Emily Lakdawalla • October 27, 2014

A feast of comet features from Rosetta at Churyumov-Gerasimenko

I have been horribly behind in posting images from Rosetta's exploration of comet 67P/Churyumov-Gerasimenko, and that's a shame, because the spacecraft has lately been exploring the comet from a range of only 10 kilometers. From that range, the NavCam gets sub-meter resolution, and we're seeing a menagerie of odd surface features. Some of them are odd because I haven't seen anything like them anywhere else, like the crustily walled circular pits in this photo:

ESA / Rosetta / NavCam / Damia Bouic

Comet Churyumov-Gerasimenko: pits, scarps, and boulders on the neck, October 18, 2014

The taken on 18 October from a distance of 9.9 km from the center of the comet, or about 7.9 kilometers from the surface, so the image covers an area about 1.4 kilometers wide. It contains a wide variety of remarkable landforms, including circular pits and scallop-shaped scarps, that result from weird comet surface modification processes.

Other features are odd because they look familiar and yet have no right being on a comet. I've called them "rhythmic ridges" in the caption to the image below, but to pretty much everyone who looks at them, they look like sand dunes. Which are just plain impossible on a body that has neither atmosphere nor much of any gravity.

ESA / Rosetta / NavCam / Damia Bouic

Jagged comet landscape, October 24, 2014

This four-image mosaic contains unusual rhythmic ridges and a jagged fissure, among other odd comet surface features. It was taken on October 24, 2014 from a distance of 9.8 kilometers from the center of the comet, or roughly 7.8 kilometers from the surface.

I called them "rhythmic ridges" because to call them dunes -- even if we all know that they couldn't have formed from wind or water saltating sand grains across a plain -- is to imply a cause. When you encounter unfamiliar worlds, it's all too easy to name things with terms that imply a cause and then fall into the linguistic trap that that sets up for you. It's how we got to seeing water in the "canali" on Mars. Of course, trying to avoid these pitfalls can give us really horrible names for geomorphic features, like the "recurring slope lineae" on Mars. "Canali" sounds so much more poetic.

Call them what you will, repeating ridges or "dunes" -- they show up in this gorgeous image, too. Also, I can't get over how well we can see into the shadowed areas with the light reflected off of the sunlit areas.

ESA / Rosetta / NavCam / Damia Bouic

Comet jets from close range, October 18, 2014

Comet Churyumov-Gerasimenko puts on a show for Rosetta's NavCam in this photo taken on October 18, 2014. It includes rhythmic ridges on the comet's neck as well as small bright streaks that could be particles ejected from the comet, streaking across the Rosetta photo.

This view shows the comet in a different, softer mood. Views like this make you think the comet is covered with softly drifting snow; it's all too easy to forget that the surface of this comet is as dark as coal. Which is why I'm grateful to Claudia Mignone for explaining how a coal-black comet can look so bright and snowy in Rosetta photos.

NASA / JPL / Elisabetta Bonora & Marco Faccin

NavCam view of comet Churyumov-Gerasimenko on October 15, 2014

The four images for this mosaic were from a distance of 9.9 kilometers from the center of the comet, or only 7.9 kilometers from the surface. The view is across the small lobe to the large lobe. Image scale ranges from about 0.63 m/pixel on the part of the comet that is close to the viewer, to about 0.93m/pixel on the large lobe, farther from the viewer.

They released a couple of images from the OSIRIS science camera recently, all from the wide-angle camera, so we're not yet seeing to the teeny scales that the science team is enjoying. I love this long-exposure one that shows collimated jets erupting in all different directions. It was taken six weeks ago -- I know the comet is much more active than this now.

ESA / Rosetta / DLR / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Comet Churyumov-Gerasimenko's jet activity on September 10, 2014

Rosetta OSIRIS wide-angle camera image of Comet 67P/Churyumov-Gerasimenko on 10 September 2014, showing jets of cometary activity along almost the entire body of the comet.

Here's a closeup OSIRIS image of more recent jet activity; you can slide back and forth between two different exposure settings, a short one to see surface features and a long one for jets and darker areas.

Before & after: Different exposure settings expose comet jets

ESA / Rosetta / DLR / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Before & after: Different exposure settings expose comet jets

Two views of comet 67P taken by the OSIRIS Wide-Angle Camera with different exposure settings reveal the surface (with an exposure of less than a second) and the jets (18.45-second exposure). Notice also the features in the shadowed areas of 67P that are lit by sunlight reflected off of sunlit surfaces. The two photos were taken on October 20, 2014.

I'm intrigued by the relatively bright-colored scarps along the neck in the shorter-exposure photo. They look like they might actually be a different color than the surrounding areas -- though it's hard to tell. It could just be a trick of the light. I cannot wait to see color images from OSIRIS. They've held all those close to the vest -- we may have to wait until the data get released to ESA's Planetary Science Archive to see them.

Finally, here is a photo that's not from the Rosetta team; it's work by space artist Michael Carroll, to give you a sense of the size of the comet. Enjoy this comparison of Churyumov-Gerasimenko and Mount Fuji. Thanks to Mikey's imagination, we get a sense of what the comet might look like in color:

© Michael Carroll

Comet comparison

Rosetta's comet, 67P/Churyumov-Gerasimenko, compared to Japan's Mount Fuji.

The landing is in only two weeks, and I'll be in Darmstadt to report it! Stay tuned for news!

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Emily Lakdawalla

Solar System Specialist for The Planetary Society
Read more articles by Emily Lakdawalla

Comments & Sharing

Comments

Jonathan Ursin: 2014/10/27 11:09 CDT

I love the picture of the comet with Mt Fuji! I do not know if there are any theories on what causes those dune like ridges. Here is mine. If a small ridge forms by random chance then the ridge will grow from solar erosion. In the mornings the slope will get extra heat due to the slope facing the sun rise. The extra heat helps the outgassing and the extra outgassing will help form the ridge. The same effect happens at sunset. That is my guess.

Mark Zambelli: 2014/10/28 04:43 CDT

My guess for the ridges... there may be horizontal outgassing from the face of a nearby scarp so it's been 'blowing' material grains and forming the 'dunes'... possibly.

Ned: 2014/10/28 09:01 CDT

Jonathan, interesting theory. Maybe something like penitentes. Another thought, would out gassing perhaps cause vibrations in the comet?

Ned: 2014/10/28 09:09 CDT

Maybe some kind of standing wave phenomena.

Jonathan Ursin: 2014/10/28 12:32 CDT

In my head I was thinking of something like dirt road washboarding, where instead of vehicles (me driving too fast) being the driving force it would be the sun. I think penitentes are a better analogy to my idea though. Nice!

Tom Hopp: 2014/10/28 03:15 CDT

Thanks, Emily, for ferreting out these photos. They are fabulous in detail. "Rhythmic ridges" has a nice ring to it. And we shouldn't pre-judge, but on a world where there are gasses gusting around, it seems dunes are a likely answer. But dunes of what? Black dust? Water ice particles? Both? It will be interesting to watch and see if they move like sand dunes do!!

Mac: 2014/10/28 08:52 CDT

Hi Emily, Do the "crustily walled circular pits" correspond to the sources of gas streams? It looks like the pits are roughly 100m in diameter... gas-emitting craters, like calderas (at risk of geo-mophizing.. (wait, it that a word?...)) If they were smaller, I'd think something like the "black smokers" on undersea vents...

Mac: 2014/10/28 08:58 CDT

Oops... geo-morphizing...

Mac: 2014/10/28 09:02 CDT

One other thing: I can't remember if it has been covered elsewhere, but is there good data as to the makeup of the gas jets? Water, hydrocarbon, other? Also, does this vary a lot with the comet, or the comet type?

Julian: 2014/10/29 02:12 CDT

"Rhythmic ridges" is a great descriptive term! Check out these rhythmic ridges (dune-like waveforms) to the right of the anode plate from about 2:45 to 3:15. https://www.youtube.com/watch?v=s7vhabGhDls&list=PL_7lM52_lMo2hUQUvkVlDmFJLv42hA5d2&index=13 Quite striking similarity there. What do you think, Emily? :)

Ken: 2014/10/29 02:17 CDT

It seems the jets emanate primarily from the neck area, so...is this why there is a neck, and will continued erosion cause the comet to eventually break in two?

CosmosQuest: 2014/10/29 08:55 CDT

To Julian: So are you suggesting something other than a wind or vibration cause those wavy Sand dune features? The video you posted just shows two electrodes and a pile of sand getting moved around....... But there are some interesting features at the end after he turns off the current. Not sure what the point of this video is and how it correlates to what's going on on the comet.. Please elaborate.

Julian: 2014/10/30 09:46 CDT

Hi CosmosQuest -- I'm wondering if ionized particles in the solar wind could have impacted 67P during former passes through the solar system. Since the comet likely does not possess a very extensive magnetic field, it wouldn't have the kind of "buffer" Earth enjoys. My thinking was inspired by passages from this NASA post on the "hazards of solar wind on the moon." http://sservi.nasa.gov/articles/hazards-of-solar-wind-on-moon/ For example, this quote seems germane to the electrical experiment demonstrated in the video I linked to above: "The [solar] wind does not flow evenly into the bottoms of the deep craters at the poles. Instead, the institute’s computer model shows its low-mass electrons easily flowing over the jagged edges of the craters and into their bottoms, creating a negatively charged cloud. The ions, a thousand times heavier than the electrons, want to follow. But it is much harder for them to negotiate the steep rim. Most do not make it. Those that do create an electron/ion separation effect, or ambipolar electric field. It is at its most extreme on a crater’s leeward edge—along the inside crater wall—and at the crater floor nearest the solar wind flow." Could a similar phenomenon be taking place within the scalloped "neck" of 67P/C-G?

Ned: 2014/10/30 10:41 CDT

The solar wind would not be the only possible source of ion creation. I would expect that during out gassing there could be a considerable amount of dust particle collisions which could generate ions.

CosmosQuest: 2014/10/30 01:34 CDT

Hi Ned & Julian, You both make excellent points regarding possible effects of electrical activity on the nucleus of a comet. I am especially amazed at the quote in the NASA article, to wit: "So astronauts or robotic rovers that venture into the craters will find a highly charged atmosphere made worse by cold that inhibits the already low conductivity of the lunar surface." What the author seems to suggest is that a robotic rover (or in the case of CG 67/P the Philae probe) may be subjected to a high-voltage discharge at touchdown....am I reading this correctly? I wonder is the ESO aware of this and making the necessary observations and taking the necessary precautions to exclude a potentially hazardous discharge to the lander?

Ned: 2014/10/30 04:20 CDT

The a fore mentioned article was for areas at the bottom of craters that were supper cold relative to the surrounding area and sheltered from the solar plasma wind. The solar plasma wind effectively grounds the surrounding area to prevent build up of large static charges. In the case of the Philae lander I think you are going to have the same thing as the comma plasma will prevent build up of large static charges relative to things immersed it's field. There is some discussion here http://www.diva-portal.org/smash/get/diva2:405851/FULLTEXT01.pdf , but it mainly deals with potential between the spacecraft and one of it's probes.

Mac: 2014/10/30 08:35 CDT

Remember, on something of this shape, "down" is going to vary quite a lot, and rarely be "normal to the surface"... Perhaps these waves are from the (very) gentle flow of dust or sand flowing "down" across the surface of the neck... By the way, it looks to my eye like this comet consists of two big pieces that came together, and then their new center of gravity pulled lighter dust and sand into their region of contact, the 'neck'. Possibly, this structure is not stable, so that settling occurs, resulting in the crack (lower rigth, 2nd photo). Also, top center of 2nd photo, it looks like a cascade of gravel or boulders "down" the photo, perhaps strewn after the two bodies came together... Emily, what is the best guess these days as to the composition of this comet? I wonder if those "holes" are partially collapsed cavities formed by the evaporation of the gas stream material (H2O?) Thanks for all this great imagery and analysis.

CosmosQuest: 2014/10/30 09:06 CDT

Mac, Glad you implored Emily to chime in regarding her opinion of the composition of this comet nucleus. have been a big fan of Emily since the days of the 2010 fly by of comet Hartley 2 by the reconfigured Deep Impact space probe. At both NASA conferences Emily's questions to the team of scientists we're absolutely the highlight of the 2 press conferences. she even made Michael Ahern- delete project scientist on the mission from the University of Maryland- squirm in his seat a bit when she pose a tough question about the curious columns of light that we saw eminate from the 'jets'. Emily sure put him on the spot. So question for you Emily will you have the same access to the ESO press conferences that you had at prior NASA and JPL conferences?

geecee: 2014/11/02 02:31 CST

After looking at the comet image with the jets being emitted from the center of the comet, wouldn't this object kind of break into two pieces after a few more times around the sun? All the images I've seen seem to have the jets coming out from the center. Just a thought. gary

CosmosQuest: 2014/11/03 10:09 CST

Emily: Are you going to attend the G+ Q&A session that the ESA is conducting this coming Friday? It appears they have invited you to attend as a guest (press?). Please let us know if you are going to be there.

Emily Lakdawalla: 2014/11/03 12:10 CST

Regarding electrical charges on the surface of the comet: I'm afraid I don't know what Philae has planned for, but I can tell you that engineers do plan for this in general. If you look at Curiosity self-portrait photos of the wheels, for example, you'll see long, skinny needle-like antennae that were designed to discharge static charges that they thought might accumulate during landing. I will be in Germany next week. I am planning to watch the G+ hangout on Friday, but I have not been invited to participate in it (nor would I expect to be!) The composition of the comet is mostly ice but the surface is a dusty crust -- the surface is way too hot for ice to be stable there. The structures we see are definitely related to outgassing but I don't know all that much about the mechanics of how those structures form. Hopefully Rosetta will get to witness some of this in action.

morganism: 2014/11/03 05:17 CST

Here is an article about the Vesta impact, and possible connections to impacts on bodies like 67p. Not as compressed as these plastic balls tho.... http://m.phys.org/news/2014-11-giant-impact-asteroid-vesta-belt.html ......

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