Stress explains the unique shape of Rosetta's Comet 67P

Feb. 19 (UPI) -- Scientists have successfully measured the stress forces that have shaped Comet 67P over millions of years.

More than two years after the European Space Agency lost contact with Rosetta, the first spacecraft to successfully land on the surface of a comet, scientists continue to gain new insights into the dramatic history and evolution of its target, Comet 67P.

Before Rosetta descended for its touchdown on the comet's surface, the spacecraft and its high resolution OSIRIS camera captured hundreds of images of the icy orb's shape and structures -- a pair of craggy, misshapen lobes conjoined by a small neck. Recently, scientists used those images to conduct stress modeling and three-dimensional analyses.

"We found networks of faults and fractures penetrating 500 meters underground, and stretching out for hundreds of meters," Christophe Matonti, researcher at Aix-Marseille University in France, said in a news release. "These geological features were created by shear stress, a mechanical force often seen at play in earthquakes or glaciers on Earth and other terrestrial planets, when two bodies or blocks push and move along one another in different directions. This is hugely exciting: it reveals much about the comet's shape, internal structure, and how it has changed and evolved over time."

The new model -- detailed this week in the journal Nature Geoscience -- located the greatest level of shear stress inside the comet's neck, the thin column connecting the two lobes.

"It's as if the material in each hemisphere is pulling and moving apart, contorting the middle part -- the neck -- and thinning it via the resulting mechanical erosion," said Aix-Marseille researcher Olivier Groussin.

Scientists surmise that the shear stress is the result of the object's rotation and the differences in the size and mass of the two lobes.

"A torque formed where the neck and 'head' meet as these protruding elements twist around the comet's centrere of gravity," Groussin said.

The new analysis showed that though the shear stress is centered in the comet's neck, it propagates throughout the comet, which suggests the comet's interior is composed of relatively brittle material.

"None of our observations can be explained by thermal processes," said researcher Nick Attree of the University of Stirling in England. "They only make sense when we consider a shear stress acting over the entire comet and especially around its neck, deforming and damaging and fracturing it over billions of years."