Surface Activity on Small Bodies and Regolith Mechanical Properties

Small bodies in the Solar System, such as comets, Centaurs, 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. These “active asteroids” have orbits prohibitive of surface water ice surviving and generating the mass loss. In addition, a number of Centaurs also display activity despite their very large perihelion distance, which also excludes water ice sublimation as an activity mechanism.

The presented work investigates surface failure as a possible origin for the dust ejection activity observed on these bodies. Such failures can happen due to rotational disruption or small instabilities in layered surfaces with different mechanical properties (e.g. cohesion). Numerical simulations have shown that surface mass shedding occurs preferentially when a lower-cohesion layer covers a higher-cohesion sublayer of regolith. The goal of our laboratory measurements is to quantify how the presence of fine grains or water ice (either as frost or as grains) in the regolith samples affects the mechanical properties of the granular material, in particular strength and cohesion.

Laboratory experiments are performed with icy regolith and include compression and shear measurements in various environmental conditions: vacuum, cryogenic temperatures (<150 K), microgravity, and combinations thereof. The tested material is composed of asteroid regolith simulant (CI), mixed with water ice particles at various mixing ratios ranging from 5 to 100% water ice. The grain size distributions range from ~0.1 to 10 mm.

We will discuss the implications of our results for the potential of regolith failure as a source of surface activity on small bodies of the Solar System.