Right now, the Japanese Aerospace Exploration Agency‘s (JAXA)
Hayabusa2 spacecraft is busy exploring the asteroid 162173 Ryugu. Like it’s predecessor, this consists of a sample-return mission, where regolith from the asteroid’s surface will be brought back home for analysis. In addition to telling us more about the early Solar System, these studies are expected to shed light on the origin of Earth’s water (and maybe even life).
Meanwhile, scientists here at home have been busy examining the samples returned from 25143 Itokawa by the Hayabusa1 spacecraft. Thanks to a recent study by a pair of cosmochemists from Arizona State University (ASU), it is now known that this asteroid contained abundant amounts of water. From this, the team estimates that up to half the water on Earth could have come from asteroid and comet impacts billions of years ago.
This study, which was the first time samples from the surface of an asteroid were examined for water, recently appeared in the journal Science Advances. The study team consisted of Ziliang Jin and Maitrayee Bose, a postdoctoral scholar and an assistant professor in ASU’s School of Earth and Space Exploration (SESE).
Artist’s impression of Hayabusa leaving Earth, a Japanese sample-return mission to the asteroid 25143 Itokawa. Credit: JAXA
The current scientific consensus is that asteroids are composed of material leftover from the formation of the Solar System. The study of these bodies is therefore expected to reveal things about its early history and evolution. What Jin and Bose found, after examining the samples provided by JAXA, was that they were enriched in water compared to the average for objects found in the inner Solar System.
And Bose indicated in an interview with ASU Now, this study was made possible thanks to the cooperation between the ASU and JAXA, though they were surprised to hear what she and Jin were looking for:
“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… Until we proposed it, no one thought to look for water. I’m happy to report that our hunch paid off.”
To study the five samples, each of which measure 50 to 250 microns in diameter (about half the width of a human hair), the team used ASU’s Nanoscale Secondary Ion Mass Spectrometer (NanoSIMS). This instrument is one of only 22 spectrometers in the entire world that can examine tiny mineral grains with a high degree of sensitivity.
The two Itokawa particles studied by Jin and Bose are tiny: for comparison, a human hair is 100 to 500 microns in diameter. ASU’s NanoSIMS instrument made it possible to extract information on the particles’ water content. Credit: Z. Jin and M. Bose/ASU/JAXA
In two of the five particles, the team identified pyroxene, a mineral which (on Earth) has water as part of its crystalline structure. Jin and Bose also suspected that the grains might contain traces of water, though they were unclear as to how much. Itokawa’s long history would have included heating events, impacts, shocks and fragmentation, all of which would have raised its temperature and caused water to be lost to space.
The NanoSIMS measurements confirmed this hypothesis, revealing that the sample grains themselves were rich in water. But what was surprising was just how rich they were. This indicates that asteroids such as Itokawa (which are considered to be “dry”) are capable of harboring more water than scientists previously thought.
Because of its composition, which is predominantly made up of silicate minerals and metals, planetary scientists have designated Itokawa as an S-class asteroid. Measuring just 500 meters (1800 ft) in length and 215 to 300 (700 to 1000 ft) in diameter, the asteroid circles the Sun every 18 months at an average distance of 1.3 AU – passing inside Earth’s orbit to a little beyond that of Mars.
Objects that are Itokawa’s size are believed to be fragments that broke off of larger S-class asteroids. Despite being small, these asteroids are believed to have kept whatever water and volatile materials (nitrogen, carbon dioxide, methane, ammonia, etc.) they had at formation. As Bose explained:
“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.”
The samples studied by Jin and Bose came from the feature called the Muses Sea, which is the smooth area in the middle of Itokawa. Credit: JAXA
From its structure, which consists of two boulder-strewn main lobes (with different densities) that are joined by a narrower section, it is believed that Itokawa is the remnant of a parent body measuring about 19 km (12 mi) in width. During its history, it would have been heated to between 550 and 800 °C (1000 and 1500 °F) and suffered multiple impacts, with one big event that broke it apart.
In the aftermath, two of the fragments merged to form Itokawa, which assumed its current size and shape by about 8 million years ago. Despite the catastrophic breakup that led to its formation and the fact that the sample grains were exposed to radiation and micrometeorite impacts, the minerals still showed evidence of water that was lost to space.
“Although the samples were collected at the surface, we don’t know where these grains were in the original parent body,” said Jin. “But our best guess is that they were buried more than 100 meters deep within it… The minerals have hydrogen isotopic compositions that are indistinguishable from Earth.”
What this shows is that asteroid impacts during the Late Heavy Bombardment (ca. 4.1 to 3.8 billion years ago) were responsible for distributing water to Earth shortly after if formed. As Bose added, this makes S-class asteroids a high-priority target for sample-return missions in the future.
“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. And we can say this only because of in-situ isotopic measurements on returned samples of asteroid regolith – their surface dust and rocks.”
Itokawa, a peanut-shaped asteroid that has different densities in its small body. Credit: ESO/JAXA
When those missions take place, ASU will likely be playing a significant role. Right now, Bose is working on the creation of a clean-lab facility at ASU that – along with the NanoSIMS – will be the first public university facility capable of analyzing samples of material obtained from asteroids and bodies in the Solar System.
Professor Meenakshi – the director of ASU’s Center for Meteorite Studies and the new director of the SESE – is also part of the analysis team that will be studying the samples returned by the Hayabusa2 mission. The spacecraft will be leaving the asteroid Ryugu in December of 2019 and is scheduled to return to Earth by December of 2020.
ASU is also responsible for contributing the Thermal Emission Spectrometer (OTES) instrument aboard NASA’s OSIRIS-REx spacecraft, which is currently conducting a sample-return mission with the near-Earth asteroid Bennu. OSIRIS-REx is scheduled to collect samples from Bennu next summer and bring them back to Earth by September of 2023.
These and other missions will expand scientist’s understanding of how our Solar System came to be, and might even shed some light on how life began on our planet. As Bose concluded:
“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. It would not be surprising if a similar mechanism of water production is common for rocky exoplanets around other stars.”