The Philae lander detaches from the Rosetta spacecraft in November 2014 (Credit: ESA/Rosetta/Philae/CIVA)

Philae, the European Space Agency probe that landed on and then bounced across the surface of comet 67P/Churyumov–Gerasimenko in November, has provided researchers with a wealth of data that they are now eagerly digging through. But they are also desperate for the probe to wake up again, because their findings have left them with as many questions as answers.

Philae’s mothership, Rosetta, is currently in the middle of a week-long search for Philae, but so far hasn’t found anything. “At the moment we’re simply listening for it,” says Rosetta mission scientist Matt Taylor, but he thinks the spacecraft is unlikely to be warm enough to wake up until May, when the seasons change on 67P and the comet’s dark side becomes illuminated – Philae is thought to be just on the wrong side of the dividing line.

Meanwhile, researchers on the mission have been presenting their findings at the Lunar and Planetary Science Conference in The Woodlands, Texas. The latest data, released today, shows that features on the surface that appeared to be sculpted by wind actually aren’t, chains of organic molecules are present on the comet, and the entire thing might be made from pebbles.


One aspect of comet 67P that has been puzzling researchers ever since Rosetta arrived in August are the many surface features that seem to have been produced by wind , like dunes similar to those seen on Earth. But the comet’s low density atmosphere shouldn’t be able to blow hard enough to make them. “You immediately go for a terrestrial analogue, but it might not be accurate to make that direct comparison,” says Taylor.

Dunes without wind

Stefano Mottola, who leads the ROLIS camera on the underside of Philae, presented pictures taken during the probe’s first descent to the surface that prove a different explanation. He highlighted Cheops, a nearly 50-metre wide boulder near the first landing site that the team is using as the basis for the coordinate system on 67P. It appears to have a moat in the surrounding dust on one side and a tail on the other, suggesting something is blowing dust across the comet’s surface.

The team saw similar features when they looked on a wider scale covering almost a square kilometre of the comet’s surface. “We identified 17 ‘wind’ tails that are all aligned in exactly the same direction,” said Mottola. “This is quite amazing.”

ROLIS images reveal 67Ps surface is made up of both small and large dust particles, but jets of gas emerging from the comet as it heats up aren’t strong enough to blow either kind across the surface – the small ones stick together too much, and the large ones are too heavy.

Instead, Mottola thinks particles launched into the air by jets are responsible for bouncing other dust from one part of the surface to another, a process geologists call saltation. “We are postulating that particles falling down by air fall smash on the bed and trigger saltation without the necessity of having some kind of wind,” he said.

Pebbles to the core

Philae’s final resting place, propped sideways against a cliff, seems to be quite different from its first landing spot. François Poulet of the CIVA camera team presented a more detailed analysis of the first pictures of Philae sitting on the surface on 12 November, with two feet on the ground and one hovering above. Parts of the surface here seem to be made of aggregated pebbles, but others are smoother. Neither are particularly dusty, he said. “There is no dust mobilisation in this part of the comet.”

The pebbles are a good sign because they fit with one model of how comets, the building blocks of planets, form. This model suggests that tiny particles in the early solar system clumped together to form centimetre-scale pebbles, but these pebbles don’t simply grow into larger and larger objects. Instead, the pebbles clump together while retaining their original shape. “This is exactly what we see with CIVA,” said Poulet. “It’s possible that the comet could be totally made of pebbles.”

Making contact with Philae to take more images would be extremely useful, he said, joking that the French president François Hollande’s new dog, named Philae after the probe, is much easier to wake up than the real thing. The lander rotated during its last few hours alive so CIVA would be able to take pictures of a different view, allowing the team to create a full 3D model of its surroundings by combining new pictures with the ones taken earlier. The change in illumination as the comet nears the sun will also reveal new details.

Philae’s botched landing is also causing issues for other instruments, like the CONSERT radar which scans the interior of the comet by bouncing radio waves between Rosetta and Philae. “It’s critical for some of their measurements to get a pinpoint accurate location for Philae,” says Taylor.

Getting a good sniff

Ian Wright, who leads the Ptolemy mass spectrometry on the lander, has had to adapt to the unpredictable mission. Ptolemy was designed to take gas samples from the surface shortly after landing, and analyse material from the comet delivered by the drill, but instead switched on during the bounce. “We had imagined it would take place on the surface, but it took place 100 metres away,” he said. “What we actually ended up doing was not something we had practiced for.”

The team’s analysis reveals a series of peaks in their mass spectrometry data, suggesting Ptolemy has sniffed some kind of repeated molecule. Wright thinks this is polyoxymethylene, a chain of hydrogen, carbon and oxygen atoms that was also seen during a flyby of Halley’s comet in 1986. One open question is what caps off the repeating chain – the current data suggests it is either hydrogen, formaldehyde, or acetaldehyde.

Direct sampling from the surface would really help Ptolemy, and clear up exactly which complex organic molecules are present on the comet. “This isn’t something that the instrument was particularly designed to do,” said Wright. “Please everyone, keep your fingers crossed for Philae waking up.”