We found water in the dust of an asteroid that is thought to be perfectly dry

For the first time, traces of water were discovered in a stony asteroid that was once thought to be dry.

The dust grains of the asteroid Itokawa actually contain a surprising amount of water, two cosmochemists report from Arizona State University in Tempe on May 1. Progress of science.

"We did not really expect water from Itokawa," said co-author Maitrayee Bose. But if similar asteroids have similar amounts of water, the rocks of space could have been a major source of water for the early Earth.

The Japanese spacecraft Hayabusa reported more than 1500 grains of Itokawa in 2010 (SN online: 14/06/10). Itokawa is what is known as a stony asteroid or an S-type asteroid, which means that he was born closer to the sun than to Jupiter. Scientists believe that Itokawa was formed from the rubble of a catastrophic impact that broke a larger asteroid.

Most of Itokawa's water could have boiled with the heat of this traumatic event as well as the sun's proximity to the asteroid. Previous studies have shown that meteorites that detach from S-type asteroids are generally dry.

Bose still decided to look for water. His lab has an instrument called NanoSIMS, capable of measuring a hydrogen atom in 100,000 other types of atoms. If Itokawa contained more water than that, she thought her team should be able to detect it.

And this was the case: in two grains of Itokawa, the team found between 680 and 970 parts per million of water approximately. In comparison, the Earth's crust contains 15,000 to 20,000 ppm of water.

Taking into account all the ways in which Itokawa could have lost water, Bose and his colleague Ziliang Jin calculated that Itokawa's parent company had 160 to 510 ppm of water. Other S-type asteroids, if they had a lot of water on average, could have provided water to rocky planets, including the Earth.

The researchers found that Itokawa not only had water, but also the water she needed to sow the Earth's oceans. The proportion of deuterium grains, a heavy or isotopic form of hydrogen, relative to the most common form of hydrogen, corresponds to the proportion found in terrestrial water. Previous work has shown that icy comets, on the other hand, have the wrong ratio of deuterium to explain the Earth's oceans (SN: 1/10/15, p. 8). But stony asteroids like Itokawa could have done the business.

"It's still dry compared to anything in our human experience," says Bose. "But it is sufficiently wet and with the correct isotopic composition, [for many such asteroids] provide half of the water on the Earth. "

"It's a very nice and painstaking job," said Tomoki Nakamura, Planetary Science Specialist, from Tohoku University of Sendai, Japan, who led the first team to study Hayabusa samples. Researchers at Arizona State University "have proposed a new interpretation of the origin of water on the Earth".

Nakamura's only concern is whether the water from the Earth's atmosphere could have contaminated the samples. He would like to see the same analysis done on samples of Itokawa that have never been exposed to the atmosphere, "although the preparation of these samples is extremely difficult," he says.

Scientists are also investigating whether other types of asteroids, such as carbon-rich or carbon-rich asteroids, have contributed to the construction of the Earth's oceans. OSIRIS-REx, NASA, and Hayabusa2, from Japan, 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 not tell you how excited I am about these missions," says Bose.