Alkali Brines Gradually Destroy Refractory Stardust on Asteroid Bennu

Asteroid Bennu is revealing a paradox at the heart of the early Solar System: the small, dark world preserved some of the most primitive material known to science, yet the same chemical environment that helped retain its ancient record may also be slowly destroying it. A study titled “Gradual Destruction of Refractory Stardust in Alkali Brines on Asteroid Bennu” examines how highly resistant grains of presolar dust could be altered by alkaline fluids inside the asteroid. These grains formed around ancient stars before the birth of the Sun, survived the violent assembly of the Solar System, and were incorporated into the carbon-rich material from which Bennu formed. Their progressive breakdown offers researchers a new way to investigate how water, salts and minerals reshaped primitive asteroids after their formation.

Bennu is a near-Earth asteroid about 500 metres across and is widely regarded as a surviving fragment of the Solar System’s earliest building materials. NASA’s OSIRIS-REx mission collected samples from its surface in 2020 and delivered them to Earth in 2023. Laboratory studies of those samples have identified abundant carbon-bearing compounds, hydrated minerals, phosphate and other components that record interactions between rock and water. The new research focuses on a particularly distinctive component: refractory stardust. These microscopic particles, sometimes called presolar grains, condensed in the outflows of dying stars or in the debris surrounding stellar explosions. Because they formed before the Sun, they carry isotopic signatures unlike those produced by ordinary Solar System processes.

“Refractory” describes a material that remains stable at relatively high temperatures compared with more volatile substances. Presolar refractory grains can include silicon carbide, oxide and silicate minerals, depending on the type of stellar environment in which they formed. Their survival in meteorites has allowed scientists to reconstruct aspects of stellar evolution using laboratory measurements of isotopes such as carbon, nitrogen, oxygen and silicon. The grains are commonly identified through anomalous isotopic ratios: instead of matching the average composition of the Solar System, they preserve fingerprints inherited from their parent stars. Yet survival is not guaranteed. Once a grain is exposed to chemically reactive fluids, its surface can dissolve, become coated, exchange elements with the surrounding solution or be replaced by new minerals.

The study’s central chemical setting is an alkali brine, a concentrated water-based solution rich in alkaline elements and ions. On Earth, brines can form when water dissolves salts from rocks and later becomes concentrated through evaporation. Inside an asteroid, similar fluids may develop when ice melts and reacts with minerals. Alkali-rich solutions can reach high pH values, meaning they contain relatively large concentrations of hydroxide ions. Such fluids can attack silicate structures by breaking bonds between silicon and oxygen, mobilizing elements into solution and precipitating secondary minerals. The precise reaction depends on temperature, fluid composition, grain size, porosity and the duration of contact, but even modest alteration over geological timescales can transform the chemical and isotopic record of a microscopic particle.

Bennu’s minerals indicate that liquid water once circulated through its parent body, probably after radioactive heating melted internal ice. The asteroid itself is thought to have formed from fragments of a larger carbonaceous body that experienced this aqueous alteration before being disrupted by impacts. In that setting, fluids could have moved through pores and fractures, reacting with soluble salts and silicate minerals as they travelled. The result would not have been a single uniform chemical event. Instead, different regions and grains could have encountered fluids with changing pH, salinity and oxidation state. A refractory presolar grain located near an active fluid pathway might therefore have been altered much more extensively than a similar grain sealed inside a relatively dry mineral aggregate.

The word “gradual” in the study’s title is crucial. Destruction of stardust in Bennu’s brines would not necessarily resemble a sudden dissolution event. It could proceed through a sequence of surface reactions. A thin outer layer might first lose mobile elements, while the interior retains its original isotopic composition. Continued exposure could roughen the grain, generate pits and cracks, or form a chemically modified rim. Secondary minerals might then grow over the altered surface, physically isolating parts of the grain while also recording the composition of the fluid. At a sufficiently advanced stage, the original particle could be partly or entirely replaced. This stepwise progression matters because researchers examining returned samples may find not only pristine presolar grains but also damaged survivors and mineralogical traces of grains that no longer remain.

Such alteration creates a challenge for interpreting the abundance of stardust in Bennu. If fewer presolar grains are found than expected from the composition of primitive meteorites, the difference may not mean that Bennu formed from material poor in stardust. Some of the grains could have been chemically erased after accretion. Conversely, grains that remain may represent the most resistant mineral types or the portions that were protected from fluid flow. Any estimate of the original presolar inventory must therefore account for selective destruction. The researchers’ focus on alkali brines provides a mechanism by which a body can begin with an ancient stellar record and later preserve only a filtered, incomplete version of it.

The findings also connect Bennu to a broader question in planetary science: how much of an asteroid’s apparent chemical primitiveness is genuinely primordial, and how much is the product of later alteration? Primitive asteroids are not untouched time capsules. They can preserve ancient solids while simultaneously hosting reactions that modify those solids. Water can create new minerals, redistribute elements and alter organic compounds without completely erasing the original parent material. In Bennu, this dual history is especially important because returned samples permit analysis at scales from whole grains to nanometre-thick reaction layers. Electron microscopy, spectroscopy and isotope measurements can reveal whether a particle formed around another star, identify chemical zoning caused by brine exposure and distinguish original material from alteration products.

The work has implications beyond Bennu. Asteroids that contain hydrated minerals and salts may have experienced comparable episodes of internal water circulation. The chemistry of those fluids would have influenced which organic molecules survived, which minerals formed and how efficiently ancient interstellar or circumstellar material was preserved. Understanding brine-driven destruction is also relevant to the interpretation of samples from other carbon-rich bodies, including material delivered by meteorites and future returned-sample missions. Bennu’s microscopic grains are therefore more than isolated curiosities: they are probes of stellar history, asteroid geology and the chemical evolution of the Solar System. By showing that even refractory stardust can be gradually consumed by alkaline fluids, the research underscores a fundamental lesson of planetary science—ancient evidence may endure for billions of years, but it survives only through a continual contest between resistance and alteration.