An image of Comet 67P taken on July 10, 2015. The white arrow shows the outburst caused by the Aswan cliff collapse. (ESA/Rosetta/NavCam)

It was 3 a.m., and astronomer Maurizio Pajola had been up for hours looking through images taken by the Rosetta spacecraft of its dumpy, duck-shaped comet. Pajola had just started a new job studying Mars' moon Phobos at NASA's Ames Research Center. The only time he could continue this work on the Rosetta mission was the middle of the night.

His eyes were beginning to glaze over when he spotted something unusual: A patch of something bright and white shined out from the comet's dim surface. This photo was from December 2015. Pajola started flipping back though the catalogue of images taken by Rosetta's high power OSIRIS instrument until he arrived at July 4. There, streaking across a cliff called Aswan in the comet's northern hemisphere, was a gash more than 200 feet long and wide enough for a person to fall through. 

The next image, taken by one of Rosetta's less powerful cameras on July 10, showed a plume of gas and dust bursting from the comet. Five days later, OSIRIS got another good look at the site in question. The cliff face had collapsed, revealing the radiant material beneath. 

In a paper published Tuesday in the journal Nature Astronomy, Pajola and his colleagues report that the event he spotted was a landslide — the first captured on a comet. The collapse of the dark organic material coating the cliff face revealed that pristine water ice lies beneath the comet's surface, the scientists say.

The landslide was the most dramatic of several geologic phenomena that Rosetta scientists have witnessed on Comet 67P/Churyumov-Gerasimenko, a lump of ice and rock about the size of Mount Fuji. In a second paper published in the journal Science, the astronomers describe how the comets' surface is constantly changing as a result of its rotation and the glare of the sun.

These images are showing that comets are some of the most geologically active things in the solar system,” Pajola said. “We see fractures increasing, dust covering areas that were not dusted before, boulders rolling, cliffs collapsing” — all on an object barely wider than the Mall is long. 


The Aswan cliff on Comet 67P pre- and post-collapse. (ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

The landslide, which took place sometime around July 10, 2015, would not have looked like a landslide on Earth. Comet 67P is so small it hardly has any gravity, so instead of tumbling down like an avalanche, much of the material that broke off from the fractured cliff face produced an “outburst.” Some 22,000 cubic meters of material, enough to fill nine Olympic swimming pools, puffing up above the surface to form a cloud of dust and gas. This suggests an unambiguous link between outbursts (which give comets their characteristic comas) and destructive events like landslides, the scientists say.

Aswan's collapse also offered a crucial peek into the comet's interior. The Rosetta spacecraft had been accompanied by a lander, Philae, which was slated to take samples from Comet 67P's subsurface. But Philae's battery ran out just two days after landing, and scientists never got their interior samples.

We have done this last job thanks to the cliff collapse,” Pajola said. “Now we can see the pristine material inside” the comet. 

Based on the exposed patch's high albedo — a measurement of how much light it reflects — the scientists say that it must be composed of water ice. Over time, the ice sublimated (transitioned straight from a solid to a gas), and the patch faded. If you looked at Comet 67P now, the only evidence of the landslide nearly two years ago would be a rubble pile at the bottom of the cliff.


An image of the Aswan cliff taken on Dec. 26, 2015, shows the bright water ice that initially caught Pajola's eye. (ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

In the Science paper, for which Pajola is a co-author, researchers describe how geologic events like the cliff collapse are driven by the heat of the sun. In summer 2015, Comet 67P was approaching its perihelion — its closest approach to the sun.

“This is the time where you get maximum activity, the time where you get maximum amount of change,” said Ramy El-Maarry, a planetary scientist at the University of Colorado at Boulder who was the lead author on the Science study. 

Since it has no atmosphere to protect it, areas of Comet 67P facing the sun are exposed to extreme heat, while those that face away are cloaked in chilly darkness. The comet's wonky shape (it's been described as looking like a rubber duck) means that sunlight can stream into unexpected places, creating dramatic temperature gradients.

The groundbreaking mission of the Rosetta spacecraft comes to end after a scheduled crash landing on the very comet it was chasing for 12 years. (Reuters)

That's what probably happened at Aswan. Though the cliff is located in the comet's northern hemisphere, which was experiencing a frigid winter. The area around the cliff was a frosty minus 140 degrees Celsius — 50 degrees colder than the coldest temperature ever recorded on Earth. But a small ray of sun reached the cliff itself, heating that spot up to 50 degrees Celsius — the temperature of Death Valley in high summer. The extreme heat probably caused the material making up the cliff to fracture, then collapse.

Though Pajola has a full-time job as a researcher at NASA, he has been involved with the Rosetta team almost since the spacecraft's launch in 2004. He continues to study data from Rosetta in his spare time, he said, “because I love science ... and because comets are really important.”

Comets and asteroids are debris leftover from the early days of the solar system, when the planets were just being formed. Scientists consider them “time capsules” from that ancient time, 4.6 billion years ago. Comets like 67P, which are thought to have originated in the water- and volatile-rich outer solar system and then migrated inward, may also have delivered water and organic molecules to early Earth.

“That's why it's important to understand how comets behave, how they work, and what is below the surface,” Pajola said.

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