Manufacturing and Characterization of Icy Simulants for Europa

In the exploration of icy ocean worlds, including Enceladus, Europa, and Titan, the absence of information related to material properties of the surface requires that sampling subsystems possess a robust capability to interact with a broad variety of potential icy terrain scenarios. To aid in the development of sampling subsystems that can navigate the challenges of surface excavation, sample collection, and sample delivery in a variety of terrain conditions, we aim to reproducibly generate plausible solid icy simulants with a range of mechanical properties at a scale relevant to the activities of sampling tools testbeds. This paper presents a simple method for relatively large-scale (kg/hr) manufacturing of granular ice based on a flash-freezing process that enables entrapment of additives within the ice particulates at the micron-scale. Employing additives based on magnesium sulfate (MgSO
4
) as an exemplary system applicable to Europa, we explore the impact of post-manufacturing densification parameters on granular ice mechanical properties. Simulants are characterized via porosity, concentration uniformity, qualitative assessment of particle characteristics, Raman spectroscopy, and mechanical properties testing (Shore A hardness and 3-point-bend). Our granular ice manufacturing process was found to produce roughly 17 kg of granular ice with MgSO
4
-based additives in a 2–3 hour time span. The concentration of MgSO
4
was uniform at the gram-scale, and average particle diameters of granular ice as-manufactured ranged from 169 μm - 317μm. Raman spectroscopy indicated the formation of a glassy phase of MgSO
4
hydrate when freshly frozen, and in consistency with literature, storage at low temperatures and subsequent (unintentional) thermal cycling resulted in the formation of epsomite (MgSO
4
•7H
2
O) (MS7) and mixed MS7+meridianiite (MgSO
4
•11H
2
O) (MS11) phases, respectively. Densification was accomplished using both small (commissioning) and large (scale of sampling activities) press molds. A test matrix generated via Design of Experiments (DoE) was used to explore the impact of time (7 days and 21 days), temperature (−10°C, −40°C, −80°C), pressure (1 MPa and 27 MPa), and MgSO
4
concentration (0.0 wt.%, 2.3 wt.%, and 16.0 wt.%); simulants with a range of spatially uniform mechanical properties (1–72 Shore A hardness, and 39–51% porosity) were produced. Images of densified particles revealed that melting was likely the primary mechanism of densification, and MgSO
4
phase characteristics suggested that formation of MS11 during densification was promoted by high MgSO
4
concentrations and high temperatures. Temperature had the highest impact on Shore A hardness. Future work aims to broaden the range of accessible simulant mechanical properties by varying granular ice particle diameter and expanding the densification parameter space. Ultimately, a range of densified samples will be tested with sampling tools and systems to understand the unique ways in which various simulant mechanical properties pose unique sample excavation, collection, and delivery challenges.