The team's findings suggest that early impacts on Earth's history by similar asteroids could have generated up to half of the water in our planet's oceans.
"We found that the samples we examined were enriched in water compared to the average of the objects in the internal solar system, ”says Ziliang Jin, a postdoctoral researcher at the ASU School of Earth and Space Exploration, and lead author of the article, published in‘ Science Advances ’.
"It was a privilege that the Japanese space agency JAXA was willing to share five Itokawa particles with an American researcher," says Maitrayee Bose, assistant professor at the school and co-author of the work.
The team's idea of searching for water in the Itokawa samples was a surprise for the Hayabusa project. "Until we proposed it, nobody thought about looking for water," Bose says. "I am happy to report that our hunch paid off."
In two of the five particles, the team identified the mineral pyroxene. In terrestrial samples, pyroxenes have water in their crystalline structure. Bose and Jin suspected that Itokawa particles could also have traces of water, but they wanted to know exactly how much. Itokawa has had an approximate history of warming, multiple impacts, shocks and fragmentation. These would raise the temperature of the minerals and expel the water.
To study the samples, each about half the thickness of a human hair, the team used the ASU nano-scale secondary ion mass spectrometer (NanoSIMS), which can measure these tiny mineral grains with great sensitivity.
NanoSIMS measurements revealed that the samples were unexpectedly rich in water. They also suggest that even nominally dry asteroids like Itokawa can, in fact, house more water than scientists have supposed so far.
Itokawa is a peanut-shaped asteroid of approximately 548 meters (200 to 300 meters wide). Surround the Sun every 18 months at an average distance of 1.3 times the Earth-Sun distance. Part of Itokawa's path takes it into the Earth's orbit and, as far as possible, extends a little beyond Mars.
Based on the Itokawa spectrum in terrestrial telescopes, planetary scientists place it in the S class. This links it to stony meteorites, which are believed to be fragments of S-type asteroids that break into collisions.
As the researchers point out, today's Itokawa is the remnant of a parent body of at least 19 kilometers that at some point warmed between 500-800 ° C. Then he suffered several big shocks from the impacts, with a final event that broke him. As a consequence, two of the fragments merged and formed today's Itokawa, which reached its current size and shape approximately 8 million years ago.
"The particles we analyzed came from a part of Itokawa called the Sea of Muses," says Bose. "It is an area of the asteroid that is smooth and dusty." Jin adds: "Although the samples were collected on the surface, we don't know where these pimples were in the original body. But our best estimate is that they were buried more than 100 meters deep.
Despite the catastrophic disintegration of the parental body and the sample grains that are exposed to radiation and the impacts of micrometeorites on the surface, the minerals still show evidence of water that has not been lost in space.
In addition, Jin says that "minerals have isotopic hydrogen compositions that are indistinguishable from Earth." "This means that S-type asteroids and common chondrite progenitor bodies are probably a critical source of water and several other elements for terrestrial planets," Bose explains.