The Philae comet lander has not spoken to Earth since a link-up through its parent spacecraft, Rosetta, on 9 July — and scientists may never hear from it again.
But a haul of seven reports analysing data that Philae collected and sent home over three days in November last year, before it went into hibernation, has revealed further puzzling information about the comet it sits on, which is whirling closer to the Sun. Some scientists say the findings suggest that the comet is not an unaltered time capsule from the dawn of the Solar System, as researchers had presumed.
It seems like the more we know, the less we know,” says Geraint Morgan, a co-investigator on the lander’s Ptolemy instrument and a physical chemist at the Open University in Milton Keynes, UK. “The comet is more complicated than we might have imagined.
The hard problem
One puzzle is that the surface of comet 67P/Churyumov–Gerasimenko is much harder than scientists had expected, according to papers published in Science on 30 July. When Philae landed on 12 November 2014, it bounced, grazed the lip of a crater, and bounced again before coming to rest. Measurements of how its legs compressed as they hit the comet1, as well as data from a hammer on Philae2 that tried unsuccessfully to penetrate the surface, show that the comet has a strong, hard crust, covered in places by a softer layer of dust and ice.
That finding could change future lander design, says Stephan Ulamec, project manager for the lander at the German Aerospace Center (DLR) in Cologne. Before the mission, some had feared that the lander might sink into several metres of soft dust. “In the future one would have to think about a mechanism that can cope with pretty hard material,” he says.
The hard surface could be the result of ice grains either compacting or crystallizing under solar radiation. Comet simulations in the 1990s showed that these processes were possible, says Karsten Seiferlin, a planetary scientist at the University of Bern and a member of Philae’s hammer team.
That the comet could have changed since its formation has implications for the wider mission, which had assumed that such bodies had existed largely unaltered since the start of the Solar System, says Seiferlin. But both the hard crust and a large variety of surface materials and structures found on the comet could be the result of recent modifications, he says.
For instance, an intriguing polymer found in the dust3 — called polyoxymethylene — may form when sunlight hits the comet’s surface and triggers reactions that link simple formaldehyde molecules into chains, researchers suggest. If the polymer covers much of the comet, it could explain the object’s dark colour, says Andrew Morse, a member of the Ptolemy team and a planetary scientist at the Open University. The polymer may also be masking signals from other interesting compounds formed earlier in the comet's history, he adds.
“It means we have to understand the physics and processes going on if we want to draw conclusions about the history of the Solar System,” says Seiferlin.
Rugged landscape