COMET 67P is having a breakout. In close-ups of its skin, the European Space Agency’s Rosetta probe has spotted ice patches and steaming pits, hinting at turmoil within.

Like other comets, 67P doesn’t reflect much sunlight – it’s blacker than coal. But the Rosetta team has seen shiny spots twinkling up from the surface that look like metre-sized ice boulders. The ice is peeking through gaps in a skin of dark dust.

But it’s 67P’s newly discovered pits that are especially interesting. As Rosetta neared the comet, it saw circular holes. “Imagine you took a steel rod and just punched it in the snow,” says Carey Lisse of the Johns Hopkins University Applied Physics Laboratory in Maryland.

Imagine you took a steel rod and just punched it in the snow – that’s like the pits found on Rosetta


Closer inspection revealed a flurry of activity. “We were seeing tiny jets of gas and dust that were escaping from the pits,” says Jean-Baptiste Vincent at the Max Planck Institute for Solar System Research in Göttingen, Germany.

The pits are too deep to be impact craters, and too big to have been excavated by the jets that spew from their walls. Instead, the team thinks they could be sinkholes where cavities inside the comet have been uncovered. “You have a ceiling over this cavity, and at some point the ceiling cannot support its own weight, and everything collapses,” Vincent says.

After that cave-in, dust settles on the cavern floor. But the walls are left dust-free, letting sunlight shine on exposed ice – which powers the jets Rosetta saw (Nature, DOI: 10.1038/nature14564).

What’s unclear is how such underground cavities could exist in the first place. It’s possible they could have been packed with ice that vaporised and escaped through the surface, leaving a hole behind.

The fresh pits suggest that 67P hasn’t spent much time near the sun, which would have caused even more erosion, Vincent says. Comet watchers are keen to see what else Rosetta will uncover. “We’re going to learn a lot more,” says Lisse.

This article appeared in print under the headline “Sinkholes and ice spotted in comet close-up”