the seasonal erosion of Hapi's smooth
terrains, where local scarp retreat gradually removes the uppermost dusty
mantle. As sublimation-regolith interactions occur on rocky planets, comets,
icy moons and KBOs, our coupled model and observations provide a foundation for
future understanding of the myriad of sublimation-carved worlds.
