Astronomer Maurizio Pajola, who had just started a new job studying Mars' moon Phobos at NASA's Ames Research Center, had been up for hours looking through images taken by the Rosetta spacecraft of its dumpy, duck-shaped Comet 67P/Churyumov-Gerasimenko. It was 3 a.m. and the only time he could continue his work on Rosetta mission.
It was a photo from December 2015 and his eyes started beaming when he spotted something that has never happened before-A patch of something bright and white shined out from the comet's dim surface. Pajola started looking through 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.
One of Rosetta's less powerful cameras captured yet another image on July 10, showing 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 on Tuesday in the journal Nature Astronomy, Pajola and his colleagues identified that the event he spotted was a landslide - the first captured on a comet as the collapse of the dark organic material coating the cliff face revealed that pristine water ice lies beneath the comet's surface. The scientists reported that this landslide was the most dramatic of several geological phenomena they witnessed on the comet and the lump of ice and rock was 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."
According to the scientists, 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, was puffing up above the surface to form a cloud of dust and gas. This suggests an monosemous link between outbursts (which give comets their characteristic comas) and destructive events like landslides.
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, now we can see the pristine material inside the comet," Pajola said.
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 one 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.
Researchers, in a Science paper which is co-authored by Pajola, 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.
Areas of Comet 67P facing the sun are exposed to extreme heat as it has no atmosphere to protect it and those that face away are cloaked in chilly darkness. The comet's rubber duck shape means that sunlight can stream into unexpected places, creating dramatic temperature gradients and 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.
Even 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