Mechanical Properties of Asteroid Regoliths: Role of Grain Size and Frost on Strength and Cohesion

Comets and asteroids are remnants of the planetesimal stage of the formation of the Solar System, marking them as information-rich objects of study. While comets and asteroids are commonly described as distinct entities, with comets formed in an ice-rich environment in the outer Solar System and asteroids formed in the warmer inner regions, some asteroids have been observed to exhibit cometary activity [1]⁠. These “active asteroids” are classified as asteroids because their dynamical and spectral characteristics clearly tie them to asteroid taxonomy classes, yet they display activity in the form of particle loss at rates ranging from 10^-2 to 10^-4 kg/s. By studying the mechanical properties of surface and subsurface regoliths, we are investigating rotational disruption as a possible origin for the dust ejection activity observed on a number of these asteroids, for which other dust production mechanisms were ruled out. We use a combination of laboratory measurements on regolith simulants and numerical simulations to determine the influence of grain size and the presence of water ice frost on asteroid surface properties. The simulants used are composed of a base powder mimicking the CI Orgueil meteoritic composition (UCF/DSI-CI-2 [2]⁠), prepared into grains of three size distributions (0.1, 1, and 10 mm, respectively). The samples are then either tested at room temperature or in cryogenic conditions with an induced frost layer forming at the grain surfaces. Measurements include compression and shear strength, which yield the material’s Young Modulus, angle of internal friction, and bulk cohesion. We also measure the angle of repose of the various samples. In order to better constrain inter-grain properties from these bulk laboratory measurements, and as a first step towards scaling to asteroid sizes, we have reproduced our laboratory measurements using a Soft-Sphere Discrete Element Method (SSDEM) [3]⁠, implemented as a computational code to simulate a granular aggregate [4]⁠. We present the measured and derived mechanical properties of our asteroid surface regolith simulants and their dependence on grain size and frost inclusion. We will also discuss preliminary conclusions on the presence of a fine grain population or water ice in the surface or subsurface of asteroids and comets.

1. D. Jewitt, H. Hsieh, J. Agarwal, others, Asteroids IV (2015) 221-241.

2. S.D. Covey, J.S. Lewis, P.T. Metzger, D.T. Britt, S.E. Wiggins, in: Earth Sp. Conf., 2016, pp. 11-15.

3. P.A. Cundall, R.D. Hart, Eng. Comput (1992).

4. D.P. Sánchez, D.J. Scheeres, Icarus 218 (2012) 876-894.