In 2020, the Japan Aerospace Exploration Agency's (JAXA) Hayabusa2 spacecraft completed its primary mission when it returned samples of asteroid Ryugu to Earth. In 2023, NASA's OSIRIS-REx also completed its primary mission by returning samples of asteroid Bennu to Earth. Scientists in labs around the world have been studying those samples and have uncovered some surprises.
The Ryugu sample contained uracil, one of the four RNA nucleotides that are essential for life as we understand it. That discovery indicates that asteroids could've played a role in delivering the raw materials for life to Earth. The Bennu sample contained its own surprise. It contained unexpected phosphate compounds, which suggested that it could be a splinter from a small, ancient body with an ocean.
These findings show how complex asteroids can be, and that they're more than just chunks of space rock.
Asteroids are the fragments from collisions involving planetesimals. Each one is a puzzle piece that can help astronomers uncover our Solar System's history. One of the key endeavours in asteroid and Solar System science is determining which asteroids shared the same parent bodies, which can help illuminate the overall history of the Solar System.
New research in The Planetary Science Journal shows that Bennu and Ryugu came from the same parent body. The research is "JWST Spectroscopy of (142) Polana: Connection to NEAs (101955) Bennu and (162173) Ryugu," and the lead author is Dr. Anicia Arredondo from the Southwest Research Institute.
Both are from the Polana collisional family in the main asteroid belt (MAB) between Mars and Jupiter. It took more than laboratory study of the samples to confirm it. The JWST played an important role, too, by obtaining both mid-infrared and near-infrared spectra from both asteroids.
"We present JWST Near Infrared Spectrograph and Mid-Infrared Instrument spectroscopy of the parent body of the family, (142) Polana, and compare it with spacecraft and laboratory data of both near-Earth asteroids," the authors write. "Spectral features at similar wavelengths in the spectra of Polana and those of Bennu and Ryugu support the hypothesis that both asteroids originated in the Polana family."
“Very early in the formation of the solar system, we believe large asteroids collided and broke into pieces to form an ‘asteroid family’ with Polana as the largest remaining body,” said lead author Arredondo in a press release. “Theories suggest that remnants of that collision not only created Polana, but also Bennu and Ryugu as well. To test that theory, we started looking at spectra of all three bodies and comparing them to one another.”
“They are similar enough that we feel confident that all three asteroids could have come from the same parent body,” Arredondo said.
Polana is much larger than both Ryugu and Bennu, at about 55 km in diameter. Bennu is only about 500 meters in diameter, and Ryugu is only about 850 meters in diameter. Polana is very dark, with an albedo of only 0.045, and is a Type F carbonaceous asteroid, a sub-group of the more common C-type asteroid.
The researchers think that after the collision that spawned them, Ryugu and Bennu were pushed out of their orbits close to Polana by Jupiter's immense gravity. As a result, the two smaller asteroids have been altered by their closer proximity to the Sun.
“Polana, Bennu and Ryugu have all had their own journeys through our solar system since the impact that may have formed them,” said SwRI’s Dr. Tracy Becker, a co-author of the paper. “Bennu and Ryugu are now much closer to the Sun than Polana, so their surfaces may be more affected by solar radiation and solar particles.
There are some differences between the three, especially around the depth and width of the 2.7 μm feature. This feature indicates hydrated minerals, or water-bearing minerals, and tells scientists something about an asteroid's history of thermal and aqueous alteration. "The differences in the depth and width of the 2.7 μm feature are more prominent between Polana and Ryugu than between Polana and Bennu. The cause of this difference is uncertain but could potentially be due to location in the early planetesimal or the effects of space weathering," the researchers write.
“Likewise, Polana is possibly older than Bennu and Ryugu and thus would have been exposed to micrometeoroid impacts for a longer period,” Becker added. “That could also change aspects of its surface, including its composition.”
The differences could also stem from differences in the parent body.
"The differences in hydration between Bennu and Ryugu do not necessarily mean that they come from different parent bodies," the authors explain. "Differences between the similarly sized Bennu and Ryugu could be due to parent body partial dehydration due to internal heating. If Bennu came from surface material and Ryugu came from inner material, the parent body impact would produce different layers of compaction, which would cause them to have different macroporosities and levels of hydration."
In their conclusion, the authors state that despite differences, they're confident that all three bodies share the same parent body. "We find that similarities in the shapes and strengths of many of the spectral features across the NIR and MIR, including the prominent OH feature at 2.72 μm, support the hypothesis that Bennu and Ryugu could have originated in the new Polana family," they write.
Some regions of the spectra require further study to understand and explain, according to the authors.
"The analysis of the returned samples from both Bennu and Ryugu is ongoing, and future developments in the understanding of how surface processes manifest in NIR and MIR spectra will give additional insights into the interpretation of our Polana spectrum," they conclude.