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Headshot of Emily Lakdawalla

At last, Ceres is a geological world

Posted by Emily Lakdawalla

25-02-2015 21:42 CST

Topics: pretty pictures, Tethys, Rhea, mission status, asteroids, Voyager 1 and 2, Dawn, Saturn's moons, asteroid 1 Ceres, Mimas, Uranus' moons

I've been resisting all urges to speculate on what kinds of geological features are present on Ceres, until now. Finally, Dawn has gotten close enough that the pictures it has returned show geology. Before, we could clearly see craters; but everything has craters. Now, with the latest images taken on February 19, we can see the shapes of those craters, and begin to interpret what they mean about Ceres' interior and geologic history. But we shouldn't get carried away, because the images still have a pretty small number of pixels; at their original resolution, the visible disk is about 210 pixels wide.

Approaching Ceres: Rotation Characterization 2


Approaching Ceres: Rotation Characterization 2
Dawn captured this image of Ceres during its Rotation Characterization 2, on February 12, 2015, from a distance of 46,000 kilometers. This image has been enlarged by a factor of two from the original data.

In fact, before I even go in to any interpretation, I want to prime my analytical eye with images of other worlds we've visited at similar resolution. Dawn is seeing Ceres for the first time much the way that the Voyager spacecraft gave us our first images of the icy moons of the outer solar system as geological worlds. So I dug in to the Voyager archives and pulled out images of icy moons similar in size to Ceres, with similar numbers of pixels across their disks. Six of the pictures below are moons of Saturn and Uranus, and three of them are of Ceres. All are presented at their original resolution. Check them out and compare and contrast!

Ceres fits right in, doesn't it? I think the very first thing I've learned by putting together this comparison is that Ceres is a lot like the mid-sized icy moons of the outer planets. Yet, just like all those moons, it has its own unique personality.

Eight icy worlds

NASA / JPL / UCLA / MPS / DLR / IDA / collage by Emily Lakdawalla

Eight icy worlds
Ceres bears many similarities to the mid-sized icy moons of the outer solar system. This collage compares Dawn images of Ceres taken on February 12, 2015, with photos of moons of Saturn and Uranus taken by the Voyager missions. The Ceres images are in the middle row. The top row contains moons of Saturn: two different views of Tethys, then Mimas and Rhea. The bottom row contains moons of Uranus: Umbriel and Oberon. All are shown at their original resolutions. The scale on the images varies, but the moons are all between 400 and 1500 kilometers across.

The top row contains moons of Saturn as seen by Voyager: from left to right, Tethys, Tethys, Mimas, and Rhea. The middle row contains three views of Ceres. The bottom row contains moons of Uranus as seen by Voyager: Umbriel and Oberon.

Some things to keep in mind as you compare them:

  • Size. The things pictured here come in three basic sizes. Mimas is smallest at about 400 kilometers diameter. Ceres, Tethys, and Umbriel are all roughly 1000 kilometers in diameter. Rhea and Oberon are about 1500 kilometers in diameter.
  • Density. Tethys, amazingly, is less dense than water, so it has to be made almost entirely of water ice with some porosity near the surface. The rest of them have some rock, possibly as differentiated cores; the higher the density, the more rock. Mimas and Rhea are 1.2 times denser than water; Umbriel is 1.4; Oberon, 1.6; and Ceres, 2.1. Since Ceres is roughly twice as dense as Tethys and roughly the same size, its surface gravity is double that of Tethys'.
  • Surface brightness. Ceres is darker than any of the moons. Its surface reflects only about 10% of the light that hits it, compared to 20% for Oberon and Umbriel, and 60%, 70%, and 80% for Mimas, Rhea, and Tethys, respectively. Exposed water ice is not stable for long periods on Ceres' dayside, because Ceres is much closer to the Sun than the moons are, which could explain the darkness; ice would sublimate away, leaving behind a surface lag of whatever other material is in Ceres' crust, including rocky and carbonaceous material, both of which are much, much darker than ice.

Okay, so what have I learned about Ceres by comparing it to these other moons? The first thing that you notice about Ceres has to be its bright spots. One of them is so bright that it was visible to Hubble. Dawn's camera has now resolved that brightest spot as two distinct bright spots within a crater. There are other bright spots elsewhere within craters. Those bright spots are fascinating; that extra bright pair is particularly eye-grabbing. Do remember that everything is relative, and Ceres' surface is really dark, so the bright spots don't have to be pure white, they could just be, say, Mimas-colored, or Rhea-colored. Whatever color they are, they are brighter than anything else on Ceres, and still too small for the camera to resolve them.

Approaching Ceres: one bright spot turns into two


Approaching Ceres: one bright spot turns into two
Dawn captured this image of Ceres during its Rotation Characterization 2, on February 19, 2015. This photo includes the enigmatic bright spot, which has now separated into two bright spots, both of them still smaller than the resolution of the camera (which is roughly 4 kilometers per pixel at a distance of 46,000 kilometers). This image has been enlarged by a factor of two from the original data.

The bright spots generally seem to have an association with craters, so I'm going to assume that they resulted from an impact exposing subsurface, brighter material until I see evidence to the contrary. I won't speculate on what that material is until Dawn gets closer. But we still don't know what is happening on Ceres yet, and have to keep open the possibility that they are volcanic. Why do I prefer the crater interpretation to the volcano interpretation? It's about my geological bias. it would be much more exciting for the bright spots to be volcano-related than crater-related; I really want it to be true that they're volcanic, because then there'd be a more exciting geologic history story to tell. So I have to calibrate my interpretation by dialing down my suspicion that they're volcanic, and insisting mentally that they must be impact-related until someone shows me incontrovertible evidence that they are volcanic. And then I will let myself be excited about volcanism on an asteroid!

Here is a little group of things that I found interesting in the Ceres images. The top row -- images (a) through (d) -- contains bright features, some of which seem to be surrounded by darker material. Hmm.

Interesting things in the February 12, 2015 Ceres pictures

Ignore the bright spots, and what do I notice next? The craters. There are lots of them, as there are lots on all the icy moons. Many of the smaller craters on Ceres are bowl-shaped, but the bigger ones have noticeably flat floors, like the ones on Tethys and Rhea. In the picture above, the two craters on the left, (e) and (f), look a lot like those on Rhea or Dione: they have steep rims, but flat floors, a hint of a central peak, and a hint, also, that the craters aren't perfectly round. On Rhea and Dione, we think that the non-roundness of the craters is caused by the existence of fractures in the crust; those fractures were zones of weakness, and the rupturing of the impact crater followed those weaknesses, making straight rim segments.

But there's one crater on Ceres that made my jaw drop: it's the big one at the center of the view above, (g). It's incredibly flat. You can barely see its rim. Its interior is very smooth and lacks any mid-sized craters, though there are many small ones. On the Max Planck website, they go so far as to say that the large crater is relatively recent, because it contains no middle-sized craters, only small ones.

The main question about such a flat crater, other than its age, is: was it born that way, or did it become flatter over time? On icy worlds, initially deeper craters become shallower when, over geologic time, the warmer ice in the world's interior flows from high places to low places, raising crater floors. But a layer of ice at the surface is so stiff that it still preserves the pre-impact topography. That's how Tethys' huge Odysseus basin still has a rim, a peak ring, and a central peak, even though its floor has risen over time so that it is now convex, continuing the curvature of Tethys' globe rather than concave, like the original bowl shape of an impact crater:

Global color view of a half-phase Tethys, with prominent Odysseus basin

NASA / JPL / SSI / color composite by Gordan Ugarkovic

Global color view of a half-phase Tethys, with prominent Odysseus basin
Color image of Tethys taken by the narrow-angle camera. IR, green and UV frames were processed to more closely match true color. The view is still slightly exaggerated color. Note the enormous (450 km diameter) impact structure named Odysseus. Also discernible is a grayish band running across the equator with slightly redder material to the south. Taken on Dec 24, 2005 at a distance of 197 000 km and phase angle of 85 degrees.

But Ceres' largest visible basin is much, much smoother than Odysseus. There is no pre-impact topography present -- no peak, no peak ring. What happened to the topography? Was it covered up by a volcanic flow, like lava flows filled in the lunar impact basins? Or was there never much any topography in the first place, because the impact happened long ago into a Ceres with a thin crust and a subsurface ocean? These kinds of features are called palimpsests; they don't exist on the mid-sized outer planet moons, but you see them on Europa and Ganymede. The round basin on Ceres is so incredibly smooth that I'm inclined to think it's been filled in, like the terrain in the photo of Triton below, but -- like I said before -- I'm going to remain doubtful of volcanism until I'm left with volcanism as the only interpretation. And we haven't gotten there yet.

High-resolution view of Triton's surface from Voyager 2

NASA / JPL / Ted Stryk

High-resolution view of Triton's surface from Voyager 2
On its closest approach to Triton on August 25, 1989, Voyager snapped several high-resolution mosaics. Triton is considered to be a Kuiper belt object that was captured into Neptune orbit, an event that would have heated it and altered its surface through cryovolcanism. As a result, there are few large craters; a few small ones are visible in this mosaic.

Is there anything besides impact features on the surface? What about tectonic features, places where the crust has fractured, faulted, folded? It's not obvious; here, we're reaching the limits of the resolution of these latest photos. But a sharp-eyed viewer at noticed what appear to be two parallel, linear grooves at the edge of the disk, topography revealed by the low-angle lighting (h). There's even a hint of a third groove running perpendicular to the other two. All of Saturn's icy moons have cracks and grooves; maybe this is another way in which Ceres is a cousin to Tethys!

Interesting things in the February 12, 2015 Ceres pictures

Or maybe not. The resolution is not quite enough to say for sure. We'll know better in April. As Marc Rayman explains in his most recent blog entries (January and February), the February 19 pictures are some of the best Dawn will have for several weeks, as the spacecraft is overshooting Ceres by a bit, traveling farther away and to the night side of Ceres before Ceres' gravity finally grabs it. The crescent views we'll get of Ceres from its night side will be really cool but probably won't reveal a ton more about its geology. As with so many things I predict about space exploration, I would be truly delighted to be wrong! Which is good, because I almost certainly am wrong about much of what I said here!

See other posts from February 2015


Or read more blog entries about: pretty pictures, Tethys, Rhea, mission status, asteroids, Voyager 1 and 2, Dawn, Saturn's moons, asteroid 1 Ceres, Mimas, Uranus' moons


David: 02/25/2015 10:33 CST

I suppose we casual observers don't have to be quite as cautious as Emily! It seems to me that even if you assume a bright exposed surface of some substance that was glare-white, it would still not explain why that substance has not been broken up by micro-impacts and covered over with dust over a long period of time. My guess is that whatever it is has been continuously refreshed by warmer interior material, which cools at the surface to produce a surface nearly as smooth as polished glass, and that what we're seeing is the reflection from that surface.

lodaya: 02/26/2015 12:02 CST

Albedo of the white spots is at least 40 per cent, according to Chris Russell, UCLA. In the top image the left edge looks whiter. I suppose this is some artifact caused by solar direction?

Mewo: 02/26/2015 01:48 CST

Those white dots are intriguing and the closer Dawn gets to Ceres the more puzzling they become. I can't wait to know what they really are.

arf: 02/26/2015 04:41 CST

Without reading too much into it, that big crater also appears to have 'splat!' marks radiating out from it

quayley: 02/26/2015 05:19 CST

Great blog for giving you some kind of comparative overview. Noticed that Chris Russell, the principal investigator for the dawn mission, says that the fact that there are two albedo features points to a volcanic origin for them. This is very exciting, not sure why it would point so strongly in that direction that the P. I. would say that, they tend to be notoriously cautious. May find out more NASA presentation on 2nd March, anyone know if any spectra have been taken yet, that would reveal a lot?

Steven Taylor: 02/26/2015 05:35 CST

Small captions CORRECTION required re first RC2 images above. For February 12 should read 19 February. (12 Feb was RC1). By the way, just love your summary - best on the web, in my opinion.

Arbitrary: 02/26/2015 06:23 CST

@Steven Taylor That is not a matter of opinion. It is a matter of fact! Lakdawalla is a huge resource for the global popular space community.

Gregk: 02/26/2015 07:48 CST

Does it look to anyone else a little "lumpier"/less round than those moons? Could it have something to do with Ceres being made of denser material?

macruzq: 02/26/2015 10:14 CST

I think that the bright glowing detail is a very polished ice or stone surface that reflects the sunlight.

ethanol: 02/26/2015 10:19 CST

The hard thing about the recent volcanism hypothesis is explaining how Ceres would still have enough internal heat to power it. No, there is only one rational explanation for the bright spots; the ancient remains of a now-dead Von Neumann probe factory.

Steven Taylor: 02/26/2015 10:30 CST

@Arbitrary I agree. (I usually add `in my opinion` to minimise rude responses when blogging - not here I hasten to add).

Dusanmal: 02/26/2015 04:18 CST

One additional bit of comparison would be interesting: what is "state of art" estimate about the age of Ceres and moons compared to it? Additional aspect of similarity or not?

petrojak: 02/27/2015 11:42 CST

I would suspect that both answers are right. Cryovolcanism aided by heat injected by a 'recent' large meteorite impact.

morganism: 02/27/2015 12:55 CST

Glad you finally climbed out on that limb Emily, and thanks for the fab comparison pics. What i would like to find out is whether the GRaND sensor is turned on yet, since it isn't expected to be used till we get to LMO, and especially if there was a spike as the white marks rotate past. If there was a "lighthouse" effect, would prob mean it is certainly volcanic. As for the parallel groves, is still favor the hypothesis of the UMSF folks who deduced they are actually crater chains caused by re-impact of ejected materials, that were in orbit as a ring. They used the Phobos images, but we seem the same formations at Vesta also. Fabulous times for planetary science !

joel: 02/28/2015 09:46 CST

Congratulations Emily for this (as usual) thorough cautious and to-the-point analysis. I had noticed too the likely cracks you point to on your figure h. Some other parallel cracks can be guessed (with caution ) left of the area you display in h (outside in the original global picture). It's about time indeed that Ceres takes its deserved place as a full-fledged planetary body from astrogeology point of view. The resemblance to Umbriel is striking indeed. But unlike Umbriel Ceres is much more accessible for exploration. A second mission to Ceres (Nautilus) with a magnetometer and analyzers able to analyze volatiles has been proposed to ESA in the on going call for missions Cosmic Vision M4.

Dougforworldsexplr: 02/28/2015 01:56 CST

Very informative and well written article Emily. I especially liked the comparative pictures of Ceres and moons of Saturn and Uranus and that you mentioned part of the reason for Ceres' dark colour is that it has Carbanaceous material. I had already found out the latter which to my mind leads to a lot of other questions. These include will we be able to identify any particular organic chemicals on Ceres and if so likely which ones and with measuring the chemical composition of Ceres' surface will we be able to relate any carbonaceous chondrite meteorites on Earth as I read that some other types of meteorites have been further confirmed to have come from Vesta after the data from the Dawn visit there gave more detailed chemistry of its surface. I would like to know that if there is any truth about organic chemicals even amino acids coming from space in the form of meteorites if Ceres could be a source of some of them.

rickray777: 02/28/2015 08:04 CST

An alien civilization watching us? Let's be realistic: the splat marks comment just might be a clue. Most likely, the bright spots are impact craters -- tiny, but still deep enough to punch through the darker layer into the brighter ice underneath.

AnneV: 03/01/2015 01:16 CST

The geometric albedos of Mimas, Rhea, and Tethys are actually quite a bit higher than the albedos quoted above. Mimas and Rhea have geometric albedos ~0.95 and Tethys' is 1.2!

Rob: 03/02/2015 09:34 CST

Does anyone know why a) the Ceres photos are so blurry even though the probe was only 50k miles away and even closer now? Our moon is 250k away and looks sharp in binoculars. b) Why aren't any pictures in color? Maybe too much data to transmit now? The Hubble photos of Ceres make it look a lot like Mars with a pinkish brown tint.

Robin081564: 03/03/2015 07:40 CST

I admire your level of self-discipline. I has to be very difficult to be in your position and withhold any sort of speculation regarding those bright spots. If I were to do as you said and just tell you simply what I see (as though looking at the images through the eyes of a child), I would have to say that it looks like the bright spots are some sort of beacon that is actually emitting light. I say this because, as the crater that contains them moves into the darkness, the light appears to continue shining. And they seem to be bright from all angles. This could be because the surface that is reflecting the light is a very large dome shape that can reflect in all directions. I am also curious as to where the rest of the images are. The crater with the bright spots appears suddenly mid screen. Where are the ones with that crater to left side as it comes out of the darkness?

MKhalid: 03/04/2015 02:57 CST

Ceres is the most amazing and I am sure craters in Ceres has small frozen water lakes in its craters. When Dawn enters its orbit and provide more details, I consider it will be the most interesting even of my life.

joel: 03/04/2015 05:04 CST

On the recently released pictures the southern hemisphere appears with many "five-to-five" oriented parallel faults/stripes along with roughly perpendicular faults/valleys drawing more or less rectangular plates like the stripes and faults of Enceladus do.

QubitsToy: 03/04/2015 09:16 CST

Since we have time to wildly guess, I am going for the Holy Grail of Science. It's massive amounts of Bioluminescent material spewing up from the heated ocean surrounding the inner core. Life on a grand scale.

leshiggins: 03/05/2015 11:18 CST

Well, I'm too old to be afraid to say it - I REALLY hope the bright spots turn out to be something artificial (as in manufactured). Yes, exo-geology is cool but I'd like this be so cool it's positively ice-age in nature. Anyone else share this thought?

QubitsToy: 05/04/2015 10:01 CDT

At long last, NASA has told National Geographic that bright spot #5 is ice plume. This is a development that I am very anxious to see photos and read the implications in one of Emily's blog post. Of all the informative web site regarding Ceres, this one is the best. I copy the link to the announcement, hopefully thus is allowed.

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