‘tis the Seasoning: Bright Spots on Ceres Are Very Probably Salt

The Dawn spacecraft has been orbiting the protoplanet^* Ceres in the main asteroid belt since March 2015, sending tremendous amounts of data back to astronomers on Earth. Ceres is an odd beast, the biggest object in the main asteroid belt between Mars and Jupiter. It’s very dark, and known be mostly rock but with a substantial amount of water ice under the surface. It has no atmosphere to speak of.

That much was known before Dawn got there. But as it arrived, pictures it sent back revealed a surprise: Among the zillions of craters saturating the surface, over a hundred of them contained very bright spots, far more reflective than the dull rock. One large crater, Occator, has two main spots so bright they dominate the wide views of Ceres.

Here’s a lovely animation of Ceres made using actual images, with enhanced colors to show brightness and material differences on the surface:

At first it was thought these bright spots might be deposits of water ice, somehow squeezed out of the interior. However, at Ceres’ distance from the Sun, it’s too warm for ice to exist on its sunlit surface for long. It sublimates rapidly, turning directly into a gas. So that can’t be what’s causing the spots.

The next suspect was salt. Minerals under the surface could mix with the water ice there, making it briny. If that stuff made it to the surface, the water would go away, leaving behind brighter salty deposits.

And exciting new research, published in Nature magazine, indicates that’s precisely what those white spots are.

Planetary scientists used data from Dawn’s Framing Camera, which has filters that allows it to separate out a variety of specific colors of materials on the surface. Different substances reflect light differently in different colors (for example, rust is red and olivine green), so by looking at the relative brightness of the surface in different colors, certain materials can be ruled out (something bright green isn’t rust, for example).

This is where things get interesting. The colors of the brightest part of Occator are consistent with it being a type of salt, most likely magnesium sulfate. They can’t rule other kinds out, but that’s the best match.

But wait! There’s more. Observations of Occator over the course of many Ceres days shows that there is a haze that appears brightest at local noon and disappears near sunset. This may be a very weak plume of briny water coming from the interior (and in fact water vapor was weakly detected around Ceres in 2014). As it happens, Occator is young as craters go (probably less than 100 million years old), and covered in cracks, so that’s consistent with material somehow getting out from the subsurface. It’s not clear why — Ceres has no moon or other nearby objects that could affect it, stretch it, via gravity like the Moon and Earth do to each other. Another mystery to solve.

But it’s there, and it may be carrying salt from the interior out onto the crater. Not only that, but looking at the brighter material farther from the center, the colors appear to change, indicating the material itself is changing with distance. That makes sense in a way, too: Lighter particles can be carried farther as material outgasses, but the weak plume (more like a slow leak of material) can’t carry heavier bits very far. So you see a variation with distance from the source.

Other spots on Ceres aren’t as bright, and may indicate age. Over time, once the leak stops, sunlight and other forces dim the spots. This puts together a plausible scenario: Dawn has a salty ice-rich mantle below the surface. Impacts break through the dark surface material, excavating the ice. The water sublimates, leaving behind the bright salts, which fade over millions of years. In the meantime, cracks can let more of the water up to the surface as the ground warms up over the daily cycle, releasing it as a thin haze, carrying salts with it that help tend the bright spots, so to speak.^†

This may not be the correct explanation, but it’s plausible. And we may find out a lot more soon: In October, Dawn used its ion engines to slowly lower its orbit. It reached that orbit on Dec. 8, and is now just 380 km above the Ceres  — closer to its surface than the space station is to Earth! It will soon begin to return its highest resolution images yet, and map the surface mineralogically to a much more accurate degree. It will also take much better spectra of the surface, revealing the composition of the materials there.

Dawn has been and remains an amazing mission, a truly outstanding example of what can be done in the name of exploration and scientific understating. And it’s got a lot more work yet to do. More answers — and more mysteries, certainly — await!

^* After thinking about this for a while now, I’ve decided to stop calling Ceres an asteroid. Planetary scientists tend to use that term for smaller chunks of rock and/or metal that are essentially debris, the building blocks of bigger objects (or left over from collisions). Ceres, and its sister Vesta, are actually more like planets, having accumulated enough mass to distinguish themselves from their smaller siblings. They were growing well when the solar system was young, but ran out of material before they could get any bigger. Planetary scientists call Ceres and Vesta protoplanets for that reason, and I like the term (as I point out in the footnote of an earlier post on this topic). It’s descriptive, and way better than “dwarf planet”, which in my opinion doesn’t make much sense. I still don’t think the word “planet” can be rigorously defined, but as a fuzzy category with adjectival prefixes it has its uses.

^†Magnesium sulfate has a laxative effect on humans. I can’t say it has the same outcome on Ceres, but there is a bit of poetry — crass, perhaps — in the analogy of it getting out of the protoplanet’s interior.

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