Planetary Sciences News

First Analysis of Asteroid Water Reveals Earth-Like Makeup

Samples returned from the surface of an asteroid show that these small bodies may have more water than previously thought and could have delivered that water to Earth.


Scientists have analyzed the water content of asteroid material in a laboratory for the first time and found more of it than they expected. What’s more, the asteroid water, collected in situ from 25143 Itokawa, has a composition almost “indistinguishable” from Earth’s oceans, the researchers wrote in a paper that published today (1 May) in Science Advances.

“We find that the deuterium-to-hydrogen ratios of Earth and Itokawa, both objects that formed in the inner solar system, are the same,” Maitrayee Bose, an isotope cosmochemist at Arizona State University (ASU) in Tempe and coauthor on the discovery, told Eos.

In the early days of the solar system, “several Itokawa-like bodies may have collided with the proto-Earth and provided water,” she added. “In other words, small asteroid bodies in the inner solar system could [have been] a source of water for Earth and other planets.”

Asteroids like this could have delivered up to half an ocean’s worth of water to Earth, the team says.

“Indistinguishable” Water Composition

High concentrations of water have been detected beneath the surface of Earth, the Moon, Mars, and a few objects in the main asteroid belt. But how that water got there, when it arrived, and how much of it there was originally are a matter of ongoing debate among planetary scientists.

“The planetary science community uses hydrogen as a proxy for water content…and the deuterium-to-hydrogen ratio to make connections between planetary bodies and question how the different objects received their water,” she said. Deuterium is an isotope of hydrogen.

The researchers analyzed two grains collected from the surface of Itokawa by the Japan Aerospace Exploration Agency’s (JAXA) Hayabusa mission. Hayabusa returned to Earth in 2010 with the first samples of asteroid material gathered in situ, about 1,500 grains of regolith.

Two grains of Itokawa surface material analyzed for its water composition
Two grains from Itokawa collected by Hayabusa. Ziliang Jin and Maitrayee Bose examined the water in these two grains and found that it had an isotope composition similar to that of Earth’s ocean water. Credit: Japan Aerospace Exploration Agency, edited by Ziliang Jin

The scientists measured the abundance of water in the grains’ pyroxene crystals and found that the normally water poor pyroxenes contained nearly 1,000 parts per million (ppm) of water when they formed.

“We found the samples we examined were enriched in water compared to the average for inner solar system objects,” lead author Ziliang Jin, a postdoctoral researcher in cosmochemistry at ASU, said in a statement.

The team also measured the deuterium-to-hydrogen ratio (dD) in the pyroxenes using secondary ion mass spectrometry. The grains’ dD is about the same as that of standard mean ocean water on Earth and some carbonaceous chondrite meteorites that date from the earliest days of the solar system.

“The minerals have hydrogen isotopic compositions that are indistinguishable from Earth[’s],” Jin said.

This is the first time that scientists have analyzed water from samples of asteroid material for its abundance and composition.

A Look Back at the Early Solar System

Itokawa likely experienced a huge collision in its past and broke off from a larger parent body.

“Although the samples were collected at the surface,” Jin said, “we don’t know where these grains were in the original parent body.” From simulations, “our best guess is that they were buried more than 100 meters deep.”

If the two grains are representative of the rest of the original asteroid, Jin and Bose said, Itokawa could have had around 500 ppm of water when it formed.

It’s “bone dry with respect to anything in our human experience,” Bose said, “but it is still wet enough and with the correct isotope composition to provide more than half of the water on Earth.”

Itokawa-like asteroids are common throughout the inner solar system from the orbit of Mercury outward to the main asteroid belt, Bose said, and can reveal a lot about the mix of chemicals that made the solar system.

“S-type asteroids like Itokawa are least modified by space and planetary processes,” Bose said, because they are typically only a few dozen kilometers in size. “The water in S-type asteroids was incorporated directly from the solar nebula” that birthed the solar system.

The combination of these asteroids being common and possibly water rich means that they could have been an important water delivery mechanism to Earth, Mars, and elsewhere in the inner solar system, the team said.

Jin and Bose plan to expand this analysis to include samples from interplanetary dust and other carbonaceous meteorites. They are also looking forward to the return of two ongoing missions, JAXA’s Hayabusa2 and NASA’s Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx), which will bring back samples from other carbonaceous asteroids in the next few years.

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

Citation: Cartier, K. M. S. (2019), First analysis of asteroid water reveals Earth-like makeup, Eos, 100, Published on 01 May 2019.
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