No Really, Look at It!

UP close and personal with Comet 67P/Churyumov-Gerasimenko.  Courtesy Rosetta/ESA.

UP close and personal with Comet 67P/Churyumov-Gerasimenko. Courtesy Rosetta/ESA.

You haven’t ever seen anything quite like this in your life. This is one of a continuing series of very cool images taken by the Rosetta’s NavCam imager from a distance of 9.8 kilometers from the center of the comet. What you see here is a region on the “neck” that connects the two parts of the comet together. There are rockpiles, outcrops, smooth areas, and — right in the center — what looks like dunes!

Dunes?  On a comet?

That’s what many of us said when we saw this image.  On one of the Facebook comet groups I’m part of, we joked around a lot about how the “spice must flow” in an obligatory “Dune” reference, but truth to tell, we were amazed to see these. What could be causing them?  It’s not like there’s an atmosphere and heavy winds on the comet’s nucleus to contribute aeolian (wind-driven) forces to the comet (although there is small atmosphere, gravitationally bound to the comet.  See the comments to this article for a more nuanced discussion of that).

But, think about it. We’re looking at an icy object that is getting close to the Sun (and thus is outgassing material from deep inside the comet due to the effects of solar heating). It will do more of this “mass loss” as it passes through perihelion (the closest point to the Sun) in its orbit. Outgassing comes from within the comet. So, chances are very good that those dunes are close to an outgassing vent. Or, at least, that’s what I’m hypothesizing. If so, then maybe the boulders were displaced by outgassing as well.

There’s a lot to learn about this comet, and the stream of images from Rosetta’s NavCam are paving the way to a greater understanding of just how comets change as they approach the Sun.  Many thanks to the ESA folks who have been sharing images and blogging about them, and to the NavCam team for making them available. We should be seeing more from the OSIRIS team (headquartered at Max Planck Institute), which has been careful about image releases. They do have an image release program in place, so stay tuned.

In the meantime, keep an eye peeled as Rosetta prepares to launch the Philae probe to the surface on November 12, 2014. The proposed landing site, currently named “J” is the subject of a naming contest, so if you’d like to see a fancier name than a single letter, enter the contest as soon as you can.It ends at midnight GMT, October 22nd!


Mimas Wobbles Due to Ocean or a Weird Core

Meet Mimas, the moon with the wobbly orbit around Saturn that might be hiding a weird core or a subsurface ocean. Courtesy NASA/JPL-Caltech/Space Science Institute

Okay, so this is kind of cool news. Astronomers studying the Saturnian moon Mimas used Cassini images of it to figure out how much it wobbles as it orbits Saturn. Turns out, it jiggles quite a bit, and that has set off a flurry of speculation about what would cause it to do that. Picture a top the shape of a Star Wars Death Star (which is kind of what Mimas looks like) wending its way around Saturn. As it does, its spin is off-kilter. And that means something’s a little odd inside Mimas.

What are the possibilities?  For one, since the wobble is about double what scientists expected it to be, whatever it is that throws the moon off as it spins has to be massive. That could mean an ocean (which would easily affect how the moon spins) or an oddly shaped core. One of the scientists suggested that if it’s the core, then it would have to be nearly football-shaped to do the trick.

How could a core get to be oblong instead of round in a world as old as Mimas? (It dates back to the earliest epochs of the solar system and is about 4 billion years old.)  One school of thought says that the core may have frozen into an oblong shape long ago, thus preserving some hint of its early history.

If Mimas is hiding an ocean beneath its cratered surface, then a little bit of math tells us that a liquid water ocean would be hidden at least 24 kilometers beneath the crust. There also needs to be some mechanism to keep the water liquid. Mimas long ago lost all the heat from its formation and its core is likely cool as well.

So, what would keep things warm enough to sustain liquid?  It turns out that tidal flexing — that is, the squeezing and contracting due to Saturn’s strong gravitational pull that Mimas undergoes as it orbits the planet — could keep things warm enough through friction to do the trick. Mimas undergoes a fairly elongated (think of it as egg-shaped) orbit around Saturn, and so at different times at its orbit it encounters changes in the gravitational pull.  This slight deviation in its orbit causes the point on Mimas’ surface that faces Saturn to vary a bit over time. If you could watch Mimas from Saturn, you’d see that wobble and notice how small areas of the surface limb shift just enough to become visible. This effect is called libration. Our own Moon has the same  motion.

So, which is it: football-shaped core or liquid ocean?  Further analysis leans toward an ocean, since models of an oddly shaped core seem to result in a different-looking Mimas than the one we really have out there. As usual, more data will help tell the story, and the Cassini Solstice mission can be counted on to crank out more images of Mimas as it pursues its wobby path around the Saturn. Stay tuned!