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Headshot of Emily Lakdawalla

Finding my way around comet Churyumov-Gerasimenko

Posted By Emily Lakdawalla

15-08-2014 12:30 CDT

Topics: Rosetta and Philae, pretty pictures, comets, comet Churyumov-Gerasimenko

Rosetta has nearly completed its first funky three-cornered orbit around in front of the comet. ("In front of?" It's been pointed out to me that the spacecraft's motion isn't enircling the comet's center of mass, but is instead maneuvering around a point located sunward of the comet.) Each day we're getting views of the nucleus from more directions. There are amazing features everywhere, but it's really hard to orient myself on this crazy-shaped world, and that makes it hard to figure out whether I'm looking at the same thing from one image to the next. I figure other observers are having the same problem, so I thought I'd put together some images that will help all of us navigate around Rosetta's new home!

This effort would be a lot easier if features had names, but of course there are no official names yet; Rosetta only just arrived, and mapping this world will take time. It's tempting to run around naming features, but that seems a bit presumptuous, and would be counterproductive once official names come out. I have to do something, though, so I'll be using unique, descriptive words to identify a few of the things that stick out at me, and hopefully at some point in the future I'll be able to attach actual official names to some of these features.

To begin with, let's orient ourselves to how this comet spins in space. The comet is made of a big lobe and a little lobe with a narrow neck between. The comet's spin axis is located at a point near where the neck connects to the big lobe. They have already defined a coordinate system for the comet, with the north and south poles corresponding to where the spin axis crops out of the nucleus. The big lobe is the zero-longitude end, and the little lobe is the 180-degree-longitude end. Take a look at how the coordinate system relates to the general shape of the comet in this animation:

Geography and sunlight on comet Churyumov-Gerasimenko

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

Geography and sunlight on comet Churyumov-Gerasimenko
A shape model for Rosetta's comet as of August 6, 2014. The colorful model on the left shows how much illumination the comet surface receives over the course of its day (red is brighter, blue is darker). On the right, a coordinate system has been superimposed. Lines of longitude converge at the north spin pole. The 180-degree longitude line runs through the narrow end of the comet. Here is a version that spins more slowly.

Now, if you look at the left side of the animation, you'll see a multicolored picture of the comet. Ignore the green dots for now; just focus on the yellow and red patterns. Those colors signify how much of the time those parts of the comet spend being sunlit. Red parts are pretty much always sunlit. You can see that there are two sunny poles, one on each lobe, that are always sunlit. If some part is always sunlit, there's a corresponding part that's always in shadow. Much of the comet above 30 degrees north is always sunlit. Therefore, we don't ever see much of the comet below 30 degrees south latitude.

So, one thing I've learned from looking at the animation is that for the most part, if I take any photo of the comet and rotate it so that the light appears to be coming from above, I will usually have oriented it so that north is approximately up. The only time this doesn't work is when the view from Rosetta is looking down onto the north pole.

Of course, as I surveyed the recent NavCam images, I realized that many of them are actually looking at a polar view. Here's a montage of all the recent NavCam images from this first orbit. The top row are approximately polar views, so I've oriented them all to have the little lobe (and 180 degrees longitude) toward the top. The bottom row are a little more oblique, so I've oriented them to have north approximately up, as best as I can figure it. (I'm waffling about which row I should've put the top right image in....)

NavCam images of comet Churyumov-Gerasimenko from Rosetta's first orbit

ESA / Rosetta / NavCam / Emily Lakdawalla

NavCam images of comet Churyumov-Gerasimenko from Rosetta's first orbit
A collection of images released by ESA to show Rosetta's view on the comet during its first orbit at an average distance of 100 kilometers. They have been reoriented to align similar features. The top row of images looks down on the comet from the north. The images were taken on August 11, 10, 9, 8, and 12. The bottom row has more oblique views, taken on August 14, 7, 13, and 6.

Now that I'm a bit more oriented, I can take on the latest image that they released from the OSIRIS camera yesterday. Actually, it's a pair of images, a left and right stereo pair. This is the first Churyumov-Gerasimenko image I've seen that fills the full OSIRIS camera frame. OSIRIS has the biggest detector of any framing camera flying beyond Earth right now: it's a 4-megapixel camera, each image being 2048 pixels on a side. Grab your red-blue glasses to enjoy the 3D view, or just enjoy the left-eye or right-eye image.

Churyumov-Gerasimenko in 3D, August 7, 2014

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

Churyumov-Gerasimenko in 3D, August 7, 2014
The two images used to make the anaglyph were taken on 7 August 2014 from a distance of 104 kilometers with Rosetta's OSIRIS narrow-angle camera. They are separated by 17 minutes.

Crossed-eye stereo

Parallel-eye stereo

Flicker gif

What are we looking at here? We're looking across the big lobe toward the small lobe. The north pole is tilted a bit toward the left.

I see three distinctly different kinds of terrain. First, there is a very, very smooth terrain. To my eye, it looks like a mantle that obscures something underneath. In one spot, I notice what looks very much like an impact crater in it. I could talk myself into believing that there were other impact craters present, whose edges have been blurred or degraded somehow.

C-G detail: smooth terrain

In some places, there are fields of boulders. There's one particularly striking field of boulders on the neck. I said I wasn't going to give anything but descriptive names to these things, but the way it lines up as a bunch of beads around the neck, I'll probably never think of this boulder thing as anything but the "necklace."

C-G detail: boulders

You can see it in several of the NavCam images, too -- an incongrouous group of oddly vertical ricks. In some places they look like they're sitting atop the otherwise smooth terrain of the neck; in other places they seem to be slightly mantled by it, emerging from it.

NavCam view of comet 67P, 1st orbit, August 8, 2014

But the terrain that keeps drawing my eye is what you see beyond the neck, on the side of the little lobe that faces inward toward the neck. It's striated, or stripey. I wouldn't call it layered. But it has a distinct fabric to it, a preferred orientation of little ridges and chasms. Possibly more than one set of intersecting fabrics. You can see the striated fabric keep popping out in the steep walls and ridges that crop out of the smoother terrain up on the sunny pole of the little lobe beyond it. That fabric has formed in a material that's coherent enough to hold together into scarps that make fascinating silhouettes against black space.

C-G detail: striated terrain

Here's a more distant view, from NavCam.

NavCam view of comet 67P, 1st orbit, August 6, 2014

Elsewhere on the comet, there are quite a few features with raised, round rims. Suspiciously round. It's hard for me to imagine how you can make such raised, round rims by any mechanism other than impact cratering.

C-G detail: round feature

But if these are impact craters, a lot has happened to them since they formed. Check out this funny feature on a stalk cropping out of the rim.

C-G detail: round feature (annotated)

What fun! Beginning on Sunday, Rosetta will transition to a slightly lower orbit, at around 80 rather than 100 kilometers, and it will spend the rest of August surveying from that altitude. Here's a timeline of the upcoming events. Stay tuned for more great photos!

After all this staring at the comet, and someone on Twitter sending me a cartoon of what he thought the nucleus looked like, I couldn't resist it any more; I had to sketch out the creature I've been seeing emerge from the pictures. Enjoy.

The vast majority of commenters on Twitter agree that it is actually this South American rodent:

See other posts from August 2014


Read more blog entries about: Rosetta and Philae, pretty pictures, comets, comet Churyumov-Gerasimenko


BBCAmos: 08/15/2014 01:42 CDT

Love the bulldog. Much more sophisticated than a duck. My eye has also been drawn to the striations on the neck. Lots of weird shapes all over the place, like the crater rims which stand so proud.

Thomas Appere: 08/15/2014 03:00 CDT

Thanks Emily for this informative article. You say that "we don't ever see much of the comet below 30 degrees south latitude." But will we see these regions after Chury has passed the Sun ?

David Hall: 08/15/2014 07:06 CDT

What do you bet that Philae lands on one of those "boring" parts like the first terrain you mentioned above. It'd still be cool, though, just not as awesome as the neck area. Probably the safest looking areas I've seen imaged so far though.

comet_follower: 08/15/2014 07:36 CDT

Very interesting variation of terrain, I doubt ESA could have lucked out with a much better target. With the freshness of the cliff terrain, my guess the comet spun up - pulled apart at the middle and then reattached relatively recently at a different orientation. With the number of lobed and binary comets we are finding this may be a common occurrence. I hope they chose to land at the interface of the top and the cliff since the cliff may be a priceless look at the interior of a comet. This stands to be the most exciting mission in a long time.

Geoffrey Bryan: 08/15/2014 09:23 CDT

Of course it's a Capybara-Gopher (i.e. CG)!

Geoffrey Bryan: 08/15/2014 09:33 CDT

Question - How do they determine the coordinate mesh surrounding the comet? Is it a gravitatinal equipotential surface?

Davesterx: 08/15/2014 09:41 CDT

Is there something that would confirm these crater-like structures are impact craters? As the surface warms and the ices sublimate, couldn't explosive events cause these craters from below? Either way, it will be exciting to learn something new about comets!

David Salo: 08/15/2014 10:11 CDT

One thing I'd be curious to know more about is the local gravity on Churyumov-Gerasimenko. I'm sure it's very slight at any given point, but obviously it's enough that all of that sand, dust, and grit doesn't fly off into space. My wish would be for a map that could graphically answer the question: if you were standing at any given point on the surface of the comet, in which direction would "down" be? I have a feeling the answer would *not* always be "at right angles to the ground," and that some points on the surface which appear flat and horizontal are, in terms of local gravity, nearly vertical cliffs.

amcshane: 08/16/2014 07:52 CDT

Great article ! Odd thing about some of the "craters" is that some of them seem very shallow. In fact some (and this will sound crazy) look like they are "balloons" that "popped" (think of Gas coming up through mud - a balloon forms and then it pops - leaving the rim and a shallow indentation). Also there does not (as far as I can see) seem to be much ejecta. Some may well be classical impact craters - but some just look wrong... Also one of the deeper indentations on the large lobe seems to have a white "fan" like covering - could it be a jet from which subsurface gas/dust escaped ? Can't wait to see even closer images when Rosetta drops to 50Km ;-)

Marguerite Barwick: 08/16/2014 12:58 CDT

Marvellous article, Emily! Thank you do much. And I ADORE the little bulldog (hamster? Coypu?) I've been privately calling this comet "Gussy", as it's Kennel Club name is far, far too long for everyday use, and Gussy the Bulldog it will firmly be for me from now on!

morganism: 08/16/2014 06:37 CDT

We could think about the formation scenario like this. The major impact crater on the ducks head was a hypervelocity one, and it came in only a few degrees off the line to the geo-center of a roughly sphereical body. We can even measure the angle from the top crater "thru" the neck. This force compresses the material, and the compression "forges" the neck column into higher density (metamorphise?)material. That energy wouldn't go all the way thru the sphere, or it would dissasociate the body. That energy has to go somewhere, so we see spalling, blasting out wedges of material, forming the neck. The striations we see are actually the cleave lines from the spalling. Methane hydrate at low pressure forms a spongelike material, with a density near styrofoam packing peanuts. This also comes out near the "rubble pile" asteroid densities, but allows a cohesive body that wont's shift around and revert to a sphere. If other impacts forge craters, with compression and melt/refreeze, then you could get "hardened" edge craters, that would be more resistant to weathering, than un-impacted surfaces. The craters actually got filled with dust before they were raised, because they were pits in the surface, with the body plane higher than the pit walls.

Jonathan Ursin: 08/16/2014 07:23 CDT

I like your idea morganism, here is my alternate interpretation. The comet is made of two materials a hard rock and a soft rock, the soft rock contains water and other stuff that is the source of the tail. The two lobes are made of the hard rock and over time they have not eroded away much. The neck is made of the soft rock material containing comet tail stuff. As the comet passes the sun over and over again the neck erodes into space, forming the tail. The smooth material that the necklace rests upon is a talus of hard rock that was embedded in the soft rock. Likewise that is the formation of the boulders, bits and pieces that couldn't get ejected out into the tail. The cliffs are the area where the comet tail is formed. Here the sun actively erodes away at the comet and when hard boulders or layers of hard dust appear they fall to the neck. Since the cliffs are resurfaced with every orbit this is why they have no cratering. Of course this is all just my theory which could be proven incredibly wrong. But I like sharing these ideas. :)

Jonathan Ursin: 08/16/2014 09:08 CDT

Bonus: Here's how to tell if my idea is correct. If we see a increase/decrease in the amount of material ejecting off the comet that correlates with the cliffs entering and exiting darkness than I'll find that to be good evidence.

Marco: 08/17/2014 02:35 CDT

Thanks Emily, your article is both informative and fun! The most interesting thing to discover, is what is below the surface. Being that the density requires the inside to be a lot of empty space, but from the outside of the comet, the surface appears contiguous, covering up any empty space, it leaves the tantalising possibility that the skin of the comet is organic and waxy. Thus, the boulders, cliffs, terraces and craters are driven from internal processes, and the organic skin blankets any large gaps, keeping the comet together despite its fractured nature inside.

marco: 08/17/2014 02:42 CDT

I was thinking from the colour, something similar to crude oil, rather than dust or soot.

glen: 08/17/2014 10:44 CDT

Another idea... apparently the EPH understanding of a comet like this is that it is a fragment of an exploded planet! As such, it would not be surprising to find features (strata, organics, high-pressure minerals, etc.) that could not possibly form in such a small isolated body. Boulders sitting on the surface were also predicted by the EPH theory. It will be interesting to see what is discovered over the next months!

Handee: 08/17/2014 04:39 CDT

Who could have thought that 67p/ Churyumov-Gerasimenko would inspire such name calling?

Handee: 08/17/2014 05:03 CDT

It seems to be getting a name change with every Rosetta orbit. If this rate of change is maintained throughout the mission, I predict it will be Bill Nye the science guy by the time it has rounded the sun

Bill Campbell: 08/17/2014 11:15 CDT

Stunning shots Emily. This is the kind of stuff that makes science great. And you guys involving us as well. Thanks for this.

Thomas Hopp: 08/18/2014 06:49 CDT

Thanks Emily. Great reporting, as always.

Xiaosi Zhou: 08/18/2014 08:17 CDT


AdamK: 08/18/2014 11:56 CDT

Quite incredible. They should have designed Philae to be able to fly into the ion emitting head of the duck and once photographs and other analytical instruments had finished their job, deliver a small nuclear device 1 kilotonne that would blow the comet apart. That way the mother ship Rosetta could see what is inside in unprecedented detail.

Scootra: 08/19/2014 02:47 CDT

Emily, Really enjoyed your article and descriptive observations. I, too, noticed the impact crater. I think the other craters are not from impact, but rather are artifacts of eruptions and sublimations of ices distributed throughout the comet body. Impact craters on other small asteroids don't look like the big craters here. Like Jonathan Ursin, I believe the comet shape is more related to erosion than collision of independent bodies. A non-uniform distribution of ices coupled with the ever changing spin axis (due to thrust effects and cg shifts due to venting fluids) may have begun the unusual erosion patterns. Then the shadowing just exacerbates the erosion like a gully on a hillside turns into a canyon. That striated cliff is actually more normal to gravity like a mesa than a cliff. The gravity in the necklace area is probably quite low due to the mass of the 2 lobes offsetting each other. Interested in your thoughts on these ideas.

Skip Morrow: 08/19/2014 05:54 CDT

Are the photographs of the comet in black & white, or is color but the comet is a really dull gray? My 10YO daughter loved the pictures, but this was her first question and I didn't know the answer. Thanks for the incredible article!

morganism: 08/19/2014 06:26 CDT

They don't have a true color camera, but use filters in front of the other cam to highlight certain spectra to identify different materials on the surface, and in the gas jets too. It really is a little darker than charcoal !

Jon Alexandr: 08/20/2014 01:31 CDT

A quick run through a paint program...

Anonymous: 08/24/2014 02:16 CDT

Could the lack of ejecta around the craters be due to there being insufficient gravity to pull it back to the surface?

Emily Lakdawalla: 08/26/2014 10:56 CDT

@Skip: These are all black and white photos. OSIRIS has absolutely gorgeous color capability but they have not chosen to release any color photos yet. As for what causes the different features -- I'm not confident enough about any possible mechanisms to speculate. Comets are weird.

Mac: 08/30/2014 02:11 CDT

Echoing David Salo's thoughts, this may be the first body we have ever encountered where we will be encountering definitely non-spherical (non-ovoid?) gravity fields... This reminds me of the weird gravitational effects from a book I read once... (RingWorld?...) where the place was a Very Big torus (maybe it was Azimov...), so once you got off the "outside", things got very weird... Like, considering the "neck" region, if you had a hole through it, down inside you should encounter (roughly) a reduced gravity area. In fact, the surface of the neck area should presumably have a somewhat lower gravity field, yes? I wonder if the powdery looking material coating the neck is a function of the lower gravity in that area... On a related topic, Emily, do you happen to know if this body is rotating "wobbly", i.e. where is the approximate center of gravity?... Brilliant work. Keep it up! Mac

John Ackerman: 09/04/2014 06:01 CDT

This looks like two asteroids that impacted one another and stuck together. The material will be found to have a full complement of elements in the periodic table, with significant iron but, because the two chunks formed in weightlessness from a vapor, the density is very low.

jimmywalter: 11/12/2014 06:44 CST

While I initially agreed with Ackerman, after reading morgan, Ursin, and Scoota I am unsure. I still lean towards two objects fused together somehow. It could be just that they collased together out of some hot cloud. The smooth parts seem flows of something, either melted rock from Impact Locations or from rock melted inside due to compression from inpacts; or slurries from melted water or other liquids when the comet cools. If the Pictures Keep coming throughout the Orbit, it will be very interesting to see what happens. But great hypotheses from all!

vlaxmanan: 11/18/2014 05:33 CST

How long does it take for Rosetta to complete one orbit? I realize the distance from the center has not been constant, with numbers as low as 10 km and as high as 30 km. Nonetheless, I would be interested in finding out about the orbit times. So far, I have not been able to find a single blog that talks about this - of course, I might have missed something. Thanks.

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