The brightest spots on dwarf planet Ceres are seen in this image taken by NASA's Dawn spacecraft on June 6, 2015. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Jun 11, 2015 Ever closer to Ceres

Sen—Dawn has now achieved its second mapping orbit at Ceres, and the mission is posting new images to the website every day. Public attention has focused on the bright spots, which have not gotten any less enigmatic with Dawn’s closer views. They dominate any photo of Ceres in which they can be seen. Consider this global view, taken on May 5, while Dawn was in its first mapping orbit:

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Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

A closer view of the spots, released yesterday, shows them separating into more spots; but it’s still not clear what they are.

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Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Chris Russell, the mission’s principal investigator, dutifully says what he can about them, but it’s not very satisfying: "The bright spots in this configuration make Ceres unique from anything we've seen before in the Solar System. The science team is working to understand their source. Reflection from ice is the leading candidate in my mind, but the team continues to consider alternate possibilities, such as salt. With closer views from the new orbit and multiple view angles, we will soon be better able to determine the nature of this enigmatic phenomenon."

I do think the spots are intriguing, but they’re far from the only intriguing thing that Dawn’s images of Ceres are showing. Now that Dawn is closer to Ceres and getting clearer views of its surface, let's examine some of its other fascinating geomorphology.

Consider this photo:

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Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

I’ll enhance it for you.

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Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/Emily Lakdawalla

The photo contains a lot of craters; clearly we’re looking at a relatively ancient surface. But there is a number of irregular things about it. Here’s an annotated version of the same image, and a list of the things I find intriguing.

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Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/Emily Lakdawalla

1. The craters are not all round

No matter what angle the asteroid impact—unless it's super, super shallow—impact craters should be round, because the physics of an asteroid strike is essentially that of a point-source explosion at the point of impact. If an impact crater is not round, its non-roundness results from an inhomogeneity in the target. For crater walls to be straight, not curved, implies preexisting straight-line zones of weakness in the target material: a fractured crust. What created those preexisting zones of weakness? What are their orientations, and why are they oriented that way? Geomorphologists will no doubt be mapping the orientations of unexpectedly straight-line crater walls to try to answer these questions.

2. There are lots of catenae

Throughout the image you can find linear chains of depressions, also known as catenae. Catenae are common in the Solar System; they form as part of the ejecta patterns of large impacts, when material blasted out of a crater breaks up in flight, spreading out along its outward path, and crashes to ground in a rat-a-tat-tat line.

Catenae may be quite straight or may be sinuous, depending on how the initial block of material was spinning as it flew. However, there are other ways to create catenae. For instance, there could be subsurface faults—the same kinds of faults that would control the shapes of the craters—and if they opened up, material could drain into them from above, creating pit chains. If a catena is suspiciously straight, it may not be an impact crater chain; it could be a pit chain. Time will tell.

3. There are splashes of lighter-colored material here and there

The bright spots distract attention from the fact that there are lots of places on Ceres where you can see anomalously bright material. It's often associated with impact craters, so maybe there's a brighter material below Ceres' surface that is being exposed and splashed out when an impact crater forms. But it's not found around every crater, so whatever this stuff is, it's not everywhere below Ceres' surface.

4. A crater with cracks in the middle?

The largest crater in this image has a set of cracks at its center. The cracks radiate out from the center. Cracks form when a brittle material is undergoing tension—that is, when it's being stretched. For cracks to form in all orientations, that means that the whole center of the crater is being stretched in all directions. What are the forces that are making the crust stretch? You'd expect this behavior if the center of the crater were doming upward—something we see on Saturn's icy moons, including Enceladus.

More to come

The Dawn mission is continuing to release images from its tighter orbit every weekday, showing how Ceres is not just a cratered globe; it has fascinating surface features that can tell a rich geologic story. It has craters with radiating troughs:

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Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

It has palimpsests, craters whose features are nearly completely effaced by later geologic processes:

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Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

It has regions that look like typical ancient cratered plains...until you notice the domical feature popping out of the surface:

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Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Can't see it? I'll point it out:

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Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/Emily Lakdawalla

It is very easy to make holes on planetary surfaces, but much harder to build mountains and domes against the force of gravity. This could be nothing more than a spot where two ancient impact craters overlapped, building an especially tall bit of rim at their intersections. Or it could be evidence for internal geologic activity.

The closer Dawn gets to Ceres, the more exciting the images become. There is a rich geologic story here, just waiting to be deciphered, translated, and told to the world.

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