NASA's Dawn mission has achieved a number of firsts, including being the first spacecraft to go into orbit around two different bodies. The second of those destinations is Ceres, a dwarf planet that is by far the largest body in the asteroid belt. That visit has now shown us that a lot of our expectations for what we would find at Ceres were wrong: it's not an icy body, but liquid water has helped shape the dwarf planet's most dramatic features.

A couple of papers that analyze Dawn data have appeared in Nature journals this week. In one case, they suggest that the dwarf planet's composition is much rockier than we expected. But the other suggests that the mysterious bright spots found in some of Ceres' craters are the result of salty brines making their way to the surface.

Our thoughts about Ceres prior to Dawn's visit were dominated by the dwarf planet's relatively low density. This suggested to many people that it must be composed largely of water, although the surface was darker than you would expect from water ice that was expected to be a thin veneer over an icy world. Craters were also expected to be relatively scarce, as water ice is semi-viscous at the temperatures (120K and up) expected to be found on Ceres.

Craters argue against ice

The reality turned out to be that Ceres is a crater-filled world, as the picture above clearly shows. In fact, many of its larger craters are pockmarked with smaller impacts that occurred after they were put in place, indicating that the original crater is very old. One of the two papers released this week compares the apparent age of the craters to the behavior we would expect from a water-ice world.

The authors modeled how deep craters would evolve over time on different areas of Ceres (as on Earth, the poles of the dwarf planet are colder than the equatorial regions). While it's possible to have an icy Ceres that supports old craters near the poles, most of the rest of the planet would see the viscosity of water ice allow the interior of the crater to slowly deform. After sufficient time, even deep craters would be reduced to depths of less than 500m if water ice is the dominant material on Ceres. A 100km-wide crater would no longer be visible after just 10 million years.

They then compared that to a catalog of actual crater depths as mapped by Dawn and showed that there's a large discrepancy: many of the actual craters, including some quite old ones, are very deep. In fact, they calculate that the persistence of these craters requires that whatever comprises Ceres' crust has to be over 100 times more viscous than water ice. "Ceres’ outer layer is therefore probably relatively ice poor, with non-ice material constituting 60–70 percent of the volume," the authors conclude.

So what is it? The authors suggest that various salt-ice mixes or clathrates (organic molecules trapped by water ice) could make the ice somewhat more robust. But it's also possible that Ceres is mostly rocky, but very porous. A loose mixture of material seen in many asteroids (called chondrites) could also account for the apparent low density.

Bright spots argue for water

If water is out on Ceres' surface, there is new evidence that it's present in the interior. This comes courtesy of the bright spots found in some of the dwarf planet's craters, which stand out so much that they were easy to spot even as Dawn was still approaching. There has been a lot of speculation about what material they might be made of, but an analysis of spectroscopic data seems to have settled the argument: they're the largest sodium carbonate deposits in the Solar System.

A material's spectrum—which wavelengths of light its absorbs or reflects—can tell us a lot about its composition. And Dawn carried an instrument that obtained spectra from light reflected off Ceres' surface. These spectra showed that the bright spots were clearly distinct from the material that forms the neighboring crater floor. The data also indicates that water can only be a few percent of the total material present.

Instead, there was a clear signal from carbonates; combined with other evidence, the authors argue that the most likely form is sodium carbonate, a chemical that's commonly used in industrial processes on Earth. We actually have to manufacture it here, because it's so water soluble that it doesn't tend to last long when formed naturally.

So how did it get to Ceres? The authors argue that it's not commonly found on asteroids or comets so probably wasn't put in place by an impact. It may be found in craters, but it's associated with fractures and the central mound of the impact, suggesting it got put in place after the impact occurred. The most likely way to form these large deposits is to have a water solution of the sodium carbonate freeze on the surface, after which the water would sublimate away due to Ceres' lack of a significant atmosphere.

The problem, as noted above, is that Ceres is too cold to have liquid water. So the authors suggest that the heat of the impact itself might have created enough liquid water to bring the carbonates to the surface. Even salty brines, however, would be likely to boil rather than flow on Ceres, so the explanation isn't entirely satisfying.

The end result of this work is that we have answered a couple of big questions about Ceres—we know what the bright spots are and how icy the dwarf planet is. But, in answering these questions, we've set up two entirely new ones. Dawn's still there, however, so there's a chance that further data will tell us what Ceres is made of and how water could flow on the surface of the dwarf planet.

Nature Geoscience, 2016. DOI: 10.1038/NGEO2743,

Nature, 2016. DOI: 10.1038/nature18290. (About DOIs).