In a groundbreaking comparative analysis of extraterrestrial samples, recent studies have unveiled striking mineralogical similarities between the near-Earth asteroids Bennu and Ryugu. Both asteroids have emerged as treasure troves for understanding the early solar system’s chemistry, exhibiting mineral assemblages that echo some of the most primitive materials found on Earth—CI chondrites. This discovery challenges longstanding assumptions about the rarity and preservation of these chemically pristine bodies, offering a fresh perspective on the diversity and evolution of asteroid populations.
At their core, Bennu and Ryugu are dominated by fine-grained phyllosilicates, specifically serpentine and saponite. These hydrous minerals reveal a history marked by extensive aqueous alteration processes, whereby water interacted with primordial rock material to foster the formation of sheet silicates. Samples from both bodies showcase comparable quantities and compositions of secondary minerals such as sulfides, magnetite, and carbonate phases. Remarkably, the mineralogy suggests that these objects underwent similar hydrothermal alterations, stitching a coherent narrative about water-driven chemical evolution across these distinct near-Earth objects.
The fine assemblage of these minerals is not limited to the aforementioned major phases. Both Bennu and Ryugu host an array of minor and trace minerals, along with late-stage, sodium-rich phases that imply evaporation processes at the end of fluid activity. This nuanced mineralogy, varying slightly between the two sample sets, likely comes down to the limited mass returned and studied—nevertheless, the overarching similarity cannot be overstated. Intriguingly, calcium-aluminum-rich inclusions (CAIs) and chondrules—typically hallmark components of many chondritic meteorites—are rare or sparse in Ryugu samples, a characteristic they share with Bennu.
Such mineralogical cohesion prompts direct comparisons with CI carbonaceous chondrites, a class of meteorites distinguished by their high content of volatile and chemically pristine material. CI chondrites, exemplified by the well-studied Orgueil meteorite, are also dominated by Mg-rich serpentine and saponite, closely resembling the silicate structure and composition found in Bennu. Moreover, both Bennu and Ryugu samples lack abundant CAIs and chondrules, consistent with observations in CI meteorites. These data consolidate the link between these near-Earth asteroids and one of the rarest and most chemically primitive classes of meteorites on Earth.
A notable departure in composition arises in the prevalence of calcium sulfates and iron oxyhydroxides like ferrihydrite within CI chondrites—minerals conspicuously absent from Bennu and Ryugu. This distinction likely highlights the pervasive effects of terrestrial weathering on meteorites retrieved on Earth. Sulfates and ferrihydrites in CI chondrites are interpreted to be products of oxidative alteration of sulfides after their fall, underscoring how Earth’s environment can mask original mineralogical signatures. Conversely, the pristine sample return missions allow scientists to study these volatile-rich materials in an untouched state, free from terrestrial degradation.
The scarcity of CI chondrites in Earth’s collection—only six unheated falls totaling approximately 21 kilograms—reflects a profound sampling bias. The similarity between returned asteroid samples and CI chondrites implies that chemically primitive and volatile-rich bodies may be far more common in the near-Earth asteroid population than previously realized. This revelation suggests that the terrestrial meteorite record fails to capture the true diversity of asteroid materials orbiting near Earth, primarily due to atmospheric survival biases and fragmentary break-up dynamics of parent asteroids in the main belt.
Further complicating terrestrial meteorite sampling is the fragile nature of these primitive materials. Both Bennu and Ryugu exhibit low thermal inertia regions indicative of highly friable material that likely disintegrates upon atmospheric entry, preventing substantial quantities from reaching the surface intact. This fragility provides a rationale for the underrepresentation of CI-like meteorites in collections and underscores the value of space missions that circumvent Earth’s atmospheric filter. Spaceflight and sample return present an unparalleled opportunity to access pristine, minimally altered materials directly from these bodies.
The mineralogy unveiled by these sample analyses provides compelling evidence for the hydrothermal histories of Bennu and Ryugu. Water-rock interactions early in the solar system’s timeline reconfigured primary minerals while accelerating the formation of secondary phases such as sulfides, magnetite, carbonates, and phyllosilicates. These processes document aqueous alteration scenarios that are remarkably consistent across different asteroid sources, offering a fresh window into the conditions that predate planetary accretion and differentiation.
The presence of late-stage sodium-rich phases formed by evaporation denotes a complex and evolving fluid chemistry within these asteroids. Such signatures evoke scenarios where localized heating events or transient fluid reservoirs led to the concentration and precipitation of sodium-bearing minerals. Decoding the genesis of these mineral phases helps reconstruct the thermal and chemical environments encountered by these asteroids, providing clues to the prebiotic chemistry that may have operated on their surfaces or interiors.
Geochemical and structural comparisons between Bennu’s sheet silicates and those found in the Orgueil CI meteorite underscore the importance of this type of sample return science. The uncanny mineralogical fidelity supports existing hypotheses about the initial building blocks of our solar system, placing Bennu and Ryugu as living museums preserving the earliest alteration narratives experienced by carbonaceous asteroids. This evidence enriches our understanding of volatile transport, aqueous alteration intensity, and the distribution of organic and inorganic phases on small bodies.
The discoveries challenge the traditional perception of carbonaceous chondrites merely as cold relics. Instead, they indicate dynamic aqueous processes influenced mineralogy ensuing the initial accretion epoch. An improved understanding of the interplay between fluid chemistry, mineral formation, and subsequent evaporation reveals complex geochemical cycling absent in previously studied meteorites, positioning Bennu and Ryugu as key benchmarks for identifying hydrothermal pathways on small bodies.
The remarkable similarity of Bennu and Ryugu’s mineralogy with CI chondrites—and the joint absence of terrestrial weathering products—reinforces the importance of controlled sample curation in space missions. Unlike meteorites on Earth that inevitably undergo oxidation and contamination, these pristine samples enable high-fidelity constraints on early solar system alteration products. Such constraints are crucial for modeling elemental fractionation, volatile retention, and the timing of hydrothermal alteration on asteroid parent bodies.
The implications of these findings extend beyond mineralogy. Understanding Bennu and Ryugu’s aqueous alteration histories informs us about the availability of water and possibly organic material in the inner solar system, shedding light on the preconditions for life’s molecular precursors. These data provide essential input for models proposing that carbonaceous asteroids may have delivered essential volatiles and organics to early Earth, contributing to habitability and biochemical evolution.
Moreover, the evidence of widespread aqueous alteration and the formation of secondary minerals such as magnetite and carbonates informs models of thermal evolution for small bodies. These processes require limited but significant internal heating and fluid mobilization, pointing to a nuanced thermal history that challenges binary views of asteroids as either entirely primitive or thermally metamorphosed. In fact, an intermediate pathway dominated by low-grade alteration appears preferable for explaining the observed mineralogy.
From a planetary defense and resource utilization perspective, the friable and volatile-rich nature of these asteroids informs mission design and hazard assessment. Their fragility complicates predictions of impact outcomes and surface interactions but simultaneously positions them as promising reservoirs of water and other volatiles for in-space resource exploitation. Detailed mineralogical and geochemical characterizations thus assist in prioritizing asteroids for future exploration and utilization.
In conclusion, the mineralogical parallels between Bennu, Ryugu, and CI chondrites offer profound insights into the early solar system’s aqueous alteration environments, volatile distribution, and chemical evolution pathways. These findings reshape our understanding of the composition and diversity of near-Earth asteroids, revealing a hidden abundance of chemically primitive, volatile-rich material previously masked by terrestrial biases. Sample return missions thus play a pivotal role in uncovering the true nature of the small bodies that orbit our planet, holding keys to understanding the origins of water, organics, and ultimately life itself.
Subject of Research: Mineralogical and geochemical characterization of Bennu and Ryugu asteroid samples with emphasis on hydrothermal alteration processes.
Article Title: Mineralogical evidence for hydrothermal alteration of Bennu samples.
Article References:
Zega, T.J., McCoy, T.J., Russell, S.S. et al. Mineralogical evidence for hydrothermal alteration of Bennu samples. Nat. Geosci. (2025). https://doi.org/10.1038/s41561-025-01741-0
Image Credits: AI Generated
