A piece of the dawn: Asteroid sample science hints at an ancient salt lake and conditions in the early solar system

WEST LAFAYETTE, Ind. — The organic ingredients to life, hints to where Earth’s water came from, and the fundamental building blocks of our planet and the solar system seem like a lot to fit in 4 ounces of dusty rock, but that’s exactly what scientists have found in samples of asteroid Bennu brought to Earth in 2023.

Michelle Thompson, associate professor of earth, atmospheric and planetary sciences in Purdue’s College of Science, was one of the first nine lead investigators from the science team to study the samples. She was on the “tiger team” — the team of experts who were given first access to the sample of the asteroid Bennu returned from NASA’s OSIRIS-REx spacecraft last year. They had 72 hours to make the preliminary measurements and assessments of the sample.

Thompson, an expert in extraplanetary bodies, chemistry and how they interact with the environment of space, explained that these results help scientists understand the makeup of the early solar system — including the mineral components Earth formed from and the organic molecules that might have led to the evolution of life upon it.

“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.”

Major research results from studying Bennu, including findings gleaned from the first feverish 72 hours of examining the sample, were recently published in the journal Nature, with Thompson as a co-author.

A tall order in a small canister

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. Results will help give scientists insight into the origin of the terrestrial planets and the earliest history of our solar system.

“This is a truly once-in-a-lifetime — maybe a once-in-several-lifetimes — experience,” Thompson said. “OSIRIS-REx was selected in 2011, the year I started graduate school, and launched in 2016, the year I finished my PhD. It reached Bennu in 2018, the year I came to Purdue. And now I am one of the first humans to get to study it. Bennu is a treasure trove of information; this is literally the project of my career.”

Two other asteroid missions — Hayabusa and Hayabusa2, both launched by the Japan Aerospace Exploration Agency — brought asteroid samples back to Earth and shared those samples with the OSIRIS-REx science team, including Thompson. NASA also shared a small amount of the asteroid Bennu sample with several other Japanese institutions, seeding scientific knowledge across the globe as asteroids cross-seed minerals throughout the solar system.

OSIRIS-REx brought back a canister containing 4.29 ounces (121.6 grams) of asteroid Bennu material — almost the same weight as a medium-sized apple, a stick of butter or a baby panda. The mission’s goal was to return 60 grams of sample, allowing scientists to get their best first look at what minerals and organic molecules make up asteroids like Bennu.

Of course, pieces of asteroids can fall to Earth on their own. But when they do, they don’t get here unscathed. Screaming through the friction of Earth’s atmosphere is a violent process, and not many rocks make it to the surface intact. And once they’re down, they are immediately contaminated by Earth’s abundant, aggressive and exuberant ecosphere.

Escorting the rock down, gently and insulated in still-pristine condition using human innovation, gave scientists a privileged look at the asteroid as it was in space, lending understanding to a wide range of planetary science.

Seeding the early Earth

Bennu is a carbonaceous asteroid, which means that the element carbon — a fundamental element found in life — makes up part of its composition. Like charcoal, a common carbonaceous material, many of these asteroids are dark, making it hard to understand their composition just by studying them with telescopes from Earth.

A team of 66 scientists from four continents studied the minerals in the Bennu samples. One of the most striking revelations from that research is the abundance of phosphate, sulfate and other salt minerals in the samples. Phosphate minerals are important to living beings on Earth, found in seeds, bones and leaves. The scientists found evidence of an ancient salt lake, 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 their inventory 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 scientific community doesn’t have a definitive understanding of how Earth got all its water,” Thompson said. “Looking at how much water is part of these minerals in the samples from Bennu gives us a clue about how much water was out in the solar system when the planets were forming. How much material was available that could have been delivered through impacts — not just organic material, but water and minerals as well.”

Comparing the composition of Bennu and other similar asteroids to icy dwarf planets, including Ceres and Pluto as well as moons of the outer solar system, implies that the building blocks for life could be widespread across the solar system — and beyond.

About Purdue University

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