The European Space Agency's Philae lander made history last November when it became the first spacecraft to land on a comet. Unfortunately, that landing turned out to be a bit of a mess. Some of Philae's hardware malfunctioned during its descent, and the lander wound up bouncing a couple of times before finally coming to rest in a spot too shadowy to charge its solar-powered batteries. So after 64 hours of collecting data, Philae powered down and went into hibernation mode.
Although data collection ended sooner than researchers anticipated, those 64 hours of study have still proven extremely valuable. Today, the journal Science published seven papers analyzing the data gathered by Philae in that short time. The research paints a comprehensive picture of the lander's rocky home, comet 67P/Churyumov-Gerasimenko — detailing its sandy surface, the composition of the comet's interior and exterior, and the organic compounds that dominate its terrain. The findings bring scientists one step closer to better understanding the origins of comets, which, can tell us a lot about how our Solar System came together billions of years ago.
Philae's bouncy landing may not have been part of the plan, but it turned out to be a small blessing in disguise for researchers. "We used it to our advantage, because now we have hardness estimates of two locations — very different ones," says study author Jens Biele, of the German Aerospace Center. His team analyzed the footprints made by Philae during each landing to determine the type of terrain underneath.
"We have hardness estimates of two locations — very different ones."
Philae's first landing site was found to be covered in a loose granular material called regolith, a little less than a foot thick; this surface material has a similar consistency to the materials covering the Moon and Mars. Biele says it's likely that the comet ejects dust flakes when heated, which then resettle on the surface to form this powdery soil.
This image shows Philae's different landing points on the comet, indicated by stars. (ESA)
The spacecraft's second landing location turned out to be significantly harder than the first. "Philae's instrument meant to hammer itself into the soil couldn’t do that there, even after three hours, because the hardness of this surface exceeded the design limit of the instrument," says Biele. "It's harder than the hardest material we had expected on comets." Biele theorizes that water ice sublimating and recondensing inside the comet forms a glue that cements the comet's materials together.
Recognizing the hard and soft parts of a comet's surface will be valuable for future missions to comets that aim to bring back samples, Biele says. "The immediate technical question is: How do you sample the hard stuff? What is best to get a sample of the comet’s surface in case it’s hard like that?"
Comet Interior & Exterior
Philae also sent electromagnetic signals through the comet's nucleus — or the "head" of the rubber ducky shaped comet. Studying the amplitude of the signals and the time it took for them to travel through the nucleus gave researchers a hint about what was inside.
"We found that it was rather homogenous," says Wlodek Kofman, director of the Grenoble Planetology Laboratory and study author. The signal data also showed that the rock is very porous, a mixture of dust and ice.
Wind tails are shown on the comet in the right image.
Two additional studies looked at close-up and infrared images to analyze the comet's exterior. Photographs of Philae's original landing site show a smooth surface with boulders and other debris jutting outward.
The pictures also show "wind tails" behind some of the jutting rocks, features that had some scientists stumped because there isn't any wind on the comet. On Earth wind tails are caused when wind carries soil across the planet’s surface. Boulders act like barriers to the direction of moving soil, causing materials to build up like a "tail" behind the rocks. Researchers speculate that the comet’s wind tails are instead caused by "splashing" — when an incoming projectile hits the comet and ejects soil particles. This particle ejection mimics the wind erosion seen on our planet.
Two additional studies focused on perhaps the most exciting elements of the comet's surface: organic compounds. Philae found 16 organic molecules in the space surrounding the comet; four of them — methyl isocyanate, acetone, propionaldehyde, and acetamide — have never been known to exist on comets before.
Learning more about the comet's organic compounds is significant because these are the molecules needed to give rise to life. "At a simplistic level, these are compounds made of carbon, hydrogen, nitrogen, oxygen," says Ian Wright, professor of planetary sciences at the Open University and one of the study authors. "They're the kinds of things you and I are made out of."
These are the molecules needed to give rise to life
Some experts theorize that comets were instrumental in bringing organic compounds to the surface of Earth billions of years ago. The idea is that water and other life-sustaining chemicals were left behind in comet impact craters, giving the earliest lifeforms the right materials to thrive.
"We're kind of looking back in time here at this stuff that would have come down," says Wright. "We're not saying life came in from the comets, but the organic materials that needed to create life came in from comets."
In other words, comet 67P/Churyumov-Gerasimenko possibly contains the same primordial soup of organic compounds that were brought to Earth when the planet was still very young. Philae has given us a sample of that material, which has been preserved in space for the last 4 billion years.