The paleo rotation plane is first mentioned in Part 26.
The 0° (zero degrees) line running horizontally across the middle of the picture is the rotation plane (i.e. equator) as it is on today’s 67P. The brown line is the hypothesised paleo rotation plane based on 11 stretch signatures laid out in a list in Part 26. Eight of those signatures are pre-shear stretch signatures. In other words, they can only be explained if the comet stretched prior to shearing. They are therefore, by their very presence, evidence for stretch before shearing.
There is a dearth of these pre-stretch signatures between 90° and 180° longitude but since this corresponds to the flat Imhotep plain, the two signatures at either end can be joined with a straight line. These signatures are Apis and the body lobe hinge gouge (Part 20).
Notice how the paleo plane runs centrally through both Imhotep and Hatmehit, as you might expect for a comet rotating and stretching about its longest axis. Today’s plane is awkwardly displaced to one side of both features. Seeing as both Hatmehit and Imhotep are hypothesised to be the site of missing slabs at the comet’s extremities due to comet spin-up, you’d expect the paleo plane to run right through the middle of both and it does.
The brown line running across the head lobe is displaced upwards, northwards, by about 15° with respect to the two lines either side of it on the body. This is probably due to it being rotated in two axes as well as translated in two axes as it rose from the body on the growing neck.
The paleo line doesn’t run exactly symmetrically either side of the current rotation plane but is very close doing so. It would be expected not to be symmetrically balanced because it is representing the rotation plane of an erstwhile comet that had somewhat more mass. This was before slabs were lost and the head lobe was sliced during shearing. Therefore the present-day centre of gravity isn’t where it was for the formerly larger comet and it follows that today’s comet couldn’t be made rotate about the paleo axis. This means that the extra skewing southward of the brown line around 60° longitude might be due to the missing mass.
The brown line wouldn’t be expected to follow a perfect sine wave like satellite ground tracks on a Mercator projected map. This is due to the comet not being a sphere.
The neck is not annotated with the paleo plane because a) it’s wholly new material and b) the head lobe was displaced northwards in the stretch. So a sloping line across the neck would be telling us nothing about the paleo plane.
Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
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All dotted annotations by scute1133.