Onions and Asteroids

Asteroid 4 Vesta (right) and an onion (Vesta image: Ben Zellner, Georgia Southern University; Peter Thomas, Cornell University; NASA)

If you want to look inside an onion, all you need to do is cut it with a knife. Easy! But, if you want to look inside an asteroid, the problem is not so straightforward. One way would be to try and piece together the asteroid using the bits of it that land on Earth. Bits of asteroid that land on Earth are meteorites, of course. This approach is like trying to work out what a nut looked like using the bits left behind after it had been whacked by a giant sledgehammer. In the case of asteroids the problem is even worse, because you only have a very tiny fraction of the remaining debris. Sounds like an impossible task. In fact, despite the difficulties, we think we know quite a lot about the internal structure of asteroids.

Just like volcanic rocks on Earth, some meteorites show clear evidence of having once been completely molten, The largest group of such “igneous” meteorites are the HEDs, which are believed to be samples from the large asteroid 4 Vesta (see previous entry). Asteroids which underwent extensive melting have the best understood internal structures. Like some sort of cosmic blast furnace, dense liquid metal sank to their cores, with less dense silicate-rich material forming the surrounding mantle and outermost crust. In many ways, this is similar to the structure of the Earth.

But the most common type of meteorites landing on Earth come from asteroids that never melted. These are the ordinary chondrites. However, while they were never molten, laboratory study indicates that they were heated to varying temperatures inside their parent asteroids. The outer layers would have remained fairly cool, with the temperature increasing progressively towards the centre. So, if you could look inside an ordinary chondrite asteroid, it might consist of a series of concentric layers, a bit like an onion. This simple “onion-shell” structure has almost certainly been messed up by later impact events, so that many asteroids are just loose lumps of shattered debris, so-called rubble piles.

Image of asteroid Itokawa taken by the Hayabusa spacecraft showing its rubble pile structure. Recent analyses of particles collected by the spacecraft indicate that Itokawa is composed of ordinary chondrite-like material. (image: JAXA)

In fact, the onion-shell model can be extended to another important group of unmelted meteorites, the carbonaceous chondrites. Our team, based mainly at the Open University, looked at various features of two carbonaceous chondrite groups, the CVs and CKs, and concluded that they could have originated within a single onion-shell asteroid that was later smashed to pieces. We went on to suggest that the Eos asteroid family could represent the remnants of this body. The original Eos asteroid is believed to have had a diameter of about 218 km and was catastrophically destroyed during an impact event 1,300 million years ago.

Now a new study has added a further twist to this tale.

Some CV chondrites, including the celebrated Allende meteorite, possess a slight remnant magnetization. A recent study, by a team from the Massachusetts Institute of Technology (MIT), has concluded that this feature was produced when these meteorites formed the outermost cold crust of an asteroid with a molten metal core. In this new model the very outer part of the asteroid would have had an onion-shell structure, while the inner portion was similar to a molten asteroid, with a metal core and silicate mantle. The MIT team estimate that this body formed early in solar system history, had a diameter of greater than 400 km, and retained a molten core for at least 10 million years. They go on to suggest that Ceres, the largest body in the asteroid belt, may have a similar structure. Ceres is the second target for the NASA Dawn mission, which it will visit in 2015 after completing its study of 4 Vesta (see last blog). While the new MIT model is certainly a controversial proposal, it has the advantage that it can be fully tested with the aid of the Dawn spacecraft. We just have to wait to 2015 to learn the outcome.

The NASA Dawn spacecraft will arrive at Ceres in 2015 (image: NASA)

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