Asteroid Bennu Samples Contain Stardust Older Than Our Solar System

The bold plan to collect material from asteroid Bennu and bring it back to Earth is already delivering.

Just as researchers anticipated, Bennu is opening a window onto the Solar System’s earliest chapter. It is far more than an inert lump of rock: Bennu holds not only material formed within our Solar System, but also components that originated beyond it.

Bennu’s orbit, NEA status and PHO classification

Bennu travels on a path that carries it close to Earth every six years. For that reason it is categorised as a near-Earth asteroid (NEA) and also as a potentially hazardous object (PHO).

OSIRIS-REx and asteroid Bennu: why NASA chose this target

When NASA selected the destination for the OSIRIS-REx mission-the project that visited Bennu and returned a sample-it followed an intensive scientific and engineering assessment of multiple candidate asteroids. Because Bennu is relatively accessible from Earth and is a primitive carbonaceous asteroid, NASA chose it as the target.

It is sufficiently large to allow the spacecraft to operate nearby and gather a sample, and spectroscopy of the surface indicated the presence of materials scientists were keen to investigate, including carbon-rich compounds and hydrated minerals.

Nearly nine years after OSIRIS-REx lifted off, the returned material is now under examination in laboratories worldwide.

Three new papers on Bennu’s origins, hydrothermal alteration and space weathering

Three recently published papers report that Bennu is built from ingredients sourced both from within and from outside our Solar System. They also describe how parts of the asteroid’s material have been modified through space weather and through interactions involving water.

• The variety and origin of materials accreted by Bennu's parent asteroid
• Mineralogical evidence for hydrothermal alteration of Bennu samples
• Space weathering effects in Bennu asteroid samples

Jessica Barnes, an associate professor at the University of Arizona’s Lunar and Planetary Laboratory, is a co-lead author on one of the studies.

"This is work you just can't do with telescopes," Barnes said in a press release.

"It's super exciting that we're finally able to say these things about an asteroid that we've been dreaming of going to for so long and eventually brought back samples from."

The Polana family, Bennu’s parent body and material from other stars

Bennu’s parent body belongs to the Polana family of asteroids. Over time, a sequence of impacts produced Bennu, and its original parent contained a blend of matter from our Solar System and from beyond it. Consequently, Bennu also includes material that formed close to the Sun, material that formed far from the Sun, and even material that originated around other stars.

That parent body assembled from this mixed inventory more than 4 billion years ago, as the Solar System itself was taking shape. The paper The variety and origin of materials accreted by Bennu's parent asteroid sets out the evidence and reasoning in depth.

"Bennu's parent asteroid may have formed in the outer parts of the solar system, possibly beyond the giant planets, Jupiter and Saturn," Barnes said.

"We think this parent body was struck by an incoming asteroid and smashed apart. Then the fragments re-assembled and this might have repeated several times."

"The first bodies to form in the Solar System acquired their materials from stars, the presolar molecular cloud and the protoplanetary disk," the authors write.

"Asteroids that have not undergone planetary differentiation retain evidence of these primary accreted materials."

Analyses of the Bennu samples indicate that a substantial portion of near-surface material has undergone hydrothermal interactions that modified isotopic compositions, chemistry and overall mineralogy. But none of the samples have been altered.

"We show that some primary accreted materials escaped the extensive aqueous alteration that occurred on the parent asteroid, including presolar grains from ancient stars, organic matter from the outer Solar System or molecular cloud, refractory solids that formed close to the Sun, and dust enriched in neutron-rich Ti isotopes," the paper states.

One especially striking outcome is how much material appears to have come from outside our Solar System. This ancient stardust is older than the Solar System itself and is distinguished by isotopic signatures that differ from Solar System material. In other words, Bennu’s “recipe” is more intricate than previously assumed.

"Those are pieces of stardust from other stars that are long dead, and these pieces were incorporated into the cloud of gas and dust from which our Solar System formed," Barnes said.

"In addition, we found organic material that's highly anomalous in their isotopes and that was probably formed in interstellar space, and we have solids that formed closer to the Sun, and for the first time, we show that all these materials are present in Bennu."

Mineralogical evidence for hydrothermal alteration of Bennu samples

Although some Bennu material has avoided modification by space weathering, chemical processes and even impacts, much of it shows clear signs of change. The second paper, Mineralogical evidence for hydrothermal alteration of Bennu samples, concludes that hydrothermal processes altered the majority of Bennu’s material.

"The mineralogical evidence indicates alteration of accreted minerals by a fluid that evolved with time, leading to etching, dissolution, and reprecipitation," the authors write.

"We think that Bennu's parent asteroid accreted a lot of icy material from the outer Solar System, which melted over time," said Tom Zega, director of the Kuiper-Arizona Laboratory who co-led the study.

Residual heat from Bennu’s formation-or heat generated by later impacts-could have melted ice within the asteroid. Water produced in this way may then have reacted with silicate minerals, driving hydrothermal chemistry that transformed the Bennu material.

"Now you have a liquid in contact with a solid and heat – everything you need to start doing chemistry," Zega said. "The water reacted with the minerals and formed what we see today: samples in which 80% of minerals contain water in their interior, created billions of years ago when the Solar System was still forming."

Space weathering effects in Bennu asteroid samples

The third paper, Space weathering effects in Bennu asteroid samples, focuses on the long-term impact of micrometeorite strikes on Bennu.

"Space weathering processes, dominated by micrometeoroid impacts and solar irradiation, modify the mineralogy and chemistry of exposed surfaces," the authors explain.

"Comparison of Bennu samples with those collected from the asteroids Ryugu and Itokawa suggest that micrometeoroid impacts might play a more active and rapid role in the space weathering of asteroidal surfaces than was initially suggested, particularly for carbonaceous bodies."

Within the returned material, some particles retain distinctive marks left by micrometeoroid impacts. These strikes-together with the solar wind-are part of what scientists call space weathering. Because Bennu has no atmosphere to shield it, its surface has been continually peppered by these tiny collisions. The findings indicate that space weathering proceeds far more quickly than had been thought.

"Melt deposits occur in <0.5% of Itokawa samples, 2% of Ryugu particles and 20% of Bennu particles (although analyses of additional material may improve these statistics)," the paper states.

"Together, these results suggest that micrometeoroid impacts play a more important role in the space weathering of asteroidal surfaces than was suggested from early observations of asteroidal returned samples."

Why returned asteroid samples matter on Earth

Most asteroid fragments that arrive at Earth are destroyed as they pass through the atmosphere. Even when meteorites do survive to reach the ground, they are still exposed to Earth’s air and can change rapidly. That is why asteroid sample return missions are so valuable for understanding the Solar System.

"Those that do make it to the ground can react with Earth's atmosphere, particularly if the meteorite is not recovered quickly after it falls," Zega said, "which is why sample return missions such as OSIRIS-REx are critical."

This article was first published by Universe Today. Read the original article.