First author

Clues about the evolution of certain objects in the Solar System are concealed in their rocky crusts. On Earth, for example, the continents and mountain ranges so familiar to us today are the result of billions of years of plate movements and collisions. Meanwhile, the Moon's crust is thought to have originated from low-density minerals floating to the top of a molten 'magma ocean' early in the satellite's history. By analysing the compositions of a pair of meteorites discovered in Antarctica, James Day, a postdoc at the University of Maryland in College Park, and his colleagues have identified a new mechanism of crust formation (see page 179). Day tells Nature why these rocks are in a class of their own.

What intrigued you about the meteorites?

They were found in 2006, and we began studying them at the end of 2007. We realized immediately that their compositions were unusual — for one thing, they contain a lot of feldspar, a type of mineral that crystallizes from magma. Feldspar is common in Earth's crust and that of the Moon, but it is rare to find such quantities in meteorites from other worlds.

Is the origin of these meteorites known?

At one stage geologists thought they might have originated on Venus or the Moon, but research has put that debate to rest. From their composition it's clear that these meteorites are very old; they formed at least 4.5 billion years ago, before Venus' crust. And their oxygen-isotope compositions are not like those of Earth or the Moon, which argues against that link. All the data we have so far are consistent with the meteorites originating from the asteroid belt.

What else is unusual about them?

When we looked at the platinum-group elements in the meteorites, we found them to be enriched in these precious metals, which are rare in the crusts of Earth and the Moon. This told us that the planetary body from which these meteorites came did not melt extensively. It has been shown experimentally that you can generate rocks just like these meteorites through partial melting of planetary-body constituents.

How common do you think this mechanism of crust formation might be?

That is a question we want to answer. Asteroids that have a lot of feldspar in their crusts are expected to reflect light from the Sun very brightly. We plan to compare spectral reflectance data for different asteroids with our data for these meteorites to see whether this type of crust formation was extensive in the early Solar System.