In an extraordinary advancement for planetary science, freshly returned samples from the asteroid Bennu have unveiled groundbreaking insights into space weathering processes that reshape our understanding of how airless bodies evolve under solar system conditions. These revelations come as a pivotal contrast to decades of remote sensing data and laboratory analogues, providing a rare window into the microstructural and chemical transformations that occur on carbonaceous asteroids over time. The implications extend beyond Bennu itself, potentially offering new paradigms for deciphering the spectral signatures of other sulfur-rich, airless rocky objects such as Mercury.
For years, the scientific community has relied heavily on spacecraft observations and laboratory simulations to infer weathering rates and spectral changes on asteroid surfaces. Space weathering is a set of alteration processes driven primarily by solar wind irradiation and micrometeorite bombardment, which modify the optical, chemical, and physical properties of regolith materials. Traditionally, models based on orbital spectrometry suggested that the principal changes on Bennu’s surface happen over timescales on the order of 100,000 years. However, precise isotopic and structural analyses of individual particles returned by the OSIRIS-REx mission indicate that these transformations may, in fact, progress an order of magnitude faster than previously assumed.
A particularly striking revelation emerges from the spin exposure ages (SEP), which gauge the duration that individual particles have been exposed to the space environment at Bennu’s surface. Analysis shows that certain particles have only been weathering for about ten thousand years—vastly shorter than the tentative estimates made from spacecraft spectral data. This accelerated timescale necessitates a reconsideration of how surface renewal processes and regolith turnover occur on such small bodies, hinting at more dynamic and possibly episodic resurfacing mechanisms than the gradual steady-state erosion generally considered.
One of the more enigmatic aspects of Bennu, often highlighted in spectral data yet now better understood through laboratory investigation, is its distinctive surface reflectance evolution. Unlike the Moon or ordinary chondrite asteroids which tend to darken and redden with space weathering, Bennu intriguingly becomes brighter and exhibits a “bluer” spectral slope over time. This behavior challenges classical paradigms and raises fundamental questions about the compositional drivers behind these trends.
Close examination of Bennu’s mineralogical inventory revealed the presence of hydrated amorphous magnesium-sodium phosphate phases. Comparable materials retrieved from Ryugu, another near-Earth carbonaceous asteroid explored by the Hayabusa2 mission, show a consistent bluing effect across visible wavelengths. This similarity strongly supports the notion that these phosphates contribute significantly to the distinct optical properties observed in both asteroids and may serve as key indicators of aqueous alteration histories as well as subsequent surface exposure regimes.
Laboratory experiments with terrestrial analogues have added layers of nuance to interpreting these spectral phenomena. The observed bluing in reflectance is often linked to fine-grained, optically opaque components embedded within the host minerals. These components include carbonaceous matter, various sulfides, and iron oxides such as magnetite. Spectral modeling has elucidated how these nano- and micro-scale opaque inclusions scatter and absorb light, thereby modifying the overall spectral reflectance characteristics in subtle but measurable ways.
A standout finding from the Bennu samples involves melt deposits capping many particles. Within these thin layers lie abundant nano-phase and micro-phase inclusions composed chiefly of FeNi metals and FeNi sulfides. The presence of these nano-inclusions is critical: spectral simulations show that troilite (FeS) inclusions larger than approximately 40 nanometers effectively induce a bluing effect across the visible to near-infrared wavelengths. This mechanism provides a robust explanation for the observed spectral trends and shifts attention away from the long-presumed dominance of nano-phase metallic iron, traditionally thought to govern space weathering effects on silicate bodies.
This paradigm shift in attributing spectral evolution to sulfide inclusions rather than solely nano-phase Fe metal bears profound implications. It suggests a reevaluation of space weathering models for carbonaceous asteroids—bodies historically underrepresented in weathering studies dominated by lunar analogues and ordinary chondrites. The findings underscore the critical role that sulfur chemistry and sulfide mineralogy play in controlling surface optical properties under solar wind exposure and micrometeorite impacts.
The implications ripple outward, offering new perspectives on spectral datasets gathered by telescopes and spacecraft over decades. For instance, Bennu’s surface color transformation, once puzzling in its departure from lunar trends, now gains a coherent theoretical framework grounded in its unique sulfide-rich mineralogy. By extension, the insights gained from Bennu provide a template for interpreting the remote sensing observations of other small bodies with similar compositions and surface processes.
The study also suggests that sulfur-enriched bodies such as Mercury might experience analogous weathering pathways, where nano- and microscale sulfide inclusions significantly modify optical properties. Considering Mercury’s harsh space weathering environment and known sulfur inventory, this work invites renewed investigation into the planet’s surface alteration mechanisms, potentially revising assumptions about its spectral and compositional heterogeneity.
From a broader geoscience standpoint, the Bennu samples underscore the efficiency and subtlety with which solar wind ions and micrometeorite impacts induce changes on airless objects. These processes not only remodel surface chemistry but also alter microstructural textures at nanometric scales, influencing magnetic, spectral, and mechanical properties. Such detailed understanding enriches models of regolith evolution across countless bodies in the solar system.
The findings also highlight the invaluable role of sample-return missions in bridging the gap between remote observations and direct laboratory analyses. Access to pristine material from Bennu offers unparalleled opportunities to calibrate remote sensing data more accurately, refine models of space weathering, and identify hitherto unrecognized contributors to spectral variability. This sets an inspiring precedent for future missions targeting other asteroid types and planetary surfaces.
Moreover, the recognition that space weathering effects occur over significantly shortened timescales suggests more rapid cycling of surface materials, implicating dynamic surface processes such as landslides, seismic shaking induced by impacts, or thermal fracturing. These mechanisms continually refresh the regolith, exposing less altered material and maintaining spectral and chemical heterogeneity on asteroidal surfaces.
In conclusion, the Bennu samples invite a profound rethinking of how carbonaceous bodies weather in space. The revelation that sulfide inclusions—not simply nano-phase Fe metal—mediate spectral bluing reshapes the conceptual framework for interpreting asteroid spectra. The accelerated weathering timeline challenges long-held assumptions about regolith aging, urging closer study of asteroid surface dynamics. Collectively, these insights deepen our comprehension of the solar system’s evolutionary narrative and highlight the continuing surprises awaiting in the study of small body surfaces.
As ongoing analyses progress, the scientific community eagerly anticipates further revelations that will articulate the complex interplay of compositional, structural, and environmental factors sculpting the surfaces of asteroids and other airless worlds. With every particle scrutinized, we edge closer to unravelling the intricate processes that have shaped planetary materials since the solar system’s infancy.
Subject of Research: Space weathering effects and timescales on the surface of asteroid Bennu, including microstructural and chemical sources linked to spectral characteristics.
Article Title: Space weathering effects in Bennu asteroid samples.
Article References:
Keller, L.P., Thompson, M.S., Seifert, L.B. et al. Space weathering effects in Bennu asteroid samples. Nat. Geosci. (2025). https://doi.org/10.1038/s41561-025-01745-w
Image Credits: AI Generated
