Dust from asteroid Itokawa contains more water than expected
Earth may have formed with its water baked in. Samples of dust from an asteroid revealed an unexpectedly high amount of water, and asteroids like it may have been the building blocks that made our planet.
In 2003, the Japanese spacecraft Hayabusa was launched toward the 500-metre-wide asteroid Itokawa. When it came back to Earth seven years later, it brought with it the first sample of asteroid dust we have ever had. Its successor, Hayabusa 2, is currently gathering samples from another asteroid.
Itokawa is an S-type asteroid. These are thought to have formed relatively close to the sun, so researchers expected them to have very little water. Because of this, many scientists have hypothesised that while Earth may have been built from S-type asteroids, it got its water later on from wetter types of asteroids and icy comets.
Maitrayee Bose and Ziliang Jin at Arizona State University analysed the water content in two mineral grains brought back by Hayabusa. They expected that there should be too little water to measure, but proved themselves wrong. “Itokawa is bone-dry with respect to anything in our human experience,” Bose says. “But it’s wetter than we expected.”
Itokawa was once part of a larger rock, probably 20 to 50 kilometres in size, says Bose. The pair estimate that this larger rock would have contained about half as much water as all of Earth’s oceans combined.
They also measured the ratio of hydrogen and deuterium, an isotope of hydrogen, in the grains. This ratio is a signature of where the water came from, and the pair found that it was the same as Earth’s water, as well as the water on the moon and Mars.
This suggests that the water on all of these bodies came from the same place, Bose says – primordial pebbles that stuck together to become asteroids like Itokawa, which crashed together to become whole worlds. In this scenario, there is no need for water to be delivered later on.
There are two missions at other types of asteroids now that will eventually bring back samples: Hayabusa 2 at Ryugu, and the OSIRIS-REx mission at Bennu. Understanding how their water is similar or different to Earth’s water could help complete the puzzle. “Getting all this variety of materials will really help us understand our story, the story of our planet, better,” Bose says.
Journal reference: Science Advances, DOI: 10.1126/sciadv.aav8106
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