Extraterrestrial dust reveals asteroid's past and future

Talk about seeing a world in a grain of sand. A sprinkling of asteroid dust that slipped into Japan's Hayabusa probe when it touched down on the asteroid Itokawa six years ago has revealed surprising details about the space rock's past and its likely future.

Hayabusa was meant to land on the 500-metre-wide asteroid in 2005 and fire projectiles into its surface, scooping up the resulting debris for later return to Earth. But the projectiles never fired, and team members had to wait five long years for the probe, which suffered numerous equipment failures, to limp back to Earth (see a picture of it after it touched down in Australia).

They hoped that some asteroid dust had managed to find its way into Hayabusa's collection chamber when the probe touched Itokawa, but even when they saw dust there they were sceptical. "It was me who opened the sample catcher," says Tomoki Nakamura of Tohoku University in Sendai. "I could not believe they were real Itokawa samples."

Now, he and dozens of other researchers are reporting their analyses of the samples, which comprise more than 1500 rocky particles from Itokawa, all smaller than 0.2 millimetres across.

Larger parent

The studies suggest that Itokawa was once part of a much larger asteroid. The conclusion is based on the fact that the particles show a range of minerals that must have reached 800 °C to form (see picture). The decay of radioactive aluminium isotopes could have created that much heat if the parent body was at least 20 kilometres across – 40 times its current size, Nakamura and his collaborators say.

"The body needs to be big; otherwise, it would lose the temperature too quickly for these processes to occur," says Trevor Ireland of the Australian National University in Canberra, who analysed some of the samples.

Indeed, data from Hayabusa itself had already determined that Itokawa has had its share of knocks in the past. The asteroid's gravitational pull on the probe revealed that Itokawa has a very low density, suggesting it is a rocky rubble pile that probably coalesced after its parent body was smashed in an impact.

Solar wind

The samples also hint at a bleak future for Itokawa. Keisuke Nagao of the University of Tokyo and colleagues studied noble gases, such as neon, in three grains. These gases can be implanted by charged particles streaming in from the solar wind and even from beyond the solar system.

The results suggest that the grains have been exposed to these charged particles for no more than 8 million years. This implies either that Itokawa coalesced 8 million years ago, or that it loses tens of centimetres of material from its surface every million years, exposing new layers of rubble in the process.

This could occur if dust grains lifted off the surface after micrometeoroid impacts simply float away from the lightweight asteroid. If Itokawa is losing its surface at that rate, it may completely disappear in less than a billion years.

Disappearing space rock

"It's a bit sad to think it will eventually disappear," says Scott Sandford of NASA's Ames Research Center in California, who analysed some of the samples.

But he adds: "The 'disappearance' of Itokawa has its compensations. For one thing, I'd rather Itokawa be whittled away then to have it run into the Earth as a single object, which would cause some serious issues for the Earth if it happened. Also, one should remember that it is the process of whittling away at asteroids like Itokawa that produces the smaller meteoroids that ultimately land on the Earth as meteorites."

This particular finding about its future may actually have been helped by Hayabusa's failure to fire projectiles into the asteroid. "Solar wind penetrates only 100 nanometres or so into rock," says Ireland. "The gun firing would have meant that the projectile penetrated into the asteroid, pushing out chips and fragments." That would have made it "very hard to isolate surface pieces, which is where the solar wind resides", he adds.

Link confirmed

The samples' composition matches that of the most common type of meteorite found on Earth, called ordinary chondrites. Since Itokawa is classified as a stony S-type asteroid – the most common kind in the inner asteroid belt, the analyses confirm that ordinary chondrites come from S-type asteroids.

The studies also highlight the importance of returning samples from extraterrestrial bodies to Earth for study. "The analysis apparatus is too heavy to carry to an asteroid," Nakamura told New Scientist.

Alexander Krot at the University of Hawaii at Manoa, who was not part of the team, says two other sample-return missions – Japan's Hayabusa-2 and NASA's OSIRIS-Rex – will blast off in 2014 and 2016 to collect samples from asteroids rich in minerals that formed in water.

These could reveal clues about "one of the most outstanding questions in planetary science – the origin of Earth's water", he says in a commentary in Science. Studies of hydrogen isotopes in the water-formed minerals in the target asteroids could help determine if Earth's water was delivered by asteroids or comets or simply came from the dust from which the planet formed.