The stuff of asteroids

Many of them are in Earth’s neighbourhood, patrolling the space between Mars and Jupiter, but there’s a lot we don’t know about asteroids.

Now new observations from the VIRTIS instrument aboard the Rosetta spacecraft are revealing what the potato-shaped asteroid Lutetia is made of.

I asked Fred Taylor of Oxford University’s Department of Physics, one of the team reporting the new findings in this week’s Science, what makes Lutetia special and what this new research tells us about our rocky neighbours…

OxSciBlog: Why is Lutetia an interesting asteroid to study?
Fred Taylor: All of the asteroids are interesting, because there is such a variety of sizes, shapes, and compositions. If we want to understand them we must study a large selection, and so far we only have close-up data on a few.

Lutetia is particularly interesting because it is one of the largest, about 130 km along its longest axis (it is potato-shaped) and a thousand trillion tonnes in mass, discovered as long ago as 1852.

Before the Rosetta encounter, Lutetia was remarkable for having an infrared spectrum (the way in which it reflects sunlight and emits heat radiation at different wavelengths) that is different from most other asteroids. We wanted to find out why this is, using the VIRTIS spectrometer. 

OSB: What do the latest VIRTIS observations tell us?
FT: The composition of the surface of Lutetia turns out to be nearly the same everywhere, whereas most asteroids and meteorites have a variety of materials exposed in different places.

The reason, it turns out, is that Lutetia is covered all over with a thick layer of dry, dusty soil that covers the hard surface. We know there is a hard surface, although we can't see it, because the density of Lutetia is very high, so most of it has to be hard rock with a significant metal - we'd expect mostly iron, but also nickel etc - content.

There's no sign of hydrated minerals or organic material, both of which are often present in other asteroids and meteorites. This, and the thick ubiquitous regolith, is what makes the spectrum of Lutetia unusual. The closest match we have found is to the so-called E-type meteorites, which are very depleted in oxygen (so the iron is present as sulphides etc rather than the more common oxides, for example), but we may not be able to confirm this without samples from Lutetia.  

OSB: How do these results help us understand other asteroids/the asteroid belt?
FT: This was the largest asteroid yet studied close up, until the Dawn spacecraft arrived at Vesta recently (and will go on to the largest, Ceres). The big asteroids (larger than 100 km in diameter) are thought to be remnants of the primordial asteroidal distribution - this means they are not by-products of fragmentation events, and their physical properties were probably determined during the accretion epoch.

They may be the only large bodies we have studied that are relatively unmodified since the early days of solar system formation (but there are others, icy rather than rocky, in the Kuiper belt out beyond Neptune that we will visit some day). 

OSB: What important questions about Lutetia remain to be answered?
FT: The next logical step would be to land on the asteroid and drill into its interior to find out what minerals are actually present, in what properties, and how they are distributed throughout the body, especially with depth, so we can see how Lutetia was originally put together. This of course is a major undertaking that will be some time in coming!

Professor Fred Taylor is based at Oxford University's Department of Physics.