Comet 67P/Churyumov-Gerasimenko may not have the most catchy name in the solar system, but there are plenty of other things that make it intriguing – not least the first observation of a landslide on a comet.
The comet ‘belongs to’ the Rosetta spacecraft, and this image taken in December 2015 first caused excitement regarding an interestiong phenomenon.
Astronomer Maurizio Pajola, who at the time had just started a job studying Mars’ moon Phobos at NASA’s Ames Research Center, noticed something bright emanating from the comet’s surface. He was looking over Rosetta photos in his own time, late at night.
Looking back through Rosetta images, including some taken by the spacecraft’s powerful OSIRIS instrument, he landed on one from July 4th which showed a 200-ft-long gap on a cliff named Aswan, located in the comet’s northern hemisphere.
Another photo showed a burst of gas and dust, which turned out to be radiant material set beneath the cliff – revealed when it collapsed.
The rubber duck comet
In recently published papers in the Nature Astronomy and Science journals, Pajola and colleagues explained how what he had seen was the first observation of a landslide on a comet. Dark organic material on the cliff face had collapsed to reveal pristine water beneath the comet’s surface.
The team also noted that the comet, which happens to be shaped like a rubber duck, has a constantly changing surface due to its rotation and the sun’s glare.
“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”.
The comet is around the size of Mount Fuji, and its mass means gravity is less than that on Earth. Therefore, unlike with landslides on Earth, there is an outburst of material rather than a rapid tumble downwards.
Such destructive events go towards explaining comets’ distinctive shapes.
Extreme temperature swings
Destructive events are most likely when a comet is closest to the sun.
“This is the time where you get maximum activity, the time where you get a 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.
Without the protection of an atmosphere, the sun’s intensity has a dramatic effect. When parts of the comet are facing away from the sun, they are colder than any temperature ever recorded on Earth. When the sun then hits these areas, the temperature shoots up to levels similar to the hottest found on Earth.
This causes fracture and collapse – in this case, the landslide.
Image credit: ESA/Rosetta/NavCam