Rosetta reveals all

Growing fractures, collapsing cliffs, rolling boulders and moving material burying some features on the comet’s surface while exhuming others are among the remarkable changes documented during Rosetta’s mission at Comet 67.

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A study published in Science 21 March summarises the types of surface changes observed during Rosetta’s two years at Comet 67P/Churyumov-Gerasimenko. Notable differences are seen before and after the comet’s most active period – perihelion – as it reached its closest point to the Sun along its orbit.

“Monitoring the comet continuously as it traversed the inner Solar System gave us an unprecedented insight not only into how comets change when they travel close to the Sun, but also how fast these changes take place,” says Ramy El-Maarry, study leader

The changes, which were either unique transient phenomena or taking place over longer periods, are linked to different geological processes: in situ weathering and erosion, sublimation of water-ice, and mechanical stresses arising from the comet’s spin.

In situ weathering occurs all over the comet, where consolidated materials are weakened – such as by heating and cooling cycles on daily or seasonal timescales – causing their fragmentation. Combined with heating of subsurface ices that lead to outflows of gas, this can ultimately result in the sudden collapse of cliff walls, the evidence of which is apparent in several locations on the comet.

A completely different process is thought to be responsible for the 500 m long fracture spotted in August 2014 that runs through the comet’s neck in the Anuket region, and which was found to have extended by about 30 m by December 2014. This is linked to the comet’s increasing spin rate in the lead up to perihelion.

Furthermore, in images taken in June 2016, a new 150–300 m-long fracture was identified parallel to the original fracture.

Close to the fractures, a 4 m-wide boulder moved by about 15 m, as determined by comparing images taken in March 2015 and June 2016. It is not clear if the fracture extension and movement of the boulder are related to each other or caused by different processes.

A substantially larger boulder, some 30 m wide and weighing 12 800 tonnes, was found to have moved an impressive 140 m in the Khonsu region, on the larger of the two comet lobes.

It is thought that the boulder moved during the perihelion period, as several outburst events were detected close to its original position. The movement could have been triggered in one of two ways: either the material on which it was sitting eroded away, allowing it to roll downslope, or a forceful outburst could have directly lifted it to the new location.

 

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