The pristine interior of comet 67P revealed by the combined Aswan outburst and cliff collapse

Abstract

Outbursts occur commonly on comets<SUP>1</SUP> with different frequencies and scales<SUP>2,3</SUP>. Despite multiple observations suggesting various triggering processes<SUP>4,5</SUP>, the driving mechanism of such outbursts is still poorly understood. Landslides have been invoked<SUP>6</SUP> to explain some outbursts on comet 103P/Hartley 2, although the process required a pre-existing dust layer on the verge of failure. The Rosetta mission observed several outbursts from its target comet 67P/Churyumov-Gerasimenko, which were attributed to dust generated by the crumbling of materials from collapsing cliffs<SUP>7,8</SUP>. However, none of the aforementioned works included definitive evidence that landslides occur on comets. Amongst the many features observed by Rosetta on the nucleus of the comet, one peculiar fracture, 70 m long and 1 m wide, was identified on images obtained in September 2014 at the edge of a cliff named Aswan<SUP>9</SUP>. On 10 July 2015, the Rosetta Navigation Camera captured a large plume of dust that could be traced back to an area encompassing the Aswan escarpment<SUP>7</SUP>. Five days later, the OSIRIS camera observed a fresh, sharp and bright edge on the Aswan cliff. Here we report the first unambiguous link between an outburst and a cliff collapse on a comet. We establish a new dust-plume formation mechanism that does not necessarily require the breakup of pressurized crust or the presence of supervolatile material, as suggested by previous studies<SUP>7</SUP>. Moreover, the collapse revealed the fresh icy interior of the comet, which is characterized by an albedo >0.4, and provided the opportunity to study how the crumbling wall settled down to form a new talus.