Imperial College London, UK

A planetary scientist learns how comet dust gets from the inner to the outer Solar System.

I was lucky enough to be part of a team studying the grains of comet dust collected last year by NASA's Stardust mission. Comets are primitive, pristine objects, and the Stardust samples are changing the way we think about how our Solar System formed.

Among many surprising findings, perhaps the most significant is that a large fraction of the dust grains are minerals formed at high temperatures — temperatures expected only in the inner Solar System. How did this stuff get out to where the comet began its life, in the cold, outer regions of the Solar System?

At the recent Lunar and Planetary Science Conference in Houston, Texas, I learned about a numerical simulation that potentially offers a neat solution (F. J. Ciesla and J. N. Cuzzi, abstract here).

Observations of dusty disks around young stars show an inward flow towards the central star. Ciesla and Cuzzi's simulation suggests that this inward transport is confined to the top and bottom of the disk. It predicts that there is a narrow region near the disk's midplane where dust flows outwards — a flow sufficient to account for the Stardust results.

So now we know that comets contain a mixture of stuff from the inner Solar System, and we have a physical model that can explain how it got there. But we're still left with one question.

Virtually everything in the inner Solar System — Earth, Mars, the Moon, almost all meteorites — is depleted in volatile elements, which can't condense at high temperatures. But the cometary dust grains don't show this depletion signature. Why not? It'll be fun finding out.