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Low, variable cohesion between Bennu regolith particles indicated by atomic force microscopy of returned samples

Atomic force microscopy of pristine regolith particles from asteroid Bennu reveals extremely low interparticle cohesion (~1.25 nN), suggesting that carbonaceous near-Earth asteroids are weakly resistant to rotational disruption and that their surfaces lack millimeter-scale particles due to these minimal cohesive forces.

Original authors: Keanna Jardine, Christian Hoover, Ronald-Louis Ballouz, Anthony Woolson, Paul Sánchez, Jens Biele, Andrew Ryan, Robert Macke, Harold Connolly, Dante Lauretta

Published 2026-09-14

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CC BY 4.0

Original authors: Keanna Jardine, Christian Hoover, Ronald-Louis Ballouz, Anthony Woolson, Paul Sánchez, Jens Biele, Andrew Ryan, Robert Macke, Harold Connolly, Dante Lauretta

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Small, rocky worlds drifting through the inner solar system often look like loose piles of gravel held together by nothing more than their own weak gravity. These are rubble-pile asteroids, bodies so fragile that a gentle spin could theoretically fling their surface stones into space. Yet, many of these asteroids spin surprisingly fast without flying apart, leading scientists to wonder what invisible glue might be holding them together. In the vacuum of space, where there is no air or water to create sticky bridges between grains, the only force capable of acting on tiny dust particles is a subtle attraction known as van der Waals force. This is a weak electromagnetic pull that occurs when atoms on the surface of one particle briefly interact with atoms on another. While this force is negligible for large rocks, it can become the dominant strength for microscopic grains, potentially acting as a microscopic cement that stabilizes the entire asteroid against the centrifugal push of its own rotation.

To understand how strong this cosmic glue really is, researchers turned to a rare gift: dust and pebbles returned from the asteroid Bennu by the OSIRIS-REx spacecraft. Unlike meteorites that have fallen to Earth, which are often altered by our atmosphere and humidity, these samples remained pristine, preserving the exact conditions of their home world. A team of scientists took these returned particles into a specialized, dry laboratory environment and used a highly sensitive instrument called an atomic force microscope to measure the force required to pull two individual grains apart. By attaching a tiny particle to a microscopic needle and bringing it into contact with another particle on a flat surface, they could directly feel the strength of the bond between them. The results revealed that the connection between Bennu's grains is incredibly weak, with an average pull-off force of just 1.25 nanonewtons. This tiny force translates to a tensile strength of 0.001 pascals, a value so small it suggests the asteroid's surface is held together by the bare minimum of attraction possible.

The measurements showed that this weak cohesion is not uniform; it varies depending on the specific shape, texture, and composition of the grains. Some particles stuck together slightly more than others, likely due to differences in their surface roughness or mineral makeup, but the overall picture was one of fragility. When the researchers compared these pristine Bennu samples to similar-looking meteorites found on Earth, they found a stark difference. The meteorites, which had been exposed to Earth's atmosphere and water, exhibited cohesion forces that were two to ten times stronger. This discrepancy suggests that the meteorites we study in labs are not perfect analogs for the actual surfaces of asteroids, as the terrestrial environment has likely altered their chemical properties and made them stickier than they truly are in space.

These findings have profound implications for how we understand the stability of asteroids. Because the cohesive force between Bennu's particles is so low, the asteroid is far more vulnerable to spinning apart than previous models predicted. The data suggests that the weak glue between grains is insufficient to hold together large boulders on a fast-spinning world, which helps explain why Bennu's surface appears to lack many small, millimeter-sized rocks; they are likely too weakly bound to survive the stress of the asteroid's rotation or the impact of micrometeorites. Furthermore, the study indicates that carbon-rich asteroids like Bennu are much less resistant to rotational disruption than their rocky, stony cousins. This difference in material strength may explain why we see fewer of these dark, carbon-rich asteroids close to the Sun, where the heating effects of the sun can accelerate their spin until they break apart. Ultimately, the experiment confirms that the structural integrity of these small worlds relies on a very delicate balance, where the faintest whisper of atomic attraction is the only thing keeping the rubble from scattering into the void.

Technical Summary: Low, Variable Cohesion Between Bennu Regolith Particles

Problem Statement
Small near-Earth asteroids (NEAs) are theorized to be "rubble piles" held together primarily by microgravity and weak interparticle cohesive forces, specifically van der Waals interactions. These forces are critical for explaining the structural integrity of fast-spinning asteroids that would otherwise undergo rotational disruption. However, theoretical models relying on lunar regolith data or terrestrial meteorite analogs may not accurately represent pristine asteroidal materials. A significant gap exists in experimental measurements of particle-particle cohesion using pristine astromaterials that have not been altered by Earth's atmosphere or hydration. Without direct measurements, the magnitude of cohesive strength on carbonaceous asteroids like (101955) Bennu remains uncertain, limiting the ability to model surface evolution, crater formation, and the stability of rubble-pile structures against radiative spin-up (YORP effect).

Methodology
To address this, the authors utilized atomic force microscopy (AFM) to measure nanoscale cohesive forces between micron-scale regolith particles returned by the OSIRIS-REx mission from Bennu.

Sample Preparation: Experiments were conducted in a nitrogen-purged, ultra-high-purity glovebox with relative humidity <1% to eliminate capillary forces from adsorbed water. Both AFM tips and sample substrates were heated to 103°C for 25 minutes to remove terrestrial molecular water before cooling and equilibration.
Experimental Setup: A tip-sample geometry was employed where individual Bennu particles were attached to tipless cantilevers ("tips") and to gold-coated substrates ("samples").
Data Acquisition: Force-distance curves were collected using silicon nitride cantilevers. For 24 unique particle-particle pairs, approximately 100 force-distance curves were measured at various spots on the samples to determine the mean pull-off force (the force required to separate the tip and sample).
Comparative Analysis: The study compared Bennu particle cohesion against three aqueously altered carbonaceous chondrite meteorites (Aguas Zarcas, Tagish Lake, and Tarda) measured under identical environmental conditions.
Analysis: Statistical distributions were analyzed using Kernel Density Estimation (KDE) and box plots. The study also modeled the implications of these forces on particle stability on a rotating asteroid surface, comparing Bennu (C-complex) to S-type asteroids.

Key Results

Cohesive Force Magnitude: The AFM measurements revealed a small average interparticle cohesive force of approximately 1.25 nN for Bennu particles, with a median of 1.15 nN. This corresponds to a calculated tensile strength for Bennu regolith of 0.001 Pa.
Distribution and Variability: The cohesion distribution is mostly unimodal but skewed toward lower forces, with a long tail of higher values. Significant variability was observed between different tip-sample pairings, suggesting that microstructural factors (particle geometry, roughness, and contact area) dominate over intrinsic material properties.
Comparison with Meteorites: Bennu particles exhibited significantly weaker cohesion than the tested carbonaceous meteorites. The meteorites showed mean cohesive forces ranging from ~2.95 nN to ~4.19 nN. Furthermore, Bennu's cohesion is two orders of magnitude lower than values typically reported for stony ordinary chondrites and achondrites.
Size Independence: Within the measured range of effective Feret radii (12–29 μm), no clear correlation was found between particle size and cohesive force. This challenges the standard scaling assumption that cohesive force scales linearly with particle radius.
Surface Stability Modeling: Modeling the force balance on a spinning asteroid indicates that the critical particle radius ( $r_t$ ) below which particles are held by cohesion rather than gravity is approximately 1.5 mm for Bennu (assuming size-independent cohesion). In contrast, an S-type asteroid with higher cohesion would have a critical radius of ~9 mm. Consequently, particles smaller than $r_t$ are roughly 100 times less bound to the surface on Bennu than on an S-type asteroid.

Significance and Claims
The paper argues that the low interparticle cohesive forces measured in pristine Bennu samples have direct implications for understanding asteroid mechanics:

Surface Processes: The weak cohesion explains the observed paucity of millimeter-scale particles on Bennu's surface, as these small particles are easily mobilized or lost due to the low binding energy.
Structural Integrity: Carbonaceous NEAs are likely weakly resistant to rotational disruption. The low cohesive strength suggests that fast-spinning carbonaceous asteroids may not be stable rubble piles, potentially explaining the observed scarcity of low-albedo NEAs in close proximity to the Sun (where spin-up is rapid).
Modeling Corrections: Previous studies that utilized lunar regolith data or terrestrial meteorite analogs likely overestimated the cohesive strength of carbonaceous asteroids by at least an order of magnitude. The authors suggest that standard scaling equations for cohesive force may be inappropriate for asteroidal regolith if the force does not scale with particle size.
Value of Returned Samples: The results highlight that terrestrial meteorites, even carbonaceous ones, may not serve as accurate analogs for the nanoscale cohesion of pristine asteroid regolith due to atmospheric entry effects, terrestrial hydration, and curation differences. Direct measurement of returned samples is essential for accurate hazard mitigation and mission planning.

Technical Summary: Low, Variable Cohesion Between Bennu Regolith Particles

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