Water has been discovered in samples collected from asteroid Itokawa by scientists at Arizona State University. The discovery lends credibility to the theory that water on Earth may have been deposited by meteor strikes.
The samples, which originate from asteroid Itokawa, were collected by the Japanese space probe Hayabusa. These findings suggest that impacts early in Earth’s history by similar asteroids could have delivered as much as half of our planet’s ocean water.
Ziliang Jin is a postdoctoral scholar in ASU’s School of Earth and Space Exploration and the lead author on the paper published in Science Advances. He says: “We found the samples we examined were enriched in water compared to the average for inner solar system objects.”
His co-author is Maitrayee Bose, assistant professor in the school, adds: “It was a privilege that the Japanese space agency JAXA was willing to share five particles from Itokawa with a U.S. investigator.
“It also reflects well on our school.”
Bose also points out that the team’s idea of looking for water in the Itokawa samples came as a surprise for the Hayabusa project: “Until we proposed it, no one thought to look for water. I’m happy to report that our hunch paid off.”
In two of the five particles, the team identified the mineral pyroxene–which are known to have water in their crystal structure. Bose and Jin suspected that the Itokawa particles might also have traces of water–but they needed to know how much.
Itokawa’s history involves heating, multiple impacts, shocks and fragmentation–all of which would raise the temperature of the minerals and drive off water.
To study the samples, each about half the thickness of a human hair, the team used ASU’s Nanoscale Secondary Ion Mass Spectrometer (NanoSIMS), which can measure such tiny mineral grains with great sensitivity.
The NanoSIMS measurements revealed the samples were surprisingly rich in water. Thus implying that even nominally dry asteroids such as Itokawa may in fact harbour more water than scientists have assumed.
A fragmented history
Itokawa is a peanut-shaped asteroid– 1,800 feet long and 700 to 1,000 feet wide– which circles the sun every 18 months at an average distance of 1.3 times the Earth-sun distance. Part of its path brings it inside Earth’s orbit and at its farthest point, it sweeps out a little beyond that of Mars.
Based on Itokawa’s spectrum in Earth-based telescopes, planetary scientists place it in the S class–linking it with the stony meteorites thought to be fragments from S-type asteroids broken off in collisions.
Bose explains: “S-type asteroids are one of the most common objects in the asteroid belt.
“They originally formed at a distance from the sun of one-third to three times Earth’s distance.”
She adds that although they are small, these asteroids have kept whatever water and other volatile materials they formed with.
Itokawa’s structure resembles a pair of rubble piles crunched together. It has two main lobes, each studded with boulders but having different overall densities, while between the lobes is a narrower section.
Jin and Bose point out that Itokawa, is it appears today, is the remnant of a parent body at least 12 miles wide that was, at some point in its history, heated between 1,000 and 1,500 Fahrenheit. The parent body likely underwent several large shocks from impacts, with one final shattering event that broke it apart.
In the aftermath, two of the fragments merged and formed today’s Itokawa–reaching its current size and shape about 8 million years ago.
Bose says: “The particles we analyzed came from a part of Itokawa called the Muses Sea.
“It’s an area on the asteroid that’s smooth and dust-covered.”
Jin adds: “Although the samples were collected at the surface, we don’t know where these grains were in the original parent body. But our best guess is that they were buried more than 100 meters deep within it.”
But despite the catastrophic breakup of the parent body and exposure to radiation and impacts by micrometeorites at the surface, the minerals still show evidence of water that has not been lost to space.
Jin continues: The minerals have hydrogen isotopic compositions that are indistinguishable from Earth.”
Bose, who is currently building a clean-lab facility at ASU, which along with the NanoSIMS will be the first such facility at a public university capable of analyzing dust grains from other solar system bodies, explains further: “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.”
She continues: “And 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.”
For planetary scientists and cosmochemists who are drawing a picture of how the solar system formed, asteroids are an essential resource. As leftover building blocks for the planetary system, they vary greatly among themselves while preserving materials from early in solar system history.
Hayabusa 2--another Japanese project– is currently at an asteroid named Ryugu, where it will collect samples and bring them back to Earth in December 2020.
In addition to this, NASA’s OSIRIS-REx sample-return mission– orbiting a near-Earth asteroid named Bennu–is scheduled to collect samples from that object in summer 2020 and bring them back to Earth in September 2023.
Bose explains: “Sample-return missions are mandatory if we really want to do an in-depth study of planetary objects.”
“The Hayabusa mission to Itokawa has expanded our knowledge of the volatile contents of the bodies that helped form Earth.
She concludes: “It would not be surprising if a similar mechanism of water production is common for rocky exoplanets around other stars.”