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

Outbursts occur commonly on comets [1], with different frequencies and scales [2,3]. Despite multiple observations suggesting various triggering processes [4,5], the driving mechanism is still poorly understood. Landslides have been invoked to explain some outbursts on comet 103P/Hartley 2 [6], 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 crumbling of materials from collapsing cliffs [7,8]. 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 [9]. 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 [7]. 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 dustplume formation mechanism that does not necessarily require the breakup of pressurised crust or the presence of super volatile material, as suggested by previous studies [7]. Moreover, the collapse revealed the fresh icy interior of the comet, which is characterised by an albedo > 0.4, and provided the opportunity to study how the crumbling wall settled down forming a new talus.