Snake Eyes on a Comet

Weird-looking Jets from Comet 67P Tell Tales of Interior Activity

The Rosetta Mission’s OSIRIS camera team released an image of jets blasting out from beneath the dusty surface of Comet 67P/Churyumov-Gerasimenko. The instant I saw the image, the jets reminded me of fangs on an alien snake or laser-like eyes on some strange sci-fi monster (how’s that for some Monday-morning pareidolia?). Actually, the truth is far more interesting than fiction.

What we’re looking at here is what happens when the Sun warms a comet. It’s a view of two jets on the side of the comet that will shortly be in darkness. And therein lies an interesting suggestion about jet activity on 67P. All along, since Rosetta arrived at the comet, we’ve been getting treated to increasing amounts of jet activity as 67P gets closer to the Sun and gets heated up. But, until recently, the jets were only active on the daylight side of the comet. Now, they’re persisting after sunset, and showing up prior to sunrise. So, what has changed?

The answer is pretty simple: as the comet gets closer to the Sun, its activity is increasing. The OSIRIS team suggests that the comet can store heat beneath the dusty crust, and the deeper layers remain warm quite a while after sunset and after the surface cools. This isn’t a new idea, since other comets have shown similar post-sunset activity, but this is the first time that astronomers have been able to get a good, high-resolution look at post-sunset jets on a comet. It’s the kind of detail that will help comet scientists understand their targets better.

Comet 67P has its closest approach to the Sun on August 13, 2015, and the Rosetta spacecraft will be tracking right along with it through this incredibly busy (and somewhat dangerous) time. For some comets, perihelion passage can break them apart. Comet 67P may not face as much danger as sun-grazing comets that come too close to the Sun. That’s because the closest it will come to the Sun is 186 million kilometers (115 million miles). Earth is 150 million kilometers (93 million miles) away, so that puts the comet’s perihelion distance at outside the orbit of Earth. Still, the comet should show quite a bit more activity, which the spacecraft will be documenting along the way. Stay tuned!