Rosetta’s Comet Looks The Way It Is Due To Stress

Comet 67P sublimating in front of Rosetta's 'eyes'. ESA/Rosetta/NavCam

Comet 67P/Churyumov–Gerasimenko, the object visited by the European Space Agency's (ESA) Rosetta probe, is literally being sculpted by stress. The latest analysis has shown that the independent motions of the two lobes of the comet are creating wide fissures that are changing the shape of the comet and might even lead to a break-up.

Comet 67P is believed to have formed in the outer regions of the Solar System by the collision of two primordial objects. As the comet moved inward, the two lobes become more active. Due to gravitational interaction and pressure on the material that makes up the comet, cracks have begun to appear around the neck region and extend out globally.  

The evidence collected by the team suggests another important fact about the comet. The researchers weren’t sure if this was going to be the case, but given the depth of the fissures, it's likely the interior of 67P is brittle. Their findings will be published in an upcoming paper. 

“We found networks of faults and fractures penetrating 500 metres underground, and stretching out for hundreds of metres,” lead author Christophe Matonti of Aix-Marseille University, said in a statement. “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 work used detailed high-resolution images taken by Rosetta's OSIRIS camera, which is capable of seeing the comet in visible light and infrared, as well as analyzing the composition of the comet as well. The team then used stress modeling and organized the images in a three-dimensional analysis.

“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,” explained co-author Olivier Groussin, also at Aix-Marseille University. “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 evolution of 67P over 4.5 billion years. C. Matonti et al (2019)

As 67P came to rest between the orbit of Mars and Jupiter, it got close enough to the Sun that it began sublimating – the transition of a substance from solid to gas, in this case, ice into vapor – and over tens of thousands of years, became what we see today.

Rosetta studied the comet for over two years, from July 2014. It eventually crash landed on the comet when it ran out of fuel on September 30, 2016.

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