Examination of a meteorite has revealed an unexpected treasure, a piece of a comet formed at the birth of the Solar System carried safely to Earth encased in part of an asteroid.
Meteorites are divided into different families based on the body they come from. Some are pieces of the Moon or other planets knocked off when larger objects struck these worlds, while others reveal large differences in the composition of the asteroids from which most meteorites come. This makes space rocks so precious people will go all the way to Antarctica to find them.
The LaPaz 02342 meteorite was collected on the icefield of the same name. Most of it is a fairly typical carbonaceous chondrite meteorite. Dr Larry Nittler of the Carnegie Institution searched it for presolar grains, remnant materials from before the Solar System even formed, which such meteorites sometimes contain. Instead, he found something even more precious, a piece of material with the composition of a comet whose ice has boiled away.
Comets formed much further out from the Sun than asteroids, and therefore contain ice that would melt away in hotter locations. We've also learned from missions to them, and from watching what happens when the ice melts, that comets have more carbon than asteroids. We've had to go into space to sample them because ice is so much less likely to survive entering the Earth's atmosphere than rock.
Encased inside an asteroid, however, cometary material has a much better chance of surviving the heat of entry, as LaPaz 02342 has proved.
"When Larry and Carles showed me the first electron images of the carbon-rich material," Arizona State University's Dr Jemma Davidson said in a statement, "I knew we were looking at something very rare. It was one of those exciting moments you live for as a scientist."
In Nature Astronomy Davidson and Nittler conclude the 0.1-millimeter-wide (0.004-inch) anomaly formed in the Kuiper Belt past Neptune, becoming a comet's building block before falling into the inner Solar System as a result of drag applied by the surrounding gas. The idea materials were transported from the outer Solar System in the early stages of its formation is favored by astronomers, but hard to prove.
The fragment is rich in sodium in addition to the carbon and oxygen concentrations that are its identifying features. It also has more of the pre-solar silicates Nittler was originally seeking than the surrounding asteroid, all of which helps build a picture of the earliest comets.
Antarctica doesn't get any more meteorites than the rest of the planet, but they are much easier to spot against the unrelenting white than when quickly covered by plants or dust.