When you spend your career hunting for stardust, you learn to be patient and expect surprises. Presolar grains — also called stardust — are tiny mineral grains that formed around dying stars before our solar system existed, rare wherever you look, typically a few for every million grains of ordinary solar system material. In Bennu's hummocky and angular particles, my colleagues and I had already found silicates, oxides, and SiC at abundances similar to those seen in moderately to heavily aqueously altered meteorites. So, when we began searching for presolar grains in ‘mottled’ Bennu particles, I expected similar numbers—a modest handful of grains, consistent with what we already knew. But that is not what happened.
The NASA OSIRIS-REx mission returned 121.6 grams of material from the near-Earth asteroid Bennu in 2023, the largest asteroid sample ever brought to Earth by a space mission. Samples from asteroid Bennu come in a few recognizable types. Most early presolar-grain work focused on "angular" and "hummocky" particles, which preserve a reasonable inventory of stardust—enough to show how Bennu's parent asteroid mixed pristine, pre-solar-system material with billions of years of aqueous alteration.
"Mottled" particles are a different story. They're rarer, they're potentially linked to bright, carbonate-veined boulders on Bennu's surface, and almost nothing was known about what, if anything, they preserved of the presolar record. That gap is what pulled me in. If the angular and hummocky particles showed how stardust survives moderate-to-heavy water-rock interaction, the mottled particles were our chance to see the other end of that spectrum — what happens when the alteration gets more extreme.
We picked nine mottled particles from three samples, polished them, and mapped them with the High-Resolution Nanoscale Secondary Ion Mass Spectrometer (NanoSIMS-HR) at the K-ALFAA facility at the University of Arizona to search for presolar grains.
What we found
The first result was blunt: no presolar silicate grains, no presolar oxide grains, across an area comparable to what had been searched in angular and hummocky particles, where a few such grains had shown up.
However, zero doesn't automatically mean "destroyed" — sometimes it just means "not there to begin with." So, the second result mattered more. Silicon carbide (SiC) is among the toughest presolar minerals, chemically resistant enough that synthetic SiC is commonly used industrially as an abrasive and in armor plating. If anything should survive aqueous alteration on an asteroid, it's SiC. In Bennu's angular and hummocky particles, and in most primitive meteorites, SiC shows up at about 30–55 parts per million. In our mottled particles, it was around 8 parts per million—five to seven times lower —and the surviving grains were noticeably smaller.
That's the moment this stopped being a routine search for presolar grains and became a mystery. What strips an extremely resistant, refractory mineral down to a fraction of its expected abundance on an asteroid that never got hotter than about 100°C?
While chasing that question, we also mapped something else in the same particles: carbon- and nitrogen-isotopically anomalous organic matter, including tiny carbon nanoglobules that trace back to chemistry in the cold outer solar nebula or the presolar molecular cloud from which our solar system formed. Given how thoroughly the presolar minerals had been stripped out, I expected the organics to have been affected too.
They didn't. We found hundreds of isotopically anomalous regions with carbon and nitrogen isotope compositions similar to those of organic matter in Bennu's other particle types, in samples from asteroid Ryugu, and in carbonaceous meteorites. Whatever had gutted the presolar silicon carbide preserved isotopically anomalous organic molecules.
That contrast — a near-total loss of presolar minerals sitting beside preserved organic matter — is the heart of this paper. Different classes of ancient material responded differently to the same fluid history, and that's what needed explaining.
The mineralogy pointed us toward an answer. The mottled particles are rich in magnesium- and sodium-bearing phosphates and other evaporite salts — signatures of a late-stage, low-temperature, alkaline brine that moved through the parent asteroid, distinct from and later than whatever fluids altered the angular and hummocky material. SiC resists most fluids because it grows a thin, protective silica layer as soon as it starts to oxidize. That layer isn't stable in alkaline water, though: hydroxyl ions and alkali cations dissolve it, exposing fresh SiC to renewed oxidation, again and again, for as long as the fluid kept moving through the rock. But none of this happens quickly. It's not a dramatic dissolution event, just slow, patient chemical erosion that needs geologic time and the right fluid chemistry to add up to anything measurable. Given both, it's apparently enough to erase most of one of the most indestructible minerals known — while organic molecules, built on a very different and evidently more resilient chemistry, come through untouched.
A slow, salty brine demise… Why it matters
Presolar grains are among the few physical samples scientists have of material from stars other than the Sun, making them valuable tracers of stellar evolution and the building blocks of the solar system. Because they can also be destroyed by processes on their parent asteroids, understanding exactly how and why they disappear — and what survives instead — helps researchers more accurately reconstruct both the stellar history and the watery history recorded in primitive asteroids like Bennu.
Zooming out, this also sharpens the picture of Bennu's parent asteroid as a place with a genuinely layered history — pristine clasts closest to the original protolith, hummocky and angular material recording earlier, gentler alteration, and the mottled particles marking a distinct, later, harsher fluid episode. Three neighborhoods on the same parent body, each carrying its own water history in what it kept and what it lost.
We started this project without knowing anything about the presolar grain inventory in mottled samples and anticipated results consistent with what we already knew from Bennu's other particle types. Instead, we found a near-absence that turned out to be the most informative result in the sample — proof that sometimes what's missing from a rock is exactly the story you were looking for.
Publication: Haenecour, P., Barnes, J.J., Bloch, E. et al. Gradual Destruction of Refractory Stardust in Alkali Brines on Asteroid Bennu. Nat Commun 17, 10249 (2026). https://doi.org/10.1038/s41467-026-76821-9.
Contact: Dr. Pierre Haenecour, haenecour@arizona.edu