The curious shape of 67P has generated a lot of informal discussion and formal hypotheses that tackle the difficult process of piecing together how the shape came to be. In this abstract we are looking primarily at the shape and features of the head lobe, and in particular, the ubiquitous strata that is evident on all parts of the head lobe.
One paper in particular (Massironi et al Oct 2015) took the computer aided nucleus-wide approach to make a judgement based on 2 alternate formation hypotheses that were judged to have suitable theoretical underpinnings (contact binary and erosion)
The approach taken in this abstract (Andrew C Cooper, Marco Parigi Jan 2015 - Nov 2015) looks to follow and trace the strata without any prejudice of formation mechanism for either the strata or the bi-lobed stretch. Parallel strata running through either lobe or the entire nucleus would lean towards 67P having been part of a far bigger body, while concentric strata, either perpendicular to the current gravity vector, or a proposed Paleo gravity vector (subject to shear, folding or subduction since formation) would lean towards a formation of strata roughly ovoid in nature prior to deformation or fracture.
Patiently following the strata lines as new images in new areas were released, revealed that strata lines that appeared parallel and flat from one angle, were turning and rising in a different angle on the adjoining face of the head lobe. Additionally, some lines crossed into Hatmehit and are evident as v-shapes that can be traced onwards where appropriate images allow.
These observations were found to be completely incompatible with parallel strata, but demonstrated that the head lobe deformed before shearing. Reversing the arrow of time brings the strata back to being concentric spheroid/ovoid shape. The resulting strata lines are compatible with minute detail of the stretch before shear based on the Paleo rotation plane and precession to the modern rotation plane due to mass loss.
The following blog posts (in reverse order) detail the evidence in terms of strata analysis and detailed analysis of previously discovered matches. The conclusion is that the only way the strata could end up in the formation it did, was if the head lobe herniated then stretched, deforming the strata before then breaking away from the body lobe, locking the strata into the new configuration. The torn and sliced strata are also further evidence of fracture of the original onion layers as part of the stretch event.
Onion layers
THE SIGNATURES OF HEAD LOBE STRETCH
Before shearing
TELL-TALE LINES ACROSS ANUKET:
THE SERQET-TO-SETH MATCHES
MISSING SLAB E- ANUBIS
THE SLAB A EXTENSION
