Asteroid Samples Were Once Part of a Wetter World

Nine months have passed since NASA’s OSIRIS-REx returned its samples of asteroid Bennu to Earth. The samples are some of the Solar System’s primordial, pristine materials. They’ve made their way into scientists’ hands, and their work is uncovering some surprises.

Some of the material in the samples indicates that Bennu had a watery past.

NASA chose Bennu for the OSIRIS-REx sampling mission for several reasons. First, it’s a near-Earth asteroid (NEA), so it’s relatively close to Earth. It’s also not very large at about 500 meters in diameter and rotates slowly enough to allow for a safe sampling procedure.

But the overarching reason was probably its composition. It’s a B-type asteroid, a subtype of carbonaceous asteroids, which means it contains organic molecules. Finding organic molecules throughout the Solar System is one way of tracing its origin and formation.

Returning samples to Earth is the best and most complete way to study asteroids. Asteroid fragments that fall to Earth are scientifically valuable. But much of their lighter material simply burns up when entering Earth’s atmosphere, leaving a huge crater in our understanding.

Space missions always seem to surprise us somehow. If they didn’t, there’d be less impetus to send them. In this case, the sample contains chemicals that OSIRIS-REx didn’t spot when it was studying Bennu.

New research in the journal Meteoritics and Planetary Science presents these findings. It’s titled “Asteroid (101955) Bennu in the laboratory: Properties of the sample collected by OSIRIS-REx.” The co-lead author is Dante S. Lauretta, the principal investigator for the OSIRIS-REx mission and the Regents Professor of Planetary Sciences at the University of Arizona Lunar and Planetary Laboratory. The paper is an overview of the sample and serves as a catalogue from which researchers can request sample material for their research.

“Finally having the opportunity to delve into the OSIRIS-REx sample from Bennu after all these years is incredibly exciting,” Lauretta said in a press release. “This breakthrough not only answers longstanding questions about the early solar system but also opens new avenues of inquiry into the formation of Earth as a habitable planet. The insights outlined in our overview paper have sparked further curiosity, driving our eagerness to explore deeper.”

“We describe the delivery and initial allocation of this asteroid sample and introduce its bulk physical, chemical, and mineralogical properties from early analyses,” the authors write in their paper. The 120-gram sample dates back billions of years. It’s pristine, meaning it hasn’t melted and resolidified since it was formed.

The astromaterials curation team at NASA’s Johnson Space Center used the Advanced Imaging and Visualization of Astromaterials (AIVA) procedure to document the condition of the sample and the sampling equipment. This was done while the sample was still inside its glovebox, which is highly reflective for this purpose. This is a meticulous process involving hundreds of images stacked together.

Overall, the sample is dark. But there are brighter materials interspersed in it. “Some stones appear mottled by brighter material that occurs as veins and crusts,” the authors write. The largest piece is about 3.5 cm long, but much of it is dust. Stones with hummocky morphologies have the lowest densities, and mottled stones have the highest densities.

“Some of the high-reflectance phases have a hexagonal crystal habit, whereas others appear as clusters of small spheres, platelets, and dodecahedral forms,” the authors write. The collection also contains some individual pieces that are highly reflective.

Overall, the material is grouped into three categories:

• Hummocky material with uneven surfaces. Their surfaces feature rounded mounds and depressions reminiscent of cauliflower. This material is generally dark but has some microscopic, brighter material.
• Angular particles that have been fractured and have sharper edges. They have hexagonal and polygonal shapes and have some layering. They’re generally dark, but some faces have a metallic lustre and specular reflections. They also have some highly reflective inclusions like the hummocky material.
• Mottled particles that are mostly darker but have layers of reflective material. The reflective material fills in small cracks in the darker material and also occurs as bright, individual flakes.

Representative samples were also analyzed at other institutions in the US using different instruments including a plasma mass spectrometer, an infrared spectrometer, and an X-ray computer tomographer. These examinations revealed other information, like particle densities and elemental abundances. In particular, it contains isotopic information for hydrogen, carbon, nitrogen, and oxygen. It also compares these abundances to those of other asteroids.

But what jumps out from this initial analysis is the sample’s serpentine and other clay minerals. Their presence is similar to what’s found on Earth’s mid-ocean ridges, where Earth’s mantle encounters water.

On Earth, contact between mantle material and ocean water also creates clays and other minerals like carbonates, iron oxides, and iron sulphides. These were also found in the Bennu sample.

But one finding stands out among the rest: water-soluble phosphates. These compounds are found throughout Earth’s biosphere and are an important component of biochemistry.

JAXA’s Hayabusa 2 mission found a similar phosphate in its sample from asteroid Ryugu. But the phosphate from Bennu is different. Unlike any other asteroid sample, it has no inclusions and different-sized grains. The magnesium sodium phosphate in the Bennu sample suggests a watery past.

“The presence and state of phosphates, along with other elements and compounds on Bennu, suggest a watery past for the asteroid,” Lauretta said. “Bennu potentially could have once been part of a wetter world. Although, this hypothesis requires further investigation.”

In their paper, the authors outline several hypotheses for Bennu’s past. One of them states that “… the dominant lithologies on Bennu’s surface have mineralogical, petrological, and compositional properties closely resembling those of the most aqueously altered carbonaceous chondrites.”

The Bennu sample also shows that the asteroid is chemically primitive, meaning it has remained largely unchanged since its formation. The rocks have not melted and resolidified since their initial creation. The asteroid’s elemental properties also mirror that of the Sun.

“The sample we returned is the largest reservoir of unaltered asteroid material on Earth right now,” Lauretta said.

The initial research also shows that Bennu is rich in carbon and nitrogen, critical clues to the asteroid’s origins. These chemicals also play a role in the appearance of life, adding to the intrigue.

“These findings underscore the importance of collecting and studying material from asteroids like Bennu – especially low-density material that would typically burn up upon entering Earth’s atmosphere,” said Lauretta. “This material holds the key to unraveling the intricate processes of solar system formation and the prebiotic chemistry that could have contributed to life emerging on Earth.”

Harold Connolly is a co-author of the study and the mission sample scientist who leads the Sample Analysis Team. He’s also a professor at Rowan University in Glassboro, New Jersey, and a visiting research scientist at UArizona. “The Bennu samples are tantalizingly beautiful extraterrestrial rocks,” Connolly said. “Each week, analysis by the OSIRIS-REx Sample Analysis Team provides new and sometimes surprising findings that are helping place important constraints on the origin and evolution of Earthlike planets.”

And this is really just the beginning. With these evaluations in hand and the sample catalogued, research scientists around the world will request samples for their own research.

Further secrets will be revealed.

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