The Dawn spacecraft is on its way to Ceres, and has started sending back new images of the dwarf planet. The images are still pretty pixelated at this point, but we can begin to see features such as craters on the surface. Once considered a planet, then an asteroid, it is now considered a dwarf planet, and Dawn’s study of Ceres is likely to find several interesting surprises.

Possible interior of Ceres. Credit: NASA/ESA/STScI

We already know, for example, that Ceres has water. Spectroscopy of its surface shows that it contains hydrated material, and we’ve observed water vapor within Ceres’ thin atmosphere. Yes, Ceres has an atmosphere, but it isn’t remotely thick like Earth’s or even that of Mars. Because of this, it’s thought that Ceres has a rocky core with a mantle of water-ice. There are even some who speculate that Ceres could have liquid water within its interior, but that isn’t thought to be very likely.

The approach to Ceres marks the second body the Dawn spacecraft has visited. It has already visited and orbited Vesta, giving us a detailed analysis of its geology and history. As it orbits and eventually lands on Ceres, it will likely do the same for the smallest of dwarf planets.

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All large asteroids have been bombarded over the ages, and as a result smaller chunks have been cast adrift in the solar system. Some of these smaller bits fall to Earth as meteorites. One of the things we notice about meteorites is that many of them have certain similarities of composition and chemical signature. As a result they can be identified into groups. This would imply that these groups have a common origin, likely a particular asteroid.

For example, the Howardite-Eucrite-Diogenite (HED) group is a type of meteorite that have long thought to originate from the asteroid Vesta. The connection was first made by Guy Consolmagno, who with Michael Drake demonstrated that the composition of HED meteorites matched the spectra of Vesta back in the 1970s. There are other smaller asteroids that have similar spectra, but Consolmagno noted that of all the HED meteorites found on Earth, none contain a mineral known as olivine, which is found in the mantle of asteroids and planets. This means the HED must have come from a large, intact HED body, which points to Vesta.

But when the Dawn mission reached Vesta, it found something unexpected. Vesta is more than 500 kilometers in diameter, which is large enough for it to differentiate. That is, during its formation one would expect iron and other heavy elements to sink to its core, surrounded by a mantle (where you would find olivine among other things) and an outer crust. But one thing Dawn noticed was two large impact craters near the south pole of Vesta. These craters were large enough that they exposed the mantle in that area. But what Dawn didn’t find was exposed olivine.

That means there’s something odd about Vesta. The impact craters exposed material as deep as 80 kilometers, which is quite deep for an asteroid. The lack of exposed mantle could mean that Vesta just has a really thick crust, but that shouldn’t be the case given its size.  But it would be the case if Vesta isn’t an intact world. Basically a proto-Vesta could have been shattered by a collision with another planetoid when the solar system was young. The stripped iron core of proto-Vesta could then re-accrete what material it could.

Of course, if Vesta was shattered early on, then the HED meteorites couldn’t have originated from Vesta. So this week Consolmagno presented a talk at the AAS Division for Planetary Sciences meeting arguing against his original theory. The HED meteorites could indeed be material chipped off Vesta from smaller impacts, but the HED material didn’t originally form as a part of Vesta.

I should point out that this work hasn’t been peer reviewed, though it has been submitted for publication. Even the idea that Vesta is a shattered body is a bit controversial, so Consolmagno’s conclusions should be considered a bit tentative. But it’s an interesting idea, and it’s a good example of how science works. If you follow the evidence, you might find that even your long standing model turns out to be shattered by new evidence. So you dust yourself off and push forward with a new idea.

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We generally think of asteroids as looking like gray rocks. While that’s true to our limited eyes, more sensitive instruments find they have a variety of colors. You can see an example of this in the image above of the asteroid Vesta. This false color image was made by observing Vesta at various wavelengths in the visible and infrared spectrum. It shows that Vesta has variations in color too subtle for us to see with our eyes.

The color variation is caused by the varying composition of Vesta. The green regions, for example, indicate the presence of iron. While Vesta is the only asteroid with such detailed color observations, other asteroids have been observed at different wavelengths to determine their overall color. It turns out the color of an asteroid correlates well with the family it is a part of.

Asteroids aren’t just scattered randomly through the asteroid belt. Because of the gravitational interactions of Jupiter and the other planets, asteroids tend to be clumped into groups or families. In 2002, the Sloan Digital Sky Survey (SDSS) observed more than 10,000 asteroids at different wavelengths to determine their overall color. What was found was that there was less color variation within a particular family than their was between families.

Asteroid families are defined by the similarity of their orbits. What this study showed is that asteroid families also share similar coloring. Since the coloring of an asteroid is determined by its composition, this means asteroid families have similar compositions. Asteroid families are chemically similar.

This has important consequences for the history of our solar system. It means that asteroid families formed within their own family. They didn’t form first and then get pushed into groups by the gravitational tugs of the planets. This means the orbital dynamics of the solar system likely stabilized before the asteroids formed.

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