In this heavily overexposed image, the jets coming from vents can be seen. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Jun 16, 2015 A comet warms

Sen—The big news from the Rosetta mission right now is of course the reawakening of the Philae lander. I’m very excited and happy about that! But don’t forget there’s an orbiter up there too, and it’s discovering new things all the time.

For example: Things on the comet 67P/Churyumov-Gerasimenko are heating up. Literally.

Comets are odd objects. Unlike asteroids, which are mostly carbonaceous or rocky (and metallic to a lesser extent), comets are more like dirty snowballs: Carbon, rock, and ice all mixed together. That ice isn’t just water ice, but also frozen ammonia, carbon dioxide, and other materials we normally think of as gasses on Earth.

Many comets spend a lot of time out past Jupiter, frozen and inert. But if their orbit brings them closer to the Sun, our star warms them up. When they get close enough, sunlight can turn the ice directly into a gas (a process called sublimation). This gas expands around the solid part of the comet, forming the fuzzy head. It carries dust with it, and the material gets left behind, forming the tail. Or tails; dust tends to follow the comet in its orbit, forming one (usually yellowish) tail, while the gas gets ionized by the ultraviolet light from the Sun, then swept away by the solar wind, forming a second (blue or green) tail.

Previous space missions have shown that the ice on comets tends to be spraying out from specific spots, vents in the surface. Rosetta showed the same for 67P; images show the gas blowing away in jets from the surface. In fact, several spots on the surface appear to be circular openings, and jets are seen blowing from them. Most likely those openings widen as the ice sublimates until it runs out. That may be why the surface of 67P has that oddly sculpted-looking surface.

Rosetta has been orbiting 67P for nearly a year now, and has mapped a lot of these jets. But until recently, they’ve only been seen when the Sun is shining directly on the surface where the vents are.

That changed in April, when a handful of jets were seen blowing out of the comet after the Sun was hidden from view. The jets were coming from the Ma’at region located on the side of the smaller “head” of the comet. There’s a depression there, almost a cave, and while the Sun was shining all around that part, the jets appear to be coming from inside the dip where the Sun was not shining. That part of the comet had experienced sunset about a half hour before—the comet spins once every 12.4 hours—but the jets were still active.

This is exciting news, because it means the comet is becoming more active. That’s right on schedule! The comet orbits the Sun once every 6.4 years, on a path that takes it out just past Jupiter to inside the orbit of Mars. Rosetta caught up with it as it was inbound, the idea being to watch as the comet “woke up” and became more active.

As the comet gets closer to the Sun, the surface is warming up, and apparently staying warmer for longer now as well, even after local sunset. This means we’ll be seeing ever more activity from it, including more jets, and strengthening of the ones we already see.

What I’d love to see is a jet turning off, where we see it spewing now, but then it weakens and stops. What happens then? Does it shut off abruptly, fade away, sputter then wind down, what? And then close-up images of the vent would show what happens as the surface erodes and what happens to the vent after the ice is all gone. Does it persist, or collapse (after all, with the ice gone, you’d expect a lot of the support would go away too)?

There are a lot of unusual features on the surface of the comet, but they were there when Rosetta got there, so it’s unclear how they formed. Some look wind-blown! There’s no air on a comet, so those must be due to venting gas. But exactly how that works is unknown.

67P is small, just a few kilometers end-to-end. But what we’re seeing here is a world, and one that changes almost day by day. Maps for such a place are temporary at best, but that’s why they’re so important: They show us shifting surface of the comet, and hint at the dynamics that lie—literally—underneath. 

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