Study sheds new light on the structure of comets

A University of Stirling planetary scientist – working with European colleagues – has helped shed new light on the physical properties and evolution of comets.

Dr Nick Attree, of the Faculty of Natural Sciences, was part of a team that, for the first time ever, discovered a fracture zone – caused by two planes shearing against one another – on a comet.

While such structures are commonly seen on Earth, the suggestion of fracturing and significant erosion on comets far out in the solar system is a relatively new phenomenon, according to the new report led by the Laboratoire d’Astrophysique de Marseille (LAM) and involving a number of international partners.

Dr Attree said: “Comets formed around four billion years ago in the outer solar system and haven’t been thought to change much until they reach the inner solar system – but this study suggests that may not be the case.

“The team examined a network of fractures on the comet 67P/Churyumov-Gerasimenko and the findings suggest that fracturing from stress is an important erosion mechanism in comets, especially in the ‘neck’ of the comet, even in its early life.

“Significantly, this is the first time that a fracture zone has been identified on a comet and the suggestion of significant erosion far out in the solar system is relatively new.”

Dr Nick Attree, of the University of Stirling, worked on the research.

The team used images from the European Space Agency’s Rosetta spacecraft’s Optical, Spectroscopic, and Infrared Remote Imaging System camera (OSIRIS) – along with modelling – to examine a network of fractures on 67P/Churyumov-Gerasimenko. The comet – first discovered in 1969 and named after its discoverers – is a regular visitor to the inner solar system.

The scientists mapped fractures in the ‘neck’ region of the comet, where the two lobes join together, and found their properties match with a sheared fault zone – a structure seen on Earth, where a shear, or sliding motion, between two planes causes stress fractures in the material.

Modelling showed that the stress was caused by the shape of the comet – with the uneven distribution of the two lobes, combined with the comet’s gravity and rotation, causing a twisting and shear at the neck. The study also found that the fractures propagate into the interior, to hundreds of metres in depth – leaving the entire inside of the neck fractured and brittle.

Comets with two lobes appear to be common, so this mechanism may be important for other objects too.

Dr Attree carried out 3D analysis and statistical work on the fractures and helped to interpret the stress model while working at LAM, supported by an EU Horizon 2020 grant through the French National Centre for Scientific Research.

Dr Attree joined the University of Stirling last year, supported by the UK Space Agency.

The paper, 'Bilobate comet morphology and internal structure controlled by shear deformation', is published in Nature Geoscience.