Water has been found in the dust of an asteroid thought to be bone-dry

For the first time, evidence of water has been found in a stony type of asteroid once thought to be bone-dry.

Grains of dust from the asteroid Itokawa actually contain a surprising amount of water, two cosmochemists from Arizona State University in Tempe report May 1 in Science Advances.

“We didn’t really expect water to be there in Itokawa at all,” says study coauthor Maitrayee Bose. But if similar asteroids have similar amounts of water, the space rocks could have been a major source of water for the early Earth.

The Japanese spacecraft Hayabusa brought back more than 1,500 grains of Itokawa in 2010 (SN Online: 6/14/10). Itokawa is what’s known as stony asteroid or an S-type asteroid, which means that it was born closer to the sun than to Jupiter. Scientists think that Itokawa formed from the rubble of a catastrophic impact that broke up a larger asteroid.

Most of Itokawa’s water could have boiled away with the heat from that traumatic event as well as the asteroid’s proximity to the sun. Previous studies have shown that meteorites that break off from S-type asteroids are mostly dry.

Bose decided to look for water anyway. Her lab has an instrument called a NanoSIMS, which can measure one atom of hydrogen in 100,000 other types of atoms. If Itokawa held more water than that, she thought her team should be able to detect it.

And it did: In two Itokawa grains, the team found between roughly 680 and 970 parts per million of water. Earth’s crust, by comparison, contains 15,000 to 20,000 ppm of water.

Taking into account all of the ways that Itokawa could have lost water, Bose and colleague Ziliang Jin calculated that Itokawa’s parent body had 160 to 510 ppm of water. Other S-type asteroids, if they had that much water on average, could have delivered water to the rocky planets, including Earth.

Not only did Itokawa have water, it had the right kind of water to seed Earth’s oceans, the researchers found. The grains’ ratio of deuterium, a heavy form or isotope of hydrogen, to the more common form of hydrogen matched the ratio found in earthly water. Previous work has shown that icy comets, on the other hand, have the wrong deuterium ratio to explain Earth’s oceans (SN: 1/10/15, p. 8). But stony asteroids like Itokawa could do have done the trick.

“It’s still dry with respect to anything in our human experience,” Bose says. “But it is wet enough, and with the correct isotopic composition, [for many such asteroids] to provide half the water on Earth” in the oceans.

“This is very nice and careful work,” says planetary scientist Tomoki Nakamura of Tohoku University in Sendai, Japan, who led the first team to study the Hayabusa samples. The Arizona State University researchers “proposed a new interpretation for the origin of water on the Earth.”

Nakamura’s only concern is whether water from Earth’s atmosphere could have contaminated the samples. He would like to see the same analysis done on samples of Itokawa that were never exposed to the atmosphere, “although this sample preparation is extremely difficult,” he says.

Scientists are also looking at whether other kinds of asteroids, particularly water-rich C-type or carbonaceous asteroids, may have helped build Earth’s oceans. Two ongoing missions, NASA’s OSIRIS-REx and Japan’s Hayabusa2, will bring back pieces of two C-type asteroids, Bennu and Ryugu, over the next five years (SN: 1/19/19, p. 20).

“I can’t tell you how excited I am about these missions,” Bose says.