August 26, 2011

Earth-Bound Asteroids Originate From Stony Asteroids


Japanese scientists have determined that the most common meteorites found on Earth are born from stony asteroids.

The Earth-bound asteroids, also known as ordinary chondrites, are among the most primitive objects in the solar system.  The researchers said that the discovery means these asteroids have been recording a long and rich history of early solar system events.

Japan Aerospace Exploration Agency (JAXA) launched the Hayabusa spacecraft in 2003 to sample the surface of an asteroid known as 25143 Itokawa.  The spacecraft reached its destination two years later, took samples of the asteroid's surface, and then headed back home to Earth.

Hayabusa reentered the Earth's atmosphere and landed in Southern Australia in June 2010, bringing delicate samples for researchers to analyze with it.

Brooks Hanson, Deputy Editor of the Physical Sciences, said in a press release: "These Hayabusa samples are the first samples of an asteroid. Not only do they provide important information about the history of the asteroid Itokawa, but by providing the needed ground truth that is only possible through direct sampling, they also help make other important samples–like meteorite collections and the lunar samples–even more useful."

Itokawa is a rocky, S-type asteroid that has a similar appearance to a rubble pile.  Researchers believe that similar asteroids are responsible for most of the small meteorites that regularly strike Earth.

The Japanese scientists were among the first to analyze the Itokawa sample.  The team used a combination of electron microscopes and X-ray diffraction techniques to study the mineral chemistry of Itokawa's dust particles.

"Our study demonstrates that the rocky particles recovered from the S-type asteroid are identical to ordinary chondrites, which proves that asteroids are indeed very primitive solar system bodies," Tomoki Nakamura from Tohoku University in Sendai, Japan, said in a press release.

The team noticed that Itokawa's region has gone through significant heating and impact shocks and concluded that the asteroid is made up of small fragments of a much bigger asteroid.

"The particles recovered from the asteroid have experienced long-term heating at about 800 degrees Celsius," Nakamura, who led the research, said in a press release. "But, to reach 800 degrees, an asteroid would need to be about 12.4 miles (20 kilometers) in diameter. The current size of Itokawa is much smaller than that so it must have first formed as a larger body, then been broken by an impact event and reassembled in its current form."

Another team of researchers from Tokyo Metropolitan University and colleagues from the U.S. and Australia cut open the tiny regolith grains returned by Hayabusa to get a look at the minerals inside them.  Their analyses show that the dust grains have preserved a record of primitive elements from the early solar system.

"The cool thing about this Itokawa analysis is the tremendous amount of data we can get from such a small sample," Michael Zolensky from the NASA Johnson Space Center in Houston, Texas, a co-author of the research, said in a press release.

"When researchers analyzed regolith from the moon, they needed kilogram-sized samples. But, for the past 40 years, experts have been developing technologies to analyze extremely small samples. Now, we've gained all this information about Itokawa with only a few nano-grams of dust from the asteroid."

Takaaki Noguchi from Ibaraki University in Mito, Japan, and colleagues said the chemical difference between the lunar dust and the Itokawa samples is one of the reasons astronomers have never been able to definitively tie ordinary chondrites to S-type asteroids before.

"Space weathering is the interaction between the surface of airless bodies, like asteroids and the moon, and the energetic particles in space," Noguchi said in a press release.

"When these energetic particles–like solar wind, plasma ejected from the Sun and fast-traveling micrometeoroids–strike an object, pieces of them condense on the surface of that object. In the vacuum of space, such deposits can create small iron particles that greatly affect the visible spectra of these celestial bodies when they are viewed from Earth."

The researchers said the dust from Itokawa has been on the surface of the asteroid for less than eight million years.  They believe that material from these types of asteroids might escape easily into space to become meteorites, traveling toward Earth.

"This dust from the surface of the Itokawa asteroid will become a sort of Rosetta Stone for astronomers to use," according to Zolensky. "Now that we understand the bulk mineral and chemical composition of the Hayabusa sample, we can compare them to meteorites that have struck the Earth and try to determine which asteroids the chondrites came from."

The research was published in the August 26 edition of the journal Science.


Image Caption: This is a Hayabusa capsule landed at Woomera in South Australia. Credit: Image © JAXA/ISIS


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