Manyt short term burst in activity have been observed by Rosetta during its two yea long close observation, between 2014 and 2016.
Using observations made on July 10th 2015 with Rosetta’s navigation camera, an international tema of scientists, whcih includes one from thye Paris Observatory, Sonia Fornasier, lecturer at the Paris Diderot University, has been able for the first time to establish a link between a burst of activity and the ccollapse of a cliff.
The work, published in /Nature Astronomy/ involves a region of the comet called Aswan.
Aswan had been chosen in 2014 as a potential landing site for Philae : it is situated on the large lobe of the comet in the Seth region, next to face à Hapi.
The collapse of the cliff followed a rougly 70 m long and 1m wide crack which had already been observed in September 2014 on the Aswan plateau.
Five days after the July 10th 2015 surge in activity, and using the OSIRIS imagint system, scientists noted a very bright region enriched in freshly exposed water ice on the edge of the 134m high cliff, where there had earlier been a crack.
The collapse produced roughly 2 000 tonnes of material, and led to the formation of a hill made up of new rocks whose size is typically between 1,5 and 3 m.
<diaporama|titre=L’effondrement de la falaise d’Aswan.|docs=4243,4244,4245,4246>
The event made possible the study of the primordial water which is within the comet and is usually covered by a thick layer of dust, which becomes thinner as the comet approacches perihelion (lien à Fornasier et al., 2016 Science, communiqué de novembre 2016).
In effect, the newly exposed material is at least six times brighter than the darker material of comet 67P and it is rich in water ice.
Succesive images have shown how the reflectivity of the cliff is related to the progressive sublimamtion of the volatile material, which has dropped by a factor of 2 in December 2015, until the almost complete sublimation of the exposed water ice in August 2016.
On the basis of the size distribution of the new rocks created by the collapse of the cliff, scientists estimate that 99% of the debris is situated at the foot of the cliff, and that 1% of the material has sublimed into space.
The collapse of the cliff took place during the cometary niht, and is the result of progressive thermal fatigue which led to the propagation of surface cracks throgh the first sub-surface layers, leading to a progressive weakening of the material and finally to its collapse.
Thermal fatigue is associated with significant diurnal and seassonal temperature variations of the comet, leading to the break-up of surface material.
Scientists estimate that the cumulative effect of important thermal gradients could be a significant factor in weakening the surface material and could be cause of many jumps in activity.
