Mechanical properties of Enceladus’ surface analogues

The Kronian moon Enceladus harbours a liquid ocean beneath its icy crust, making it of particular scientific interest. Data collected by the Cassini orbiter suggest the presence of the chemicals necessary to develop life as we know it, making it a key driver behind ESA’s Voyage 2050 programme. Libration-enhanced tidal heating keeps the moon’s interior sufficiently warm to sustain its liquid ocean (Hemingway et al., 2018). The icy crust thins at its southern pole, where fissures, dubbed the ‘Tiger Stripes’, jet water from the liquid ocean to the planet's surface, akin to geysers (C. C. Porco et al., 2006; Hemingway and Mittal, 2017). The jetted liquid water expands into droplets and freezes as it experiences the vacuum of space. The water-ice grains are distributed; the heavier grains fall back to the moon’s surface, while the lighter ones escape Enceladus and feed Saturn’s E-ring (Postberg et al., 2018). Hence, Enceladus’ surface undergoes a continuous renewal from water-ice grain depositions either from direct plume deposition or E-ring bombardment (Southworth et al., 2018). Near the plume’s source, the deposition could amount to 1mm/year with particle sizes ranging from 0.6 to 15 micrometre (Spitale et al., 2015).

These water-ice grains are unconsolidated and undergo a consolidation process over long periods of time (Blackford, 2007; Molaro et al., 2019), called sintering, which largely follows the processes described by Swinkels and Ashby (1981). Initially, two grains are brought into contact, followed by a neck-growth stage driven by multiple mechanisms until the grains become difficult to distinguish over time. The pores become isolated and closed off as they shrink over a longer period, resulting in the densification of the bulk, see figure 1.

Figure 1: The blue circles represent water-ice grains sintering together over time. The initial stage highlights an initially small neck formation, which grows in stage 1, through two highlighted transport processes until the pores coalesce in the final stage.

The grain size and local temperature significantly affect the sintering time. The average surface temperature of 72K prevents significant sintering over the geological timescale of the moon (Molaro et al., 2019). However, the elevated temperatures near the plumes would allow micron-sized crystalline water-ice grains to sinter over relatively short geological timescales (~15 years) and withstand pressures on the order of ten megapascals, indicating that Enceladus’ surface may exhibit locally different mechanical properties (Southworth et al., 2015; Choukroun et al., 2020). Furthermore, the porosity of the bulk material affects the bulk thermal properties through conduction and radiation, affecting the sintering rates between the grains (Ferrari and Lucas, 2016).

To understand the sintering processes and determine the mechanical properties of Enceladus’ surface, laboratory experiments have been used to study water-ice produced in low-temperature environments. Results indicate that icy granular surfaces are mechanically weaker at low temperature than at warmer temperatures (Gundlach et al., 2018; Molaro et al., 2019; Choukroun et al., 2020; van Veen, 2025; Fabbretti, 2026). Vapour transport is the dominant consolidation mechanism in early sintering stages, while surface diffusion may become dominant at much lower temperatures. In vacuum conditions, Fabbretti (2026) found an increased sintering rate between water-ice grains compared to the atmospheric findings of van Veen (2025). This indicates that the sintering behaviour of water-ice grains operates differently in the absence of an atmosphere.

This study aims to understand the mechanical properties of Enceladus’ surface analogues as well as low-temperature and low-pressure sintering of water-ice grains. We performed experiments in our PISCES (Plumes and Ices Simulation Chamber for Enceladus and other moonS) to investigate the sintering behaviour of water-ice grains under vacuum conditions. This novel experimental chamber is used to reach sub-mbar pressures and low temperatures (Bourgeois and Cazaux, 2025). Fine grains are produced through direct spray into the chamber or outside using liquid nitrogen, and are exposed to a near vacuum where they are mechanically evaluated. A penetrometer setup is used to measure the cone penetration resistance of the bulk material in a vacuum with respect to the grain size, porosity and sintering time. The L4 mission is in the works with respect to ESA’s Voyage 2050 programme. It is set to explore the Kronian moons with an orbiter and a lander (Helbert et al., 2025). Understanding the surface’s mechanical properties will help find an adequate landing site for the mission.