ABSTRACT.
We present an analysis of the layered structure on67P/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 stratamust be geologically young (~106 y). The Hathor cliffexposes 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 theassociated intra-strata material below)of~14 m and estimate its average horizontal dimensionat290 ± 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). Prominentlineaments 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 m.y1 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 frontsare controlled by the radial outflow of CO coupled with the likely existence of a coarse layered structure in the primeval material below the front.
Prominentlineaments were formed after the strata, and we propose that they are the result of extensional 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 bethe 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” may lead to internal layer geometries that vary from one JFC nucleus to another depending on their original shape and internal structure.
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