Two cosmochemists at Arizona State University made the first measurements of water contained in samples of the surface of an asteroid. The samples came from the Itokawa asteroid and were collected by the Japanese space probe Hayabusa.
The team's findings suggest that the initial impacts on Earth's history by similar asteroids could have generated up to half of the planet's ocean water.
"We found that the samples we examined were enriched in water compared to the average of the objects in the inner solar system," says Ziliang Jin. A postdoctoral scholar at ASU's School of Earth and Space Exploration, he is the lead author of the article published on May 1 in Advances in science reporting the results. His coauthor is Maitrayee Bose, an assistant professor at the school.
"It was a privilege that the Japanese space agency JAXA was willing to share five Itokawa particles with a US investigator," Bose said. "It also reflects well in our school."
The team's idea of searching for water in the Itokawa samples was a surprise to the Hayabusa project.
"Until we proposed, no one thought of looking for water," says Bose. "I'm happy to report that our feeling was worth it."
In two of the five particles, the team identified the mineral pyroxene. In the terrestrial samples, pyroxenes have water in their crystalline structure. Bose and Jin suspected that the Itokawa particles might also have traces of water, but they wanted to know exactly how much. Itokawa had a difficult history involving warming, multiple impacts, clashes and fragmentation. These would increase the temperature of the minerals and expel the water.
To study the samples, each one-half the thickness of a human hair, the team used the ASU Nanoscale Secondary Ion Mass Spectrometer (NanoSIMS), which can measure tiny 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, may actually hold more water than scientists assume.
Itokawa is a peanut-shaped asteroid about 1,800 feet long and 700 to 1,000 feet wide. It circulates 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 Earth's orbit, and in the farthest part it sweeps a little beyond Mars.
Based on the Itokawa spectrum on Earth-based telescopes, planetary scientists place it in class S. This links it to stony meteorites, which are thought to be fragments of S-type asteroids broken in collisions.
"S-type asteroids are one of the most common objects in the asteroid belt," says Bose. "They originally formed at a distance from the Sun one third to three times the distance from Earth." She adds that, although they are small, these asteroids kept the water and other volatile materials they formed.
In the structure, Itokawa resembles a pair of debris stacked together. It has two main lobes, each with boulders, but with different general densities, whereas between the lobes is a narrower section.
Jin and Bose point out that today's Itokawa is the remnant of a parent body at least 12 miles wide that at some point was warmed between 1,000 and 1,500 degrees Fahrenheit. The parents' body suffered several large impact shocks, with a final break event that separated it. In the aftermath two of the fragments merged and formed today's Itokawa, which reached its present size and shape about 8 million years ago.
"The particles we analyzed came from a part of Itokawa called the Muses Sea," says Bose. "It's an area on the asteroid that's smooth and covered in dust."
Jin adds, "Although the samples were collected on the surface, we do not know where these grains were in the original parent body. But our best guess is that they were buried more than 100 meters deep."
He adds that despite the catastrophic disruption of the parents' bodies and the exposure of the sample grains to radiation and the impacts caused by micrometeorites on the surface, the minerals still show evidence of water that was not lost in space.
In addition, Jin says, "minerals have isotopic compositions of hydrogen that are indistinguishable from Earth."
Bose explains: "This means that S-type asteroids and the parent bodies of common chondrites are probably a critical source of water and various other elements for terrestrial planets."
She adds: "And we can only say this because of in situ isotopic measurements on regimented asteroid samples – their surface dust and rocks.
"It makes these asteroids a high priority for exploration."
Scouting for samples
Bose notes that she is building a clean laboratory facility at ASU, which along with NanoSIMS (partially funded by the National Science Foundation) would be the first such facility in a public university capable of analyzing dust grains from other organs of the solar system.
Another Japanese mission, the Hayabusa 2, is currently in an asteroid called Ryugu, where it will collect samples, bringing them back to Earth in December 2020. The director of the ASU Meteorology Center, Professor Meenakshi Wadhwa, is a Initial Analysis. team for the Hayabusa mission 2.
ASU is also aboard NASA's OSIRIS-REx sample return mission, which is in orbit of an Earth-bound asteroid called Bennu. Among other instruments, the spacecraft carries the OSIRIS-REx Thermal Emission Spectrometer (OTES), designed by ASU Regents Professor Philip Christensen, and built at the School. OSIRIS-REx is scheduled to collect samples from Bennu in the summer of 2020 and bring them back to Earth in September 2023.
For planetary scientists and cosmochemists who are drawing a picture of how the solar system formed, asteroids are a great resource. As building blocks for the planetary system, they vary widely among themselves while preserving materials from the early history of the solar system.
Says Bose, "Sample return missions are mandatory if we really want to do an in-depth study of planetary objects."
"Hayabusa's mission to Itokawa has expanded our knowledge of the volatile contents of the bodies that helped shape the Earth. It would not be surprising if a similar water-producing mechanism were common for rocky exoplanets around other stars."
Particles collected by Hayabusa give absolute age of asteroid Itokawa
Z. Jin el al., "New clues to the ancient water in Itokawa" Advances in science (2019) DOI: 10.1126 / sciadv.aav8106, https://advances.sciencemag.org/content/5/5/eaav8106
Researchers find water in samples of the asteroid Itokawa (2019, May 1)
recovered on May 1, 2019
This document is subject to copyright. In addition to any fair dealing for private study or research,
may be reproduced without written permission. Content is provided for informational purposes only.