No Really, Look at It!
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!
“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.”
Actually, that IS like it. A very rarefied outflow of gas lofting tiny dust grains in a low gravity environment. Grains that aren’t fountained off the nucleus above escape velocity settle back and collect in the neck region, where they will be subject to saltation processes, where grains that are expelled in relatively low-velocity transverse trajectories near the strike others situated around the outflow. The neck region produces a transient ‘atmosphere’ every perihelion passage that shoves solid grains around on its surface. It may be startling, but it is one of those things that in retrospect might have been predicted by entirely known factors if imagination was not habitually stultified by established conceptual models of what comets ‘should’ be like – that dust grains could sort themselves in wave-like patterns that not only resemble but in fact are dunes, which we traditionally regard as a phenomenon restricted to planetary atmospheres. Our crude and categorically-restricted conceptual models of the conditions under which dunes may form are marvelously corrected and the envelope of possibility expanded. That is a perfect example of the tonic of space exploration.
BTW, the morphology and texture of the surface with its numerous circular ‘crater-like’ subsidence features reminds me of an analog I had once encountered many years ago as a child. In a field next to a shopping center development workers had dumped lots of discarded asphalt that was still hot together with great piles of snow from a freak snowstorm that had been bulldozed off to the side after paving a very large parking lot. I spent weeks exploring those mounds. At first I was fascinated by how they hissed and puffed, and over the following days as they cooled they developed a morphology and surface texture that were very reminiscent of those on 67P – particularly the subsidence basins and smoothed regions or ‘crusts’ that appeared to ‘drape’ over portions of the mounds. Its not the first time I was struck by this resemblance – it first came from images of both comet Tempel 1 and comet Hartley 2 by the Deep Impact/EPOXI spacecraft mission. While looking at images of Tempel 1 with an astronomer friend I casually mentioned that the surface reminded me of asphalt, and he didn’t find that very appropriate. When 67P loomed close enough for Rosetta to obtain hi-res images, that friend called me and said, “Hey, now I know what you meant!”
Adoph,
Of course you are correct in describing the outgassing and its effects in more detail than I included. I was thinking when I wrote the sentence that it’s not an atmosphere like Earth’s or even Mars’s per se, or like what we think of when we say “atmosphere” as this constant blanket of gases that is held by gravity. It’s clear that there is an atmosphere at the comet, however thin. So, of course, the term “atmosphere” is broad enough to include the rarefied blanket around the comet as it outgasses. On the planetary spectrum, there’s Mercury, with the most tenuous of atmospheres that really comes from the Sun. I’d characterize the comet’s atmosphere as constantly changing in all respects. And there’s Pluto, with an atmosphere that puffs up from the ground up as it gets closer to the Sun and then freezes out as it moves away. Like Pluto, when the comet gets far enough from the Sun, that little blanket will freeze out and/or blow away for good.