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Based on the data that Philae beamed down prior to going into hibernation, scientists believe the landing site on Comet 67P/C-G is made of a layer of dust 4 to 8 inches thick covering solid ice.
At Philae’s final landing spot, the MUPUS probe recorded a temperature of –153°C close to the floor of the lander’s balcony before it was deployed. Then, after deployment, the sensors near the tip cooled by about 10°C over a period of roughly half an hour. “We think this is either due to radiative transfer of heat to the cold nearby wall seen in the CIVA images or because the probe had been pushed into a cold dust pile,” says Jörg Knollenberg, instrument scientist for MUPUS at DLR.
The probe then started to hammer itself into the subsurface, but was unable to make more than a few millimetres of progress even at the highest power level of the hammer motor. “If we compare the data with laboratory measurements, we think that the probe encountered a hard surface with strength comparable to that of solid ice,” says Tilman Spohn, principal investigator for MUPUS.
Looking at the results of the thermal mapper and the probe together, the team have made the preliminary assessment that the upper layers of the comet’s surface consist of dust of 10–20 cm thickness, overlaying mechanically strong ice or ice and dust mixtures.
In many ways, this result is a testament to the magnificence of science and the industrial revolution. The methods and technology that made it possible for scientists to predict the make up of comets (dirty snowballs) were developed in the period from the 16th to the 19th centuries, hundreds of years before it was even possible to see Comet 67P, no less land on it and sample its surface. And what do we find when we do land there? The data gathered beforehand from far away is confirmed, as precisely as one can imagine.
Update: Another of Philae’s instruments also detected organics on the surface, though the reports so far are very vague.