Researchers find water in samples of the Itokawa asteroid

Two cosmochemists from Arizona State University made the very first measurement of the amount of water contained in samples taken from 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 similar impacts at the beginning of the Earth's history with similar asteroids would have provided up to half of our planet's seawater.

"We found that the samples we examined were enriched in water compared to the average of the objects in the internal solar system," Ziliang Jin explains. Postdoctoral researcher at the School of Earth exploration and space of the USS, he is the principal author of the paper published on May 1 in Progress of science report the results. His co-author is Maitrayee Bose, an assistant professor at the school.

"It was a privilege for the Japanese space agency JAXA to be willing to share five Itokawa particles with an American investigator," Bose said. "It reflects our school as well."

The team's idea of ​​looking for water in the Itokawa samples was a surprise for the Hayabusa project.

"Until we suggest it, nobody thinks of looking for water," Bose says. "I am pleased to announce that our intuition has paid off."

The team identified pyroxene, a mineral substance, in two of the five particles. In terrestrial samples, pyroxenes have water in their crystalline structure. Bose and Jin suspected that Itokawa 's particles might also have traces of water, but they wanted to know exactly how much. Itokawa had a rough history involving heating, multiple impacts, shocks and fragmentation. These would increase the temperature of the minerals and drive away the water.

To study the samples, each representing about half the thickness of a human hair, the team used the nano-scale nanoscale ion mass spectrometer (NanoSIMS) of the ASU, which can measure with great sensitivity these tiny mineral grains.

NanoSIMS measurements revealed that the samples were of unexpected richness in water. They also suggest that even nominally dry asteroids, such as Itokawa, may actually contain more water than scientists have assumed.

Fragmented world

Itokawa is a peanut-shaped asteroid measuring about 1,800 feet long and 700 to 1,000 feet wide. He circled the Sun every 18 months at an average distance of 1.3 times the Earth-Sun distance. Part of the Itokawa route takes it to the interior of Earth's orbit and further afield it sweeps a little beyond that of Mars.

Based on the Itokawa spectrum in terrestrial telescopes, planetary scientists place it in the S class. This links it to stony meteorites, which would be fragments of S-type asteroids broken up in collisions.

"S-type asteroids are one of the most common objects in the asteroid belt," Bose says. "They initially formed at a distance of one-third to three times the distance from the Earth." She adds that although they are small, these asteroids have preserved the water and other volatile matter with which they have formed.

Itokawa structurally looks like a pair of rubble piles together. It has two main lobes, each studded with rocks but having different overall densities, while the section between the lobes is narrower.

Jin and Bose point out that today 's Itokawa is the remnant of a parent body at least 12 miles wide that was heated at one time between 1,000. and 1500 degrees Fahrenheit. The parent body has suffered several significant shocks as a result of the impacts, with one last catastrophic event that has broken it. As a result of these events, two of the fragments merged to form today's Itokawa, which reached its current size and shape about 8 million years ago.

"The particles we analyzed came from a part of Itokawa called the Sea of ​​Muses," Bose said. "It's a smooth and dust-covered area on the asteroid."

Jin adds, "Although the samples were collected at the surface, we do not know where these grains were in the original parent body, but our best guess is that they were buried at more than 100 meters deep. "

He adds that, despite the catastrophic collapse of the parent body and that the sample grains are exposed to radiation and micrometeorite impacts to the surface, the minerals still show traces of water not lost in the space.

In addition, says Jin, "the minerals have an isotopic composition of hydrogen that is indistinguishable from the Earth."

Bose explains: "This means that S-type asteroids and parent bodies of ordinary chondrites are probably a critical source of water and several other elements for terrestrial planets."

She adds, "And we can only say that thanks to the in situ isotopic measurements made on regolith samples of returned asteroids – their dust and superficial rocks.

"This makes these asteroids high priority targets for exploration."

Scouting for samples

Bose notes that she is currently building an ASU-specific laboratory, which, along with the NanoSIMS (funded in part by the National Science Foundation), would be the first institution of its kind in a public university capable of developing a technology. analyze the dust grains from other bodies of the solar system.

Another Japanese mission, Hayabusa 2, is currently on an asteroid named Ryugu, where it will collect samples, which they will bring back to Earth in December 2020. The director of the Meteorite Studies Center of the ASU, Professor Meenakshi Wadhwa, is a member of the initial analysis. chemistry team for the Hayabusa 2 mission.

ASU is also participating in NASA's OSIRIS-REx sample return mission, which revolves around a near-Earth asteroid, Bennu. Among other instruments, the spacecraft carries the OSIRIS-REx Thermal Emission Spectrometer (OTES), designed by Professor Philip Christensen of the ASU Regents and built at the school. OSIRIS-REx is expected to collect Bennu samples in the summer of 2020 and bring them back to Earth in September 2023.

For planetary scientists and cosmochemists who paint a picture of the formation of the solar system, asteroids are an excellent resource. Remains of building blocks for the planetary system, they vary greatly between them while preserving the early materials of the solar system's history.

According to Bose, "The return missions of samples are mandatory if we really want to study in depth the planetary objects".

And she adds, "The Hayabusa Mission in Itokawa has expanded our knowledge of the volatile content of the bodies that have contributed to the formation of the Earth.It would not be surprising if a similar mechanism of water production be common to the rocky exoplanets surrounding other stars. "