Australasian Science: Australia's authority on science since 1938

Inside an Asteroid

By Stephen Luntz

An interview with the only Australian co-author of the mission that has just reported on the analysis of samples collected from the Itokawa asteroid.

The only Australian author on a series of papers revealing the first results from the Hayabusa mission has explained how the findings expand our knowledge of the formation of the solar system.

Prof Trevor Ireland of the Australian National University was a co-author on four of the six papers published in Science from initial analysis of the sample returned from the asteroid Itokawa. He says, “We can now unequivocally link the asteroids we see in space with meteorites that we collect on land.”

While the idea that meteorites come from asteroids has been accepted for a long time, there has been some debate as to which sorts of asteroids produce particular classes of meteorites. The Hayabusa mission found a match between the surface of Itokawa and LL chondrites. Ireland says he doubts many LL chondrites come from Itokawa itself, but they must come from similar asteroids.

Ireland says the discovery that Itokawa is only around eight million years old in its current form – tiny in solar system time – is not surprising, as once asteroids leave the main belt their orbits become perturbed by the planets, giving them a limited life expectancy

Ireland says the findings leave open many questions, some of which may be answered through further analysis of the sample returned by Hayabusa, while others will have to await future missions.

Rather than being a solid object like many asteroids, Itokawa was observed to be what Ireland calls a “pile of rubble” from the collision of two asteroids. “It will be interesting to see if we can distinguish grains that came from the original bodies,” Ireland says.

Chemical analysis of one of the 1500 grains in the sample found it depleted in iridium. Since iridium is the classic marker indicating the presence of meteor impact the discovery of a class of asteroids low in iridium would be revealing, but Ireland says we should not jump to conclusions. “Down at particle range you get a lot of heterogeneity.”

“It should be noted that the original intent was for a gun to fire into the surface of Itokawa when Hayabusa touched down, with the ejected sample collected into the sample chamber,” Ireland notes. However, the gun failed to fire. This deprived researchers of a bulk sample. The benefit however, is that everything we have obtained is from the surface, revealing the effects of exposure to the solar wind and cosmic rays. “It would be ideal if we had two samples – one from the surface and one from further in,” Ireland says. “As it is we will have to wait for a future mission.”

Another finding is that Itokawa has less oxygen 16, relative to other isotopes of oxygen, than is observed on Earth. Ireland calls this “One of the fundamental problems of cosmic chemistry. It’s just been found the sun is enriched in Oxygen 16 compared to Earth, while Mars and Vesta have different ratios. Some process is fractionated the oxygen during the solar system’s formation.”