Landslide Spotted on a Comet for the First Time
published during a waning crescent moon.

Comets are notoriously unstable, perpetually venting gas, sublimating ice, and crumbling under solar radiation. But it was never clear what set off the sudden outbursts of powerful jets of gas and dust. Now for the first time, a spacecraft caught a landslide on a comet and were able to link it to an outburst. The landslide also allowed scientists to analyze the freshly-exposed material of the scarp face.


Aswana cliff before collapse on July 4, 2015 and after collapse on July 15, 2015. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; F. Scholten & F. Preusker

In a study published on March 3, 2017 in Nature Astronomy, researchers with the European Space Agency’s Rosetta mission described their observations of a landslide and outburst on comet 67P/Churyumov–Gerasimenko. In September 2014, scientists spied a 70-meter long, 1-meter wide fracture running along Aswan cliff. The crack separated a 12-meter wide overhanging block from the main plateau. The block was still attached to Aswan cliff as of July 4, 2015.

The Rosetta spacecraft captured photographs of a large plume of dust originating near Aswan cliff on July 10, 2015. Researchers estimated the comet ejected half a million to a million kilograms of gas and dust.


Aswan cliff on before collapse July 10, 2015 [left]; after collapse on July 15, 2015; and as the scarp dulls in brightness by December 26, 2015 and August 6, 2016 [right]. Arrows mark fracture, exposed water-ice, and delineate the new cliff top. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

On July 15, 2015, the spacecraft was in position to re-photograph Aswan cliff. Now the block was gone, the cliff collapsed into a landslide and exposing a fresh, sharp scarp of bright ice. Volume analysis using digital terrain models and anaglyph images indicate approximately 22,000 cubic meters of material broke off the cliff. Boulders formed a talus cone along the base of the 134-meter tall cliff.

Similar to on Earth, most of the material broke into smaller pieces with debris piling up into an untidy heap. Approximately 99% of the cliff collapse broke into boulders between 0.5 and 10 meters diameter, with the remaining 1% ejected into space in the dust plume observed by Rosetta five days earlier.


The cliff with fracture before collapse [left], the outburst of a plume of dust on July 10, 2015 [center], and the cliff after collapse [right] with bright freshly-exposed cliff face material in the scarp [top] and reshaped cliff top [bottom]. Orientations vary between images with the same boulder circled in each. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Scientists continued observations of the cliff, watching the fresh bright ice evaporate and dull under dust over the following months.

Just over a year later on August 6, 2016, the brightness had faded back to the average brightness of the comet with only one large bright boulder left.


Anaglyph images of Aswan cliff before and after collapse, analyzed to estimate the volume of the collapsed overhang. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; M. Pajola

This is the first unambiguous link between a cliff collapse on a comet and an outburst of gas and dust. Explaining the outburst required mission scientists to theorize a new mechanism for outbursts, as previous theories required the breakup of pressurized crust or the presence of supervolatile materials.

The two-year Rosetta mission ended on September 30, 2016 by deliberately crashing the spacecraft into the comet 67P/Churyumov–Gerasimenko.