Spectral Signatures of Resurfacing Mechanisms on Europa and Enceladus: A Laboratory Approach

Europa and Enceladus are ocean worlds that likely harbour liquid water beneath an ice shell. Both moons show evidence of active geological exchange between their interiors and their surfaces (Nimmo and Pappalardo, 2016). The ice shell forms the interface between the ocean and the surface, and the processes that shape it can carry information about the state of the interior and the composition of the ocean. Near-infrared reflectance spectroscopy of the surface ice could offer a remote window into these processes. Existing datasets from Cassini/VIMS, Galileo/NIMS, and JWST provide observations of the surfaces of these moons (Jaumann et al., 2008; Cruz Mermy et al., 2025; Cartwright et al., 2025). Upcoming missions including JUICE, Europa Clipper, and the L4 mission to Enceladus will have the chance to target areas that exhibit resurfacing activities. By combining laboratory experiments with existing data, this study aims to build a spectral framework that can guide the interpretation of future mission observations.

Mechanisms

The open question is whether the different mechanisms by which ice is brought to or formed at the surface leave distinguishable spectral signatures. Three end-member resurfacing processes thought to operate on ocean worlds are considered in this study. Plume grain deposition, in which eruptions deposit fine ice particles onto the surface, has been observed on Enceladus by Cassini (Kempf et al., 2010; Schenk et al., 2018) and tentatively been detected on Europa (Roth et al., 2014). Thermal diapirism, in which warm buoyant ice rises slowly through the shell and modifies the near-surface layer, is theorised to explain several surface features on Europa (Pappalardo and Barr, 2004). Lastly, crack-fed liquid effusion is considered, in which pressurised liquid water ascends through fractures and freezes at or near the surface (Fagents, 2003; Lesage et al., 2021). Each of these mechanisms operates under different temperature and pressure conditions and on different timescales. Each mechanism is expected to produce ice with a distinct physical structure, differing in crystallinity, grain size, and porosity. These differences could influence near-infrared reflectance spectra. This study takes a first step towards investigating whether they are large enough to be detectable.

Europa and Enceladus represent two end-members for this question. Enceladus has a surface dominated by nearly pure water ice, continuously refreshed by active plume grain deposition from the south polar terrain (Schenk et al., 2018), making it the most direct analogue for laboratory experiments with pure water ice. Europa, by contrast, has a heavily processed surface where magnetospheric irradiation and non-ice impurities rapidly obscure freshly formed ice (Carlson et al., 2009), meaning formation signatures are expected to be shorter-lived. Together these two moons cover the range of surface environments in which resurfacing might be identified.

Methods

Experiments are conducted in the PISCES vacuum chamber (Plumes and Ices Simulation chamber for Enceladus and other moonS) at TU Delft. This is a cryogenic vacuum facility capable of reproducing the low-pressure surface conditions, across a range of temperatures representative of Europa and Enceladus (Bourgeois and Cazaux, 2025). Within this chamber, each of the three resurfacing mechanisms is recreated at small scale using pure water ice.

The ice formation method is optimised through a parametric study in which container properties, liquid volume, and initial water temperature are varied to identify the conditions that produce the most representative ice samples. The behaviour of water freezing under vacuum conditions has been studied by Brož et al. (2025), and their observations provide a point of comparison for the ice formation in PISCES.

The resulting ice samples are characterised with a focus on features sensitive to crystallinity, grain size, and ice structure (Mastrapa et al., 2008; Stephan et al., 2021). Our approach is comparative: the relative differences in spectral properties are quantified, to assess whether they can be distinguished from one another. As a final step, the laboratory findings are compared against spacecraft observations to assess whether the differences are detectable in existing observational data. 

This work aims to establish whether the mechanisms that bring subsurface material to the surface of ocean worlds leave identifiable spectral signatures. Such signatures, if detectable, could offer a means to interpret surface observations in terms of the state of the ocean and the interior beneath.