Characterizing the frictional properties of Phobos’ regolith using theIDEFIX WheelCams

Introduction: The JAXA Martian Moons Exploration (MMX) mission [2] will deploy the French-German rover Idefix (Fig. 1) to the surface of Phobos [3]. The Idefix rover will act as a scout for the main goal of the mission: perform a sample return from the surface of Phobos. This will also mark the first attempt at wheeled-locomotion in a low-gravity environment, offering the opportunity to investigate both the surface of Phobos and the behaviour of regolith at the surface of small bodies [2; 10]. Idefix, the WheelCams and its science objectives: The MMX rover IDEFIX is tasked to provide data on the regolith properties thanks to scientific instruments that will deliver high-resolution images, measurements of the thermal properties, and Raman  spectroscopy, respectively thanks to the NavCams and WheelCams cameras, the miniRAD radiometer and the Rax raman spectrometer [3]. Our main interest is the WheelCams instrument, which will aim at the wheels of the rover and capture wheel-regolith interactions. These images will be used to characterize the properties of the regolith such as the size distribution and the morphological parameters [4]. The goal is to thus infer parameters such as the angle of internal friction, in order to estimate the strength of Phobos’ regolith.

Figure 1: The MMX rover Idefix. a) A photo of the delivered MMX rover IDEFIX in the clean room (Credit: JAXA). b) CAD of IDEFIX in the on-surface configuration. Field of views of miniRAD and WheelCAMs are displayed in yellow and red respectively (Credit: CNES, from Michel et al. (2022)).

Morphological parameters: A pipeline has been developed to quantitively analyse such parameters of individual grains [5]. However, waiting for the MMX mission to launch, this pipeline has been tested and approved on images from other small bodies and especially from Dimorphos (Fig. 2), the target of the NASA DART mission [1]. It was the first detailed morphological analysis of these bodies, allowing the bulk internal friction angle of the boulders at the surface of Dimorphos to be constrained (Fig. 2) and providing insights to our understanding of the formation of this binary asteroid.

Figure 2: (Left) Boxplots of the internal friction angles values from the boulders >30 px analysed on the images of Dimorphos, Itokawa, Ryugu and Bennu and the average and standard deviation amongst the 4 asteroids. (Right) last image taken of Dimorphos before impact with the DART spacecraft. Boulders selected and analysed are coloured in blue and red. The red boulders indicate the smaller resolved boulders (<30 px). Figures from Robin et al. (2024).

A digital shear box for material characterisation: In order to further our understanding on the behaviour of granular material at the surface of small bodies, we also perform Discrete Element Method simulations with the Project Chrono code[9; 8] (Fig. 3) Wheel simulations have been previously performed to understand the sinking and driving behaviours of an MMX rover wheel in different simulated granular materials [7]. In Sunday (2022) the sinking and driving behaviour of the IDEFIX rover was studied for different values of microscopic (particle-particle) friction and cohesion. However, it is also important to understand how the sinking and driving vary with the bulk properties, as it is the bulk properties that will be more readily inferred in-situ. Therefore, to quantify the macroscopic parameters, we perform shear tests, similar to laboratory experiments (Fig. 3), in order to determine the internal friction angle and the bulk cohesion of the studied granular materials.

Figure 3: Visualisation of the stress during shear tests simulations of frictional particles without cohesion. (Left) The granular material before shear. (Right) The same simulation after a displacement of 10% of the box. We can see the shear band, where most of the shear strain is located.

We performed simulations for a low friction material (μ = 0.16), a high friction material (μ = 0.4), and a high friction material with cohesion (μ = 0.4 , C = 68.9 mN at Earth gravity and C = 0.04 mN at Phobos gravity). In addition, we also used the most likely Phobos-regolith parameters a defined in the MMX ERD (μ = 0.4, C = 0.02 mN at both Earth and Phobos gravities). By running several simulations with different normal loads and retrieving the maximal shear stress endured by the granular material, after a displacement of 10% of the box, we can use the Mohr-Coulomb law to infer the internal friction angle and the bulk cohesion of the material (Fig. 4).

Figure 4: (Left) Shear stress on the pushing wall as a function of the shear box displacement. The value of shear stress for a given load at a 10 mm (i.e. 10%) displacement corresponds to a data point in the Mohr-Coulomb plot. (Right) Shear stress plotted as a function of normal stress for rough glass beads. An internal friction angle of 23.15° is measured with a bulk cohesion of 1.7 kPa, which are expected results for this material in this range of normal stress [6; 11].

 

Conclusion: These shear box simulations will allow macroscopic characterization of frictional and cohesive properties based on a specified set of microscopic parameters. The results presented at this conference will then be the framework to study sinkage and driving performance as a function of the macroscopic properties.