What Finding Water on an Ancient Asteroid Means for Life on Earth

This is much more than just a pile of asteroid dust.

itokawa asteroid

Using objects from space to study Earth's early history, two scientists have taken the first-ever measurements of water contained in samples from the surface of an asteroid. The cosmochemists at Arizona State University believe their findings suggest that asteroid impacts early in the planet's history had major implications for its earliest life, possibly delivering as much as half of our planet's ocean water.

"We found the samples we examined were enriched in water compared to the average for inner solar system objects," says Ziliang Jin in a press statement. Jin is a postdoctoral scholar in ASU's School of Earth and Space Exploration and the lead author on the paper reporting the results.

The two were studying an asteroid known as Itokawa, named after the father of Japanese rocketry . The Japanese space agency JAXA with the asteroid in 2005, when its Hayabusa spacecraft touched down twice on the asteroid and collected dust. The spacecraft brought its samples back to Earth nearly a decade ago in 2010. Its mission was one of the most dramatic in scientific space history, that almost prevented the spacecraft from returning home.

hayabusa model spacecraft
A half-scale model of the Hayabusa spacecraft.
Pavel HrdličkaWikimedia Commons

"It was a privilege that the Japanese space agency JAXA was willing to share five particles from Itokawa with a U.S. investigator," says co-author Maitrayee Bose.

The samples from Itokawa have been studied several times since 2010. It's a peanut-shaped asteroid about 1,800 feet long and 700 to 1,000 feet wide, and it circles the sun every 18 months. Scientists the asteroid is not quite solid, but rather a rubble pile of rocks loosely held together by their mutual gravity. The dust showed that Itokawa is part of what remains from a much larger object shattered by a previous collision—but nobody had thought to check it for traces of water.

"Until we proposed it, no one thought to look for water," says Bose. "I'm happy to report that our hunch paid off."

First, the dust confirmed what scientists had already thought, that Itokawa was an S-type asteroid. "S-type asteroids are one of the most common objects in the asteroid belt," says Bose. "They originally formed at a distance from the sun of one-third to three times Earth's distance."

While small, they keep whatever water and other volatile materials from their formation.

The Itokawa of today is very different than the Itokawa of the past. In its earliest days, it was part of a parent body least 12 miles wide and heated between 1,000 and 1,500 degrees Fahrenheit. After several impacts, it finally shattered, with two fragments becoming today's asteroid.

"Although the samples were collected at the surface, we don't know where these grains were in the original parent body," Jin says. "But our best guess is that they were buried more than 100 meters deep within it."

Despite its violent creation and travel through the harsh radiation of space, the minerals still show evidence of water.

"This means S-type asteroids and the parent bodies of ordinary chondrites are likely a critical source of water and several other elements for the terrestrial planets," says Bose. "We can say this only because of in-situ isotopic measurements on returned samples of asteroid regolith—their surface dust and rocks. That makes these asteroids high-priority targets for exploration."

Currently, JAXA is continuing its exploration of asteroids with Hayabusa2, which is exploring how the high-flying objects deal with impacts of their own, like the one that affected Itokawa's parent. That means firing explosive projectiles at asteroids, and they've already been surprised at their effectiveness.


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