Studying the Sun by peering closely at microscopic metal grains collected from the Moon may seem to be an odd approach. But just such an experiment has allowed Trevor Ireland and his colleagues at the Australian National University in Canberra to get results that run counter to theories of how our Solar System formed.

The Sun holds precious clues to the origins of our Galaxy. Its surface is made up of the same mixture of gases and dust from which the Solar System grew about 4.6 billion years ago. As it is not possible to study a chunk of the Sun, researchers have opted for the next best thing: the solar wind. This steady stream of particles shoots out from the Sun at more than a million kilometres per hour.

Like many researchers in his field, Ireland wants to analyse the composition of this wind. In particular, he is keen to assess the relative abundance of the various isotopes of oxygen, as that could shed light on the mechanism behind the formation of the Solar System. In samples from primitive meteorites, for example, which are thought to be descendants of the Sun's gases, there is about 5% more of the oxygen-16 isotope compared with samples from planetary rocks. So by looking at isotopic ratios in the solar wind, it should be possible to see whether the Sun is more like a meteorite or a planet.

Ireland will soon get his chance: from December 2001 to April 2004, NASA's Genesis spacecraft collected samples of this wind, which are now available for analysis. Ireland's team is currently building the necessary instruments to analyse these samples. In the meantime, the group did a trial run of the experiment by looking for traces of the wind on the Moon's surface. In late 2003, Ireland asked NASA for some of the dust collected by Neil Armstrong from the Moon's surface.

Trevor Ireland (right) with co-worker Peter Holden.

“We thought it was a one-off simple experiment just to see whether we could look at the isotope composition,” says Ireland. “At the time, I could think of many reasons why it would not work.”

For one thing it, was not clear that he could find anything to analyse that would be sufficiently free from contaminating oxygen from other sources. He chose to home in on tiny metal spheres whose cores, he reasoned, should be free of oxygen unless they had been bombarded by the fast-travelling solar wind. Isolating 30-micrometre balls (100 times smaller than a pin head) from Moon dust proved to be the most painstaking part of the experiment, says Ireland.

In the end, he found a handful of particles that seemed to fit the bill, and studied them using a powerful mass spectrometer called SHRIMP, which used a focused caesium beam to erode the surface of the metal balls. The results, published on page 776 of this issue, were completely unexpected: the values for oxygen isotopes did not agree with either meteoritic or planetary composition.

“We are confident of our results, but they do not fit with the current model of how the Solar System formed,” says Ireland. “It is not clear what the causal link between the Sun and the planets is.” Ireland expects that the finding will be met with some scepticism — at least, he smiles, until it is reproduced using the samples from Genesis.