Grain Shape and Size Reveal Why Asteroid Bennu's Surface Is So Weak

Many small asteroids, including Bennu, Ryugu and Itokawa, are not solid rocks but loose piles of dust, rock and boulders held together mainly by their own gravity and weak cohesive forces between grains. Researchers have studied how these "rubble-pile" asteroids hold together for more than a decade, but earlier simulations could only treat the fine grains as perfect spheres, even though real grains are angular and irregular. A new study in Nature Communications, led by Paul Sanchez of the University of Colorado Boulder, built a framework that accounts for realistic particle shapes.

The team modeled a small piece of an asteroid rather than a whole body: a matrix of fine, cohesive grains packed between two one-meter boulders, forming a "granular bridge." Pulling the boulders apart until the bridge broke gave a measure of tensile strength. Researchers ran 78 simulations using grains that ranged from perfect spheres to 10 different angular, elongated or flattened shapes, with sizes from 2 to 5 centimeters. Both gravity and the weak attractive force called van der Waals attraction between particles were built into the model.

The simulations showed that strength depends on two things: how big the grains are, and how far their shape departs from a perfect sphere. Smaller grains pack more tightly and create more contact points, making stronger bridges. Angular grains can also touch along edges or faces rather than a single point, which has a similar strengthening effect. Sanchez said either measurement alone, grain size or grain shape, is enough to predict an asteroid's strength, which is what makes the framework general.

Applying the framework to Bennu, whose surface grain properties were measured after NASA's OSIRIS-REx mission returned samples from the asteroid in 2023, the researchers found its surface material is about 50 times weaker than ground coffee. The work builds on research Sanchez and colleagues began in 2010, when they proposed that van der Waals forces between fine grains could hold small asteroids together even where gravity alone is too weak.