Scientists Find Water-Bearing Minerals in Samples from Asteroid Itokawa

A duo of researchers from the School of Earth and Space Exploration at Arizona State University has made the first-ever measurements of ancient water contained in samples collected by JAXA’s Hayabusa mission from the peanut-shaped asteroid (25143) Itokawa. The results appear in the journal Science Advances.

The S-type asteroid Itokawa. Image credit: JAXA.

Itokawa is an S-type asteroid about 1,800 feet (549 m) long and 700 to 1,000 feet (213-305 m) wide.

Discovered in 1998 by the LINEAR project, this asteroid was given the provisional designation 1998 SF36 and was officially named after the Japanese rocket scientist Hideo Itokawa.

The asteroid 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 farthest, it sweeps out a little beyond that of Mars.

It 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.

Planetary scientists think that Itokawa is the remnant of a parent body at least 12 miles (19 km) wide that at some point was heated between 1,000 and 1,500 degrees Fahrenheit (538-816 degrees Celsius).

The parent body suffered 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, which reached its current size and shape about 8 million years ago.

In 2010, JAXA’s Hayabusa mission collected and returned more than 1,500 particles from a part of Itokawa called the Muses Sea.

“We found the samples we examined were enriched in water compared to the average for inner solar system objects,” said Dr. Ziliang Jin, lead author of the study.

The two Itokawa particles studied by the team are tiny: for comparison, a human hair is 100 to 500 microns in diameter. Image credit: Ziliang Jin Maitrayee Bose, ASU / JAXA.

Dr. Jin and his colleague, Dr. Maitrayee Bose, found a mineral called pyroxene in two Itokawa particles.

In terrestrial samples, pyroxenes have water in their crystal structure. The researchers suspected that the Itokawa particles might also have traces of water, but they wanted to know exactly how much.

The measurements revealed the samples were unexpectedly rich in water. They also suggest that even nominally dry asteroids such as Itokawa may in fact harbor more water than scientists have assumed.

“Itokawa minerals contain water contents of 698 to 988 parts per million weight, after correcting for water loss during parent body processes and impact events that elevated the temperature of the original parent body,” the scientists said.

“Although the samples were collected at the surface, we don’t know where these grains were in the parent body. But our best guess is that they were buried more than 328 feet (100 m) deep within it,” Dr. Jin said.

“Despite the catastrophic breakup of the parent body, and the sample grains being exposed to radiation and impacts by micrometeorites at the surface, the minerals still show evidence of water that has not been lost to space.”

“In addition, the minerals have hydrogen isotopic compositions that are indistinguishable from Earth.”

“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,” Dr. Bose said.

“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.”

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Ziliang Jin Maitrayee Bose. 2019. New clues to ancient water on Itokawa. Science Advances 5 (5): eaav8106; doi: 10.1126/sciadv.aav8106