Dust Samples from 500-metre-long Asteroid, 25143 Itokawa, Analyzed
| |Dust grains scooped from a 500-metre-long asteroid have been linked to the most common type of meteorites found on Earth, report a team of Japanese scientists today.
The samples were collected from the asteroid's surface by Japan's Hayabusa spacecraft after a seven-year journey from Earth.
The near-Earth asteroid, measuring 200 metres across, is known as 25143 Itokawa, and this is the first time that samples have been retrieved from the surface of any extraterrestrial body other than the Moon.
This particular asteroid was chosen because scientists suspected from spectroscopic similarities that asteroids of Itokawa's type — known as S-type because of their siliceous (stony) content — were the source of the vast majority of meteorites that end up on Earth, known as ordinary chondrites.
Even though S-type asteroids represent only a fraction of all the different types within the asteroid belt located between Mars and Jupiter, the theory made sense.
S-types are commonly found on the inner fringes of the belt, so scientists assumed that any chips blown off them would often hit Earth.
However, no one had been able to verify the assumption because asteroids had previously only been studied by telescopes and passing spacecraft.
"Now we have a direct link, not conjecture," says Don Brownlee, an astronomer at the University of Washington in Seattle, who was not part of the study team.
The scientists were also able to measure the effects of 'space weathering' — the process by which the surfaces of asteroids are altered by cosmic rays, the solar wind and micrometeoroids.
These changes, along with the amounts of helium, neon and argon in the rock, can be used to determine how long an asteroid's surface has been exposed to space.
The result was surprising: according to a team led by Keisuke Nagao of the University of Tokyo, the dust collected from Itokawa had been exposed to this radiation for no more than eight million years.
This relatively short period of space weathering indicates that the asteroid is eroding at the rate of several tens of centimetres every million years, thus exposing a fresh surface to be weathered.
"This is a quite unexpected result," says Alexander Krot, a meteorite expert at the University of Hawaii at Manoa, Honolulu.
"This asteroid's lifetime is quite short. In several hundred million years, it will just disappear."
Another important find is that early in their history the minerals in the dust grains collected from Itokawa have been heated enough to have metamorphosed.
A team led by Tomoki Nakamura from Tohoku University in Sendai, Japan, reports that particles recovered from the asteroid seem to have experienced long-term heating at about 800 °C.
Heat would have been supplied within the asteroid by the radioactive decay of aluminium-26, a short-lived isotope that was a primary source of heating in the first few million years of the Solar System.
But a small asteroid such as Itokawa wouldn't have contained enough aluminium-26 to generate sufficient heat to metamorphose rock.
"To reach 800 °C, an asteroid would need to be about 20 kilometres in diameter," Nakamura says.
This suggests that the present asteroid is a fragment of a larger body. "It must have been broken by an impact, and reassembled in its current form," he says.
But not all of the grains brought back to Earth show the same degree of heating. "This is a complicated body," Brownlee says, "some of which is more processed [by heating] and some less processed."
At the same time, notes Humberto Campins, a planetary scientist at the University of Central Florida, Orlando, the fact that the spacecraft was able to collect small grains from the surface indicates that they are being continually produced by some process, possibly seismic or from micrometeorite impacts.
"That's also very interesting," he says, adding that is it good news for future asteroid-sampling missions, such as NASA's OSIRIS-REx, for which he is a member of the science team.
The mission's greatest success, however, could be seen as a technological one. Despite a litany of woes — ranging from Hayabusa being hit by a near-crippling solar flare shortly after launch to uncertainties about whether the sample container had properly sealed before the spacecraft lifted off for its return to Earth — the mission certainly produced results.
But the findings still present a puzzle. Although Itokawa has been shown to be a source of ordinary chondrites, the asteroid's mineralogical make-up isn't of the same type as that found in the most common chondrite meteorites, says Campins.
Most meteorites, he says, are H or L chondrites — indicating 'high' or 'low' iron content. Itokawa is LL, meaning it has a very low iron content — and LL chondrites are the least abundant of all the ordinary chondrites found on Earth.
"The Itokawa mission confirms a bunch of things, and also confirms a puzzle," Campins says.