On the origin of internal layers in comet nuclei

The independence of the strata systems in the two lobes of 67P (Massironi and 58 colleagues. 2015. Nature 526, 402 - 405) is explained by an assumed, low speed, break-up of the nucleus from a bi-lobate shape into a low energy close binary, soon after the time of onset of crystallization, as the surface is weakened. Eventually, the comet returns to a physically connected bi-lobate shape well after the crystallization of the interior of both lobes is complete (in about ∼2000 y). This “phase-change mechanism” during the Centaur phase may lead to internal layer geometries that vary from one Jupiter Family Comet (JFC) nucleus to another depending on their original shape and internal structure.