A comet once thought to be a quiet dirty snowball cruising through the solar system becomes quite active when seen up close.

Photography from the Rosetta mission reveals dancing gravel, whirling icy debris and transient, movable “depressions” on the smooth terrain of Comet 67P/Churyumov–Gerasimenko (Comet 67P). Alex Hayes ’03, M.Eng. ’03, associate professor of astronomy, presented the research at the American Geophysical Union’s Fall Meeting Dec. 10.

Credit: ESA/Rosetta/MPS

Hayes described how the process of sublimation, which drives a phenomenon known as scarp migration, can carve up comets and other worlds across the universe. He and his colleagues summarized their findings in a paper, “Migrating Scarps as a Significant Driver for Cometary Surface Evolution,” published in Geophysical Research Letters (September 2019).

Sam Birch, Ph.D. ’17, a postdoctoral researcher in astronomy and the lead author of the published work, said: “We expected that the rocky gravel material of the smooth terrains – terrain that covers half the comet – to be inert and devoid of ice. But these regions retained a lot of ice and were surprisingly the most active parts of the comet. We caught this movement in the act and we have now developed a model for how this material can erode seasonally.”

Shaped like a barbell, with two lobes and a neck, one of the largest smooth terrain deposits is on its neck, in a region on the comet called Hapi. As the comet speeds through the solar system at 84,000 miles per hour, it returns to rendezvous with the sun every 6.45 years. During this return, the sun heats the Hapi region for just a few months, forcing it to shed its icy material.

The Rosetta probe that captured images of the shedding process was launched in 2004, by the European Space Agency, in partnership with the Jet Propulsion Lab, in Pasadena, California. The spacecraft caught up with the comet and accompanied it on an inbound leg to the sun a decade later, in 2014, when it snapped close-up surface images for more than two years.

As the comet gets closer to the sun, changes on the comet’s surface begin to emerge. They start out looking like divots on a golf course. “They’re just little holes, like a Nike swoosh,” Birch said. “And they expand and grow, and they start sweeping across the entire Hapi region and remove a whole layer of material.”

Hayes told his fellow scientists that the processes seen on the comet are familiar on Earth. Much like ocean waves carve out grassy sections of beach, leaving a sandy overhang, icy jet streams carve out the comet.

“The Rosetta mission really changed the game. Now we can actually apply principles of geology in ways we hadn’t been able to before for comets and other small bodies,” said Birch.

Rosetta acquired substantial data regarding 67P’s surface changes on daily and monthly time scales. “This is just the beginning,” said Hayes. “The Rosetta mission has enabled the emergence of a new field in small body geology. For the first time, we can interrogate comet surfaces at the scales necessary to resolve the processes that drive surface evolution. I look forward to seeing what else the data can show us.”

The other Cornell researchers who co-wrote the paper are research associate Paul Corlies, Ph.D. ’19, and Steve Squyres, '78, Ph.D. '81, professor emeritus of astronomy.