It’s been nearly six years since a robot called Philae became the first humanmade object to land on the surface of a comet. Though Philae is now dead, the tracks it left as it careened across its target, comet 67P/Churyumov-Gerasimenko, have revealed that the comet’s ancient ice is surprisingly soft and fluffy.
“It’s softer than the lightest snow, the froth on your cappuccino, or even the bubbles in your bubble bath,” Laurence O’Rourke, a European Space Agency (ESA) scientist at the European Space Astronomy Centre in Madrid, told Nature.
The discovery of this velvety comet ice is part of a wider investigation of “the previously undiscovered site of the second touchdown, where Philae spent almost two minutes of its cross-comet journey,” according to a study, led by O’Rourke, published on Wednesday in Nature.
For years, O’Rourke and his colleagues have been reconstructing Philae’s footprints across the surface of comet 67P. The lander, which was delivered by ESA’s Rosetta orbiter, experienced a rough touchdown—more accurately, touchdowns—when harpoons designed to anchor it to the comet didn’t fire.
As a result, Philae bounced twice, skidded a bit, smacked into a boulder, and came to rest off-course and at a sideways angle, preventing it from conducting some of its mission objectives.
Because of the low gravity at the comet, these accidental acrobatics were gentle and played out slowly over the course of two hours. When the lander finally settled in the shadows under a cliff in the comet’s Abydos region, it was still able to communicate and take pictures, but the full trajectory of its journey across comet 67P has remained mysterious ever since.
O’Rourke and his colleagues meticulously studied Rosetta’s images of the area surrounding Philae’s landing site. They pinpointed “an unusual ice feature” in a region between two boulders called Skull Top Ridge, and concluded that “only Philae’s presence” could explain it, according to the study.
The team was then able to calculate the strength of the ice based on the depth of Philae’s impression and the time it took to leave the mark on Skull Top Ridge, which the lander recorded in its magnetic sensor data.
“At the end of the crevice, Philae made a 0.25-metre-deep impression in the boulder ice, providing in situ measurements confirming that primitive ice has a very low compressive strength (less than 12 pascals, softer than freshly fallen light snow),” the team said in the study.
The lander also exposed primitive water ice that dates back to the comet’s formation in the early solar system, 4.5 billion years ago. So, while Philae was not able to use its drill to probe comet 67p as planned, it accidentally provided scientists with another way of extracting the comet’s secrets.
Philae’s encounter with the boulders “has, in fact, excavated the ice” just underneath the comet’s surface crust, O’Rourke said in a video about the discovery.
“It has actually broken the top covering of the dust,” he continued, “and exposed something that was hidden inside for billions of years. You’re seeing, really, the interior of a comet that has not been exposed since its formation.”