For the first time, scientists have seen a large volume of exposed ice on the surface of a comet. This groundbreaking finding could reveal secrets about the early Solar System and our own beginnings.
Two papers published today reveal separate findings, made on Comet 67P/Churyumov-Gerasimenko by ESA's Rosetta spacecraft. One is published in Nature Astronomy, which saw ice exposed after a huge landslide. The second paper, detailing changes seen across the whole comet as it approached the Sun, is published in Science.
The lead author on that first paper is Maurizio Pajola, a scientist at NASA’s Ames Research Center in California who spends his day researching Mars and its moon Phobos. But in his spare time, Pajola and a number of colleagues studied images from Rosetta’s OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) camera in 2015, and found an incredible landslide accompanied by an outburst of material. “[This research] was done for fun, that’s the beauty of it,” he told IFLScience.
What they saw was a region in the northern hemisphere of 67P experiencing a large landslide and accompanying outburst in July 2015. Images revealed a large crack on a cliff called Aswan, measuring 70 meters (230 feet) long and 1 meter (3.3 feet) wide. Later images then showed this portion of the cliff had collapsed, falling 134 meters (440 feet) and creating a stream of debris known as a talus.
The cliff seen before (September 19, 2014) and after (June 8, 2016) the collapse of the Aswan cliff. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The collapse is believed to have happened in July 2015. About 22,000 cubic meters (780,000 cubic feet) of material – equivalent to nine Olympic swimming pools – fell from the cliff, revealing a large portion of bright material underneath that was likely embedded in the cliff wall, hidden from the Sun’s rays.
This material had a lower limit albedo – or reflectivity – of 40 percent, much brighter than the surrounding surface (3 to 6 percent), suggesting it is composed largely of water ice. Five months later, the albedo of the ice had halved, and after a year it was gone completely, suggesting the ice sublimated (turned from solid to gas) under the heat of the Sun.
The researchers think this is ice from the comet’s interior (or nucleus), believed to be a reservoir of ice from the early Solar System. It is the first time ice from a comet’s nucleus has been seen in such quantities.
“It’s the final proof there are volatiles frozen under the surface,” Pajola said. “It’s important because comets are likely the building blocks [of the Solar System], together with asteroids.”
Here you can clearly see the fracture line on September 21, 2014, before the cliff collapsed. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The reason that the cliff collapsed, exposing this pristine material, is not entirely clear. But the researchers think it may be due to a massive change in temperature that occurred on the northern hemisphere as the comet rotated and swung around the Sun.
Measurements suggest temperatures in this region increased from -140°C (-220°F) to 50°C (120°F) in just 20 minutes, which could have caused the cliff to fracture and break off. The event demonstrates that comets are some of the most geologically active bodies in the Solar System.
It also, for the first time, links known outbursts of Comet 67P with surface events. Rosetta observed many outbursts from the comet during its time in orbit, when material including water and dust was thrown from the surface. This landslide provides one method for how this material can be thrown into space, with about 1 percent of the collapsed material being lost to space.
“For the first time we’ve seen a direct link between an outburst and a cliff collapse,” said Pajola. “A cliff collapse was one possibility, but we’ve never had direct proof before.”
This was the outburst that accompanied the cliff collapse, seen on July 10, 2015. ESA/Rosetta/NavCam – CC BY-SA IGO 3.0
This research was all possible thanks to visible light images from Rosetta’s OSIRIS camera. But the spacecraft also had an infrared spectrometer, called VIRTIS (Visible and Infrared Thermal Imaging Spectrometer), that could have studied the ice even more closely.
At the moment, the researchers don’t know if the instrument was pointed in this direction at the right time to capture the collapse and see the ice. More likely, it was pointed towards the larger southern hemisphere. However, Pajola said he would work with the Rosetta team to find out if there is any data on this region; much of the data from the mission is still to be analyzed.
The second paper in Science, on which Pajola is also a co-author, observed how changes across the whole comet can reshape its surface. Researchers saw boulders tumbling, more cliff collapses, and even a vast crack lengthen on the comet's neck that may one day split it in to.
“As comets approach the sun, they go into overdrive and exhibit spectacular changes on their surface,” said Ramy El-Maarry from the University of Colorado, Boulder, the study's lead author, in a statement. “This is something we were not able to really appreciate before the Rosetta mission, which gave us the chance to look at a comet in ultra-high resolution for more than two years.”
Rosetta’s mission at Comet 67P came to an end in September 2016, when the spacecraft was gently lowered to the surface and switched off. But there is clearly still much science to come from the mission as data is pored through and analyzed. And even people working on it just through their love of science can make incredible findings.
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