On 24 September 2023, a capsule landed in Utah containing samples from the OSIRIS-REx mission to asteroid 101955 Bennu. It yielded 121.6 grams of rock, the largest mass returned from an asteroid to date. Curators and scientists in labs around the world have been carefully preparing and analysing the material since its return. In this issue of Nature Geoscience, we include articles reporting some of the first results obtained from the returned material, highlighting the mineralogical changes Bennu has experienced since its formation more than 4.5 billion years ago.

Credit: NASA/Erika Blumenfeld and Joseph Aebersold

OSIRIS-REx followed the Japan Aerospace Exploration Agency (JAXA) Hayabusa-2 mission that returned material from asteroid 162173 Ryugu in 2020. Both missions sampled some of the Solar System’s most primitive materials, similar to carbonaceous chondrite meteorites. These are water-rich aggregates of dust and mineral grains that also contain organic carbon compounds and didn’t undergo extensive heating or melting on their precursor — or parent — asteroids. The rare CI (Ivuna-type) carbonaceous chondrites are the most chemically primitive of these, with elemental compositions closely matching the Sun.

Sample return missions from primitive asteroids are critical to unlocking the formation and subsequent modification of materials comprising our early Solar System, as highlighted in previous Nature Geoscience editorials1,2. Compared to meteorites collected on Earth, material sampled in situ and delivered by return missions is not altered by entry through the atmosphere or terrestrial weathering. Furthermore, while the spacecraft has an array of sensors for characterizing the surface of the asteroid, the high-precision analyses of tiny minerals and organic molecules needed to unravel the origins of asteroids can only be made in Earth-based laboratories.

The results published so far have been intriguing. McCoy and colleagues3 reported evidence for salt minerals formed during the evaporation of a brine during the early evolution of Bennu’s parent body, before it was broken apart by a later impact, raising the possibility that early asteroids were locations for prebiotic organic synthesis. Meanwhile, Glavin and colleagues4 analysed organic molecules in Bennu material, detecting a range of compounds including amino acids, amines, and polycyclic aromatic hydrocarbons. They point out that these were formed and altered by low-temperature reactions on the parent asteroid, possibly in ammonia-rich fluids. Together, these studies suggest Bennu was fluid-rich shortly after it was formed.

Writing in this issue, Zega and colleagues look in more detail at the fluids circulating on Bennu’s parent asteroid. Using mineralogical evidence, they show how the composition of the fluid responsible for alteration may have changed with time, highlighting similarities between the inferred temperature conditions of alteration on Bennu, Ryugu and CI chondrite meteorites.

Space weathering of minerals exposed on the surface of Bennu was also found to be extensive, with Keller and colleagues finding that the asteroid’s surface is a dynamic environment. Combining their results with those from Ryugu, they infer a larger role for micrometeorite impacts in space weathering than previously suggested, and highlight that space weathering on carbonaceous bodies is fundamentally different from that occurring on the Moon or other asteroid types.

Overall, it appears that the chemical composition of Bennu is strikingly similar to the asteroid Ryugu5 — despite the two bodies appearing very different on the surface — as well as CI chondrites. This suggests that this material is more common than its representation in meteorite collections on the Earth implies, and the possible implications of this finding are discussed in a Comment by Takaaki Noguchi. Meanwhile, our sister journal Nature Astronomy recently published an article on the sources of the materials accreted to Bennu’s parent body in the early Solar System. Here, Barnes et al. find differences between the Bennu, Ryugu and CI-chondrite materials that point to compositional heterogeneity in the Solar System region where these bodies formed.

Together, these studies showcase the value of sample return missions — and the careful mineral-scale analyses they enable — for understanding the asteroids which ultimately contributed to building the Earth. Finally, we also highlight a recent image that witnesses how solar systems like our own were born, showing the condensation of the very first solid minerals from the gas that surrounds a newborn star. As Tamara Goldin notes, this image — and the minerals contained in early-formed asteroids like Bennu — capture the very beginnings of geoscience.