Feeling stressed? You’re not alone. ESA’s Rosetta mission has revealed that geological stress arising from the shape of Comet 67P/Churyumov–Gerasimenko has been a key process in sculpting the comet's surface and interior following its formation.

The researchers used stress modelling and three-dimensional analyses of images taken by Rosetta’s high resolution OSIRIS camera to probe the comet’s surface and interior.

Small, icy comets with two distinct lobes seem to be commonplace in the Solar System, with one possible mode of formation a slow collision of two primordial objects in the early stages of formation some 4.5 billion years ago. A new study using data collected by Rosetta during its two years at Comet 67P/C-G has illuminated the mechanisms that contributed to shaping the comet over the following billions of years.

“We found networks of faults and fractures penetrating 500 metres underground, and stretching out for hundreds of metres,” says lead author Christophe Matonti of Aix-Marseille University, France.

“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 model developed by the researchers found shear stress to peak at the centre of the comet’s ‘neck’, the thinnest part of the comet 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,” explains co-author Olivier Groussin, also of Aix-Marseille University, France.

“We think this effect originally came about because of the comet’s rotation combined with its initial asymmetric shape. A torque formed where the neck and ‘head’ meet as these protruding elements twist around the comet’s centre of gravity.”

The observations suggest that the shear stress acted globally over the comet and, crucially, around its neck. The fact that fractures could propagate so deeply into 67P/C-G also confirms that the material making up the interior of the comet is brittle, something that was previously unclear.

"None of our observations can be explained by thermal processes,” adds co-author Nick Attree of the University of Stirling, UK. “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.”

Sublimation, the process of ices turning to vapour and resulting in comet dust being dragged out into space, is another well-known process that can influence a comet’s appearance over time. In particular, when a comet passes closer to the Sun, it warms up and loses its ices more rapidly – perhaps best visualised in some of the dramatic outbursts captured by Rosetta during its time at Comet 67P/C–G.

The new results shed light on how dual-lobe comets have evolved over time.