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<em>Science</em>: Philae Probe Results Shed Light on the Nature of Comets

A year after the Philae probe landed on comet 67P/Churyumov-Gerasimenko, researchers analyzing the unique data transmitted back by the probe have determined that the comet's surface is significantly harder than expected in some areas and contains organic compounds that were previously unknown to exist on comets.

Before the landing, researchers had limited knowledge about the comet's surface and many theories about it were based on assumptions or unconfirmed data. Only a probe with the ability to analyze samples and conduct measurements on site could reveal the true nature of the comet.

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As it prepared to land, the Philae probe captured this image of 67P from a distance of approximately 3 kilometers above the comet's surface. | ESA/Rosetta/Philae/ROLIS/DLR

"Before the landing of Philae, we believed cometary surfaces might be very soft…Some colleagues even feared the lander may sink deeply into the surface at touchdown," said Stephan Ulamec, a Philae project manager at The German Aerospace Center. "Of course, we were aware of our limited knowledge…nobody could have been sure if the surface would be soft."

In their study published in the 31 July issue of Science, Ulamec and colleagues describe in detail the critical moments when Philae made contact with 67P, only to bounce off a granular surface with a shallow depth and finally settle on an even harder surface farther away.

Although this initial touchdown did not go as planned, researchers still managed to salvage data from the "collision." Based on the bouncing trajectory and confirmed by analyzing the depth of the lander footprint with imaging tools, Ulamec and colleagues believe that Philae's landing feet first came in contact with a soft granular surface, which was approximately 0.25 meters (0.82 feet) thick, with a harder layer below. The strength of the surface was much stronger at the final site than at the initial touchdown site, possibly contributing to why only one leg of the lander was able to partially anchor to this latter surface. Ulamec said the results demonstrate just how variable comet surfaces can be.

Analyses of a variety of images of the comet by Jean-Pierre Bibring of the Institut d'Astrophysique Spatiale, Stefano Mottola of the German Aerospace Center, and their respective colleagues reveal that 67P's surface is highly fractured, with a variety of grain scales and reflective rock structures. Its landscape is shaped by erosion that occurs from " splashing," the ejection of soil particles by the impact of an incoming projectile.

To determine the composition of the comet's head, Wlodek Kofman of Université Grenoble Alpes and colleagues directed electromagnetic signals from Philae through the comet to Rosetta, the space probe that delivered Philae, on its opposite side, The team used these measurements, which analyze the permittivity or resistance of the electrical field, to determine that 67P is more ice than dust, and has a porosity between 75 and 85%.

The high-resolution camera ROLIS took photos of Philae's descent to 67P on 12 November 2014. | ESA/Rosetta/Philae/ROLIS/DLR

Using a different approach to explore the comet's nature, Fred Goesmann of the Max Planck Institute for Solar System Research and colleagues examined the surface composition of 67P using the COmetary SAmpling and Composition (COSAC) instrument, which collects and identifies organic compounds in the comet.

COSAC collected molecules from 10 kilometers (6.2 miles) above the comet surface, after the initial touchdown, and at the final resting site. This process detected 16 organic compounds, four of which were previously unknown to exist on comets. Since some scientists believe comets delivered materials to Earth that were important for its chemical and biological evolution, the results could contribute to a deeper understanding of the history of life on Earth.

The unprecedented data that Philae has sent back brings us closer than ever before to understanding the nature of comets, Ulamec said. "We learned so much about comets, that future missions can be adapted better to this environment."