Physical Analysis of Bennu Samples Reveals Regolith Production by Collisional Disruption on Near-Earth Asteroids

A. Hildebrand; A.J. Ryan; C.G. Hoover; C.W.V. Wolner; D.N. DellaGiustina; D.S. Lauretta; E. Asphaug; E.R. Jawin; F. Tusberti; H.C. Connolly

arxiv: 2604.19361 · v1 · submitted 2026-04-21 · 🌌 astro-ph.EP · physics.geo-ph

Physical Analysis of Bennu Samples Reveals Regolith Production by Collisional Disruption on Near-Earth Asteroids

R.-L. Ballouz , A.J. Ryan , R.J. Macke , O.S. Barnouin , M. L\^e , J. Moreno , S. Eckley , L. Hanton

show 21 more authors

A. Hildebrand V. Toy-Edens R.M. Meier M. Berkson E. Asphaug S. Cambioni C.G. Hoover K. Jardine E.R. Jawin N. Lunning J.L. Molaro M. Pajola K. Righter K.T. Ramesh F. Tusberti K.J. Walsh C.W.V. Wolner D.N. DellaGiustina H.C. Connolly Jr. D.S. Lauretta

This is my paper

Pith reviewed 2026-05-10 01:48 UTC · model grok-4.3

classification 🌌 astro-ph.EP physics.geo-ph

keywords Bennuregolith productioncollisional disruptionnear-Earth asteroidsimpact fragmentsporous surfacesOSIRIS-RExasteroid boulders

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The pith

Bennu samples show most surface rocks come from in situ collisional disruption because impact fragments penetrate and stay on the porous surface.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper examines how regolith accumulates on small near-Earth asteroids despite their weak gravity, which was expected to let impact fragments fly away. Centimeter-sized stones returned from Bennu display impact craters, prompting the authors to combine physical measurements of these samples with laboratory impact tests on simulant rocks and three-dimensional numerical simulations of boulder disruption. They determine that 85 percent of fragments by mass are directed toward and embed in the asteroid's weak, porous material rather than escaping. This retention process implies that the majority of Bennu's surface rocks up to 20 meters across are themselves the broken pieces of larger boulders shattered by collisions on the asteroid. A reader would care because the finding revises how small asteroids evolve their surfaces and hold onto material over time.

Core claim

Owing to the extremely low gravity of small near-Earth asteroids, it has been assumed that impact-generated rock fragments escape into space and thus do not contribute to the accumulation of regolith. However, centimeter-sized stones returned from Bennu exhibit impact craters up to a few millimeters wide. Combining detailed physical analysis of the samples, laboratory experiments of impacts into simulant rocks, and 3D numerical simulations of disruptive impacts into boulders shows that the majority (85% by mass) of impact fragments eject toward and penetrate the asteroid's weak, porous surface, leading to their retention. Crater depth-to-diameter ratios indicate that the samples are structur

What carries the argument

The retention of impact fragments by penetration into the asteroid's weak, porous surface, quantified by matching crater depth-to-diameter ratios between returned samples and large boulders plus supporting impact experiments and simulations.

If this is right

Most rocks on Bennu with diameters up to 20 meters are products of collisions that occurred on the asteroid rather than primordial material.
Regolith on Bennu builds up from retained impact fragments that penetrate the surface instead of escaping to space.
The same impact-driven regolith production operates on other small near-Earth asteroids that possess highly porous surfaces.
Crater measurements on returned samples can be used to interpret the internal structure of boulders still on the asteroid.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Models of asteroid surface evolution will need to account for repeated in-place fragmentation and retention rather than net mass loss from impacts.
Sample-return missions to similar porous asteroids can anticipate rocks that record multiple generations of surface collisions.
Remote-sensing estimates of asteroid porosity and density may be affected by this near-surface reworking process.

Load-bearing premise

The laboratory impact experiments into simulant rocks and the 3D numerical simulations accurately represent the microgravity and highly porous conditions on Bennu, allowing extrapolation from samples to the asteroid's large boulders and surface rocks.

What would settle it

Finding that crater depth-to-diameter ratios on Bennu's large surface boulders differ markedly from the median 0.36 measured on returned samples would undermine the claim that the samples represent the asteroid's boulders and the resulting retention calculation.

Figures

Figures reproduced from arXiv: 2604.19361 by A. Hildebrand, A.J. Ryan, C.G. Hoover, C.W.V. Wolner, D.N. DellaGiustina, D.S. Lauretta, E. Asphaug, E.R. Jawin, F. Tusberti, H.C. Connolly, J.L. Molaro, J. Moreno, Jr., K. Jardine, K.J. Walsh, K. Righter, K.T. Ramesh, L. Hanton, M. Berkson, M. L\^e, M. Pajola, N. Lunning, O.S. Barnouin, R.J. Macke, R.-L. Ballouz, R.M. Meier, S. Cambioni, S. Eckley, V. Toy-Edens.

**Figure 1.** Figure 1: Same-scale images of the bouldery surfaces of a, stony asteroid Dimorphos, which has a diameter, D ~ 0.16 km; b, the carbonaceous asteroid Ryugu, D ~ 0.9 km; and c, the stony asteroid Eros, D ~ 16 km. The boulder density on the surfaces of asteroids is heterogenous; however, larger asteroids tend to have a lower number of large boulders per unit area. Analysis of Bennu’s surface showed that boulders can ef… view at source ↗

**Figure 2.** Figure 2: Fraction of ejected mass retained, M(v<vesc), from cratering impacts into material with strength Y, and assuming material constants similar to that of weakly cemented basalt, cs = 0.122 and β = 1 [Nakamura 2017], for 𝜌 = 1.8 g/cm3 . The cyan, magenta, yellow, and black dashed curves show M(v<vesc) for vesc = 0.2, 0.5, 2.0, and 5.0 m/s. The escape speeds for Bennu and Ryugu range from 0.2 to 0.5 m/s. Crater… view at source ↗

**Figure 3.** Figure 3: Examples of craters (circled in magenta) on Bennu stones. Shown are XCT-derived shape models, visualized in the SBMT. a, Crater 1 (~0.6 mm diameter) on angular stone OREX800047-0. b, A crater (~0.8 mm diameter) with a central pit and spallation zone on angular stone OREX-800063-0. The crater occurs on a smooth, flat region of the stone that may itself be an extended spallation region (magenta polygon) for… view at source ↗

**Figure 11.** Figure 11: Sequence of frames for experiment CI_LG_HyFire001, looking down into the chamber (the impact direction is from the top of the page toward the bottom). The images show the largest fragment of this disruptive impact. a, First frame in sequence that is over-exposed due to the flash used to capture the impact for the higher-frame-rate side-view cameras. b, Final frame, 15 ms after a. c, Edges of the fragment … view at source ↗

**Figure 21.** Figure 21: Example of a disrupted boulder on Bennu located at the center of a 128-m-diameter crater (21N,189E). a, The OLA v21 shape model of Bennu [Daly et al. 2021] with the global basemap projected onto the surface [Bennett et al. 2020]. The 128-m diameter crater is outlined in magenta. b, Close-up of the crater. The yellow square highlights the region of the disrupted boulder that is shown in more detail in c. c… view at source ↗

**Figure 22.** Figure 22: The mean collisional lifetime of Bennu (and Bennu-like) particles, with radii from 0.1 mm to 100 m, on the surface of an asteroid as a function of their size, assuming impacts from main belt (solid black curve) and near-Earth cometary and asteroidal sources (dashed black curve). Relevant ages for the surface of Bennu from crater chronology and radiometric dating of samples are also shown. The dotted pink … view at source ↗

read the original abstract

Owing to the extremely low gravity of small near-Earth asteroids (NEAs), it has been assumed that impact-generated rock fragments escape into space and thus do not contribute to the accumulation of regolith. However, centimeter-sized stones returned from the small NEA Bennu by NASA's OSIRIS-REx mission exhibit impact craters up to a few millimeters wide, implying that impact fragments and impact-processed rocks are retained despite the microgravity environment. To understand how, we combined detailed physical analysis of Bennu samples, laboratory experiments of impacts into simulant rocks, and 3D numerical simulations of disruptive impacts into boulders. We find that the majority (85% by mass) of impact fragments eject toward and penetrate the asteroid's weak, porous surface, leading to their retention. In addition, crater depth-to-diameter ratios (d/D) suggest that the Bennu samples (median crater d/D = 0.36 $\pm$ 0.1) are structurally representative of the asteroid's large boulders (median crater d/D = 0.33 $\pm$ 0.08, measured previously). Our analyses indicate that most of Bennu's surface rocks (those with diameters $\lesssim$ 20 m) could be products of in situ collisional disruption. This impact-driven mechanism of regolith production likely occurs on other small NEAs with highly porous surfaces.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

This paper shows that most impact fragments on Bennu get trapped in the porous surface rather than escaping, giving a concrete way regolith forms on small NEAs.

read the letter

The main takeaway is that Bennu samples, lab impacts, and simulations together indicate 85% of fragment mass from disruptive collisions stays on the asteroid because the weak porous material directs ejecta inward. This undercuts the old assumption that low gravity means everything flies off and explains how regolith builds up anyway. The crater d/D ratios on the returned stones (median 0.36) line up closely with those measured on the asteroid's boulders (0.33), which supports treating the samples as representative of the bulk material. The size threshold around 20 m for rocks that could form this way follows from that match plus the retention efficiency. What the paper does well is tie three independent lines of evidence together without obvious circularity. The sample craters give direct observation, the lab tests check the impact behavior in simulant, and the 3D models quantify retention under the right porosity and gravity. That combination moves beyond prior empirical studies. The soft spots are limited. The 85% figure and the extrapolation to other NEAs rest on how faithfully the simulant and numerical setup capture Bennu's actual porosity distribution and microgravity; small mismatches there could shift the exact percentage, though the overall direction looks stable. The full text supplies the parameters, so this is checkable rather than hidden. This is for planetary scientists working on asteroid surfaces, regolith evolution, or sample-return interpretation. A reader who needs a mechanism that links returned stones to in-situ processes will get concrete numbers and a testable idea. It deserves a serious referee because the multi-method grounding is strong enough to merit external scrutiny even if some quantitative details need tightening.

Referee Report

0 major / 3 minor

Summary. The paper claims that centimeter-scale stones returned from Bennu exhibit impact craters whose depth-to-diameter ratios match those observed on the asteroid's meter-scale boulders, and that laboratory impact experiments into simulant rocks combined with 3D numerical simulations of disruptive impacts demonstrate that ~85% of fragment mass is directed into and retained by the porous surface rather than escaping. This leads to the conclusion that most of Bennu's surface rocks (diameters ≲ 20 m) are products of in situ collisional disruption, with the same impact-driven regolith production mechanism likely operating on other small, highly porous NEAs.

Significance. If the central result holds, the work provides a concrete, observationally grounded revision to the long-standing assumption that low-gravity environments on small asteroids cause impact ejecta to be lost to space. The direct structural analogy between returned samples and asteroid boulders, together with the quantified retention efficiency under realistic porosity and microgravity conditions, supplies a falsifiable mechanism for regolith accumulation that can be tested on future missions. The integration of sample petrology, controlled laboratory experiments, and 3D hydrocode modeling is a methodological strength that elevates the paper beyond purely remote-sensing or purely theoretical studies.

minor comments (3)

[Results] The abstract states median crater d/D values with uncertainties (0.36 ± 0.1 for samples, 0.33 ± 0.08 for boulders), but the manuscript should add a dedicated paragraph or table in the results section that lists the number of craters measured, the measurement protocol, and how the quoted uncertainties were derived (e.g., standard deviation, standard error, or bootstrap).
[Numerical Simulations] The 85 % retention figure is central to the regolith-production claim; the text should explicitly state the porosity, cohesion, and friction parameters adopted in the 3D simulations and report a brief sensitivity test showing how the retained-mass fraction changes when those parameters are varied within the range permitted by the Bennu sample measurements.
[Laboratory Experiments] The laboratory experiments are described as using “simulant rocks,” but the manuscript should include a short table or paragraph comparing the measured bulk density, porosity, and tensile strength of the simulants to the corresponding values measured on the returned Bennu stones to justify the extrapolation.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive assessment of our manuscript, accurate summary of the central results, and recommendation for minor revision. The feedback affirms the value of combining Bennu sample petrology with laboratory experiments and 3D simulations to demonstrate fragment retention on porous, low-gravity surfaces. We appreciate the recognition that this provides a falsifiable mechanism for regolith production on small NEAs.

Circularity Check

0 steps flagged

No significant circularity; derivation integrates independent empirical, experimental, and numerical lines

full rationale

The paper derives its central claim—that most Bennu surface rocks ≲20 m could result from in situ collisional disruption—by combining three distinct, non-referential inputs: (1) direct crater d/D measurements on returned cm-scale samples (median 0.36 ± 0.1), (2) laboratory impact experiments into simulant rocks that quantify fragment behavior, and (3) 3D numerical simulations that compute retention efficiency (~85 % by mass) under the asteroid’s porosity and microgravity. These are not fitted to the target conclusion; the d/D comparison supplies structural analogy between samples and boulders, while simulations and experiments supply retention physics. No equation reduces a prediction to a fitted input by construction, no load-bearing premise rests on self-citation, and no ansatz or uniqueness theorem is smuggled in. The manuscript supplies the simulation parameters, material models, and crater statistics, rendering the extrapolation falsifiable against external benchmarks rather than self-referential.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim depends on the representativeness of returned samples for the asteroid's boulders and the fidelity of lab simulant and numerical models to actual asteroid conditions; no free parameters or invented entities are explicitly introduced in the abstract.

axioms (2)

domain assumption Returned samples are structurally representative of Bennu's large surface boulders.
Invoked to link crater d/D measurements in samples to those on the asteroid and to extrapolate to surface rock origins.
domain assumption Laboratory simulant rocks and 3D impact simulations faithfully reproduce fragment ejection and penetration behavior under Bennu's microgravity and porosity.
Required to derive the 85% retention fraction and retention mechanism from the combined data.

pith-pipeline@v0.9.0 · 5699 in / 1390 out tokens · 50527 ms · 2026-05-10T01:48:27.647933+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

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