The deep fractures on the surface of comet 67P/Churyumov-Gerasimenko were generated by shear stress

An article published in the journal “Nature Geoscience” shows an analysis of the erosion processes that have generated deep fractures and faults in the nucleus of comet 67P/Churyumov-Gerasimenko. A team of researchers led by geologist Christophe Matonti of the Aix-Marseille University, France, examined images captured by ESA’s Rosetta space probe’s OSIRIS camera to perform a geological and morphological analysis identifying two fracture formation processes. According to the researchers, surface fractures are generated by temperature changes while the deeper fractures are generated by shear stress.

The mission of the Rosetta space probe ended on September 30, 2016 with its landing on the surface of comet 67P/Churyumov-Gerasimenko, an impact that destroyed it since it wasn’t designed for that type of maneuver. Instead, the research continues thanks to the wealth of information on the processes that occur on a comet during its period of activity and more. In this case, the researchers focused on the faults sometimes hundreds of meters deep to understand the processes that generated them.

The Rosetta space probe spent just over two years orbiting comet 67P/Churyumov-Gerasimenko capturing very high-definition images of the surface of its two-lobe nucleus using its OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) camera. This made it possible to obtain details that were unthinkable before that mission of the existing faults. Already in the past polygonal fractures due to cooling were examined but there are others much deeper, up to 500 meters, particularly in the neck area, where the two original objects that merged to form this comet collided.

The models developed by the researchers indicate that at least a part of those deep fractures was generated by shear stress and probably the cause is to be found in the tension existing between the two lobes that form the nucleus of comet 67P/Churyumov-Gerasimenko. On Earth in case of earthquakes or in glaciers there can also be two bodies that push and move along one another in different directions, generating shear stress. The center of gravity and the point of contact between the two lobes of the cometary nucleus are different and this leads to torque with consequent stress.

Olivier Groussin of Aix-Marseille University, one of the authors of this research, explained that it’s as if the materials in each hemisphere were pulling and moving away from each other, generating the torque in the neck. According to the researchers, this effect is due to the rotation of the comet combined with its asymmetric shape.

The image (ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; C. Matonti et al. (2019)) shows the process of shear stress. Comet 67P/Churyumov-Gerasimenko is shown in the diagrams on the left from above and from its side. The frames on the right enlarge the squares indicated on the neck of the comet. The red arrow indicates the same point in both images, seen from different perspectives. The red lines indicate fractures and faults generated by shear stress. The green lines indicate the terraced layers. The two central images indicate this part of the neck seen by the OSIRIS camera.

This research suggests that shear stress would also act at great distances from the Sun in very long periods, billions of years after the formation of a comet, while the erosion that follows the sublimation of frozen materials is dominant on a scale of millions of years. Even in this case, the neck is the most affected region.

These conclusions on comet 67P/Churyumov-Gerasimenko could also be applied to other two-lobed bodies. At this moment, there’s a particular interest in Ultima Thule, even if its shape turned out to be flattened. We’ll have to wait for more detailed photos that NASA’s space probe New Horizons is slowly sending to see if the history of these two objects shows similarities from the erosion point of view.

ESA had the advantage of having its Rosetta space probe orbiting comet 67P/Churyumov-Gerasimenko mapping it completely and repeatedly. Gabriele Cremonese of the Italian National Institute of Astrophysics of Padua, another of the authors of this research, pointed out that this is the first time that planetary geologists could perform such detailed analyzes of a minor body. It’s another testimony of the extraordinary mission accomplished by Rosetta.

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