On the origin of internal layers in comet nuclei

We present an analysis of the layered structure on 67P/Churyumov–Gerasimenko’ s Hathor cliff and propose a mechanism (self-sustaining, dual-mode propagation of amorphous to crystalline ice phase-change fronts into the nucleus interior) for its origin. If this is a viable mechanism, the strata must be geologically young (∼10⁶ y). The Hathor cliff exposes strata and orthogonal prominent linear structures deep into the small lobe over a radial distance of ∼650 m. We find an average radial thickness of a stratum (defined as the upper stratum boundary and the associated intra-strata material below) of ∼14 m and estimate its average horizontal dimension at 290 ± 110 m. Structures in intra-strata material are highly varied ranging from thin, often distorted, layers (∼1 - 3 m thick), to block-like and globular-shaped features (typically ∼1.5 m radius). Prominent lineaments are typically separated by ∼80 ± 30 m and are ∼5 m, or less, wide. Guided by the results of calculations by Tancredi, Rickman and Greenberg (1994, Astron. Astrophys. 286, 659 – 682) on the dual-mode propagation of such fronts in a simulated comet nucleus, we propose that the bi-modal rates of propagation of these fronts (an essentially stationary “quiescent” mode lasting ∼10 y (or more), alternating with an “active” rapid spurt propagating at ∼100 my⁻¹ over ∼10 - 20 m (or more) in a few months) leads to the establishment of alternating strata boundaries (in the quiescent mode), and the intra-strata material (during the active mode). To achieve the apparent global coordination of the strata we hypothesize that the direction of propagation of the fronts are controlled by the radial outflow of CO coupled with the likely existence of a coarse layered structure in the primeval material below the front. Prominent lineaments were formed after the strata, and we propose that they are the result of tensional stress due to CO and other super-volatile gases released during the phase-change process. The array of different structural forms found in the intra-strata material of 67P are postulated to be the result of various modes of fluidization of the material during the active propagation mode. 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.