Planetary scientist decodes clues in Bennu’s surface composition to make sense of far-flung asteroids

(Press-News.org) WEST LAFAYETTE, Ind. — New results from OSIRIS-REx, NASA’s first asteroid sample return mission, reveals why some gray asteroids reflect light at different wavelengths, like red or blue, more strongly. How these asteroids reflect light at red and blue wavelengths can give deeper insights into the evolution of rocky bodies in the solar system.

It also enables future research. By having a better understanding and comparing what telemetry and telescope data say about an asteroid with what its actual surface particles say about it will enable future astronauts, scientists and explorers to navigate to and select asteroids for research or mining with greater certainty.

Michelle Thompson is an expert on asteroids and one of the international team of scientists studying the sample of the asteroid Bennu brought back by the OSIRIS-REx mission. OSIRIS-REx, NASA’s first mission to acquire a sample from an asteroid and deliver it to Earth, is the culmination of more than a decade of work by a team of hundreds. OSIRIS-REx’s name, which stands for Origins, Spectral Interpretation, Resource Identification and Security–Regolith Explorer, encapsulates the program’s goals.

Thompson, associate professor of earth, atmospheric and planetary sciences in Purdue’s College of Science, studies space weathering — the interaction between the skin of rocky bodies and the environment of space. Her research has led her to ponder the moon, Mercury and asteroids, among other rocky bodies in the solar system. Her most recent magnus opus has been Bennu — the asteroid visited by NASA’s OSIRIS-REx mission, which brought home some of the oldest and most pristine asteroid samples ever studied.

“Sample return missions are a cornerstone of planetary science,” Thompson said. “They give us snapshots of the chemistry and the composition of the very early solar system. They let us look at the building blocks of the planets and inventory what was there. We can also compare Bennu’s samples to samples from Japan’s Hayabusa missions and get a better understanding of how these asteroids change and evolve and what we can tell about asteroids from the surface of the Earth compared to when we look at the samples themselves.”

This study is part of a trio of newly published papers based on analysis of Bennu samples by worldwide experts, including Thompson. Together, the research shows that Bennu is a mixture of materials from across and even beyond our solar system, whose unique and varied contents have been transformed by interactions with water and space weathering.

Then in a mirror dimly, now face-to-face It’s not economically or physically feasible to visit every one of the 1.45 million known asteroids, or even a quorum of them, in the solar system. Being able to extrapolate and understand the nature of various asteroids by analyzing them from the safety of our home planet is key to understanding the myriad rubble pile asteroids.

OSIRIS-REx is humanity’s third asteroid sample return mission, after Hayabusa and Hayabusa2 visited asteroids Itokawa and Ryugu respectively. One of the things Thompson found fascinating is that since both asteroids Ryugu and Bennu are carbonaceous, rubble pile asteroids that date from the birth of the solar system; a natural assumption would be that they would reflect light the same way. But they don’t. In fact, Thompson says, when viewed through spacecraft telescopes, Ryugu looks faintly red — its spectrum slopes upward — and Bennu looks blue — its spectrum slopes downward.

“The question has been why,” Thompson said. “Why are their spectra different if they have the same kind of minerals? Going into the sample return, we thought maybe they might be experiencing these space weathering processes in different ways. Maybe we see different characteristics in one sample compared to the other because of this surface exposure, but what we’re actually seeing is that’s not the case. They are very, very similar in terms of the way that they experience space weathering.”

Rubble pile asteroids undergo cycles that periodically refresh the surface of the ancient asteroid, changing the way it looks to the eyes of a telescope or to human eyes looking through that telescope. Grains collected from the surface of Ryugu have been exposed to space for a few thousand years, but Thompson and colleagues found that surface grains from Bennu samples have been braving the void of space for tens of thousands of years.

“And so instead of looking at two different trajectories for how this process is operating on these bodies, instead we’re seeing two different points in one cycle,” Thompson said. “Their ‘colors’ are changing, meaning their spectral properties are changing relative to their surface exposure age.”

Being able to collect data visually, telescopically and remotely and correlate it to sample data — literally ground-truthing the data — enables scientists to extrapolate concrete knowledge to a much larger range of bodies in the solar system, perhaps expanding even to other bodies lacking an atmosphere including some moons and dwarf planets.

Salt and spice and everything nice Earlier this year, a multinational team of scientists reported the discovery of salts in the Bennu samples. Among these salts were phosphates, which are important to life on Earth and critical to metabolism and DNA. The scientists found evidence of an ancient brine — an environment well suited to kick-start some of the precursor compounds for the chemistry of life.

Understanding these minerals and the organic molecules in the samples are critical for understanding what elements were present in the early solar system.

“Looking at the organic molecules from Bennu, we are getting an understanding of what kinds of molecules could have seeded life on early Earth,” Thompson said. “Information about what compounds, what elements are there and in what proportions. We won’t find life itself, but we’re looking at the building blocks that could have eventually evolved into life.”

The same ingredients are, of course, still part of the Earth today, but they have been mixed and melded and changed over the eons by forces both biological and geological. In contrast, the materials in the Bennu samples have been kept pristine. Their state allows scientists to look back in time to what the solar system was like before the planets as they exist today formed.

“Asteroids are relics of the early solar system,” Thompson said. “They’re like time capsules. We can use them to examine the origin of our solar system and to open a window to the origin of life on Earth.”

About Purdue University Purdue University is a public research university leading with excellence at scale. Ranked among top 10 public universities in the United States, Purdue discovers, disseminates and deploys knowledge with a quality and at a scale second to none. More than 107,000 students study at Purdue across multiple campuses, locations and modalities, including more than 58,000 at our main campus in West Lafayette and Indianapolis. Committed to affordability and accessibility, Purdue’s main campus has frozen tuition 14 years in a row. See how Purdue never stops in the persistent pursuit of the next giant leap — including its comprehensive urban expansion, the Mitch Daniels School of Business, Purdue Computes and the One Health initiative — at https://www.purdue.edu/president/strategic-initiatives.

Paper Sulfide Minerals Bear Witness to Impacts Across the Solar System

Nature Communications

doi.org/10.1038/s41467-025-61201-6

Media contact: Brittany Steff, bsteff@purdue.edu

Note to journalists:

High-resolution versions of the photo in this story showing Michelle Thompson, along with photos from OSIRIS-REx and of the asteroid Bennu, are available via Google Drive. 

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