Self-Assembling Ice Membranes on Europa: Brinicle Properties, Field Examples, and Possible Energetic Systems in Icy Ocean Worlds

Abstract

Brinicles are self-assembling tubular ice membrane structures, centimeters to meters in length, found beneath sea ice in the polar regions of Earth. We discuss how the properties of brinicles make them of possible importance for chemistry in cold environments-including that of life's emergence-and we consider their formation in icy ocean worlds. We argue that the non-ice composition of the ice on Europa and Enceladus will vary spatially due to thermodynamic and mechanical properties that serve to separate and fractionate brines and solid materials. The specifics of the composition and dynamics of both the ice and the ocean in these worlds remain poorly constrained. We demonstrate through calculations using FREZCHEM that sulfate likely fractionates out of accreting ice in Europa and Enceladus, and thus that an exogenous origin of sulfate observed on Europa's surface need not preclude additional endogenous sulfate in Europa's ocean. We suggest that, like hydrothermal vents on Earth, brinicles in icy ocean worlds constitute ideal places where ecosystems of organisms might be found.

Entrainment and Dynamics of Ocean‐Derived Impurities Within Europa's Ice Shell

“…Our estimates of lens evolution (Section 3.3.3) include the chemical evolution of the water bodies and can be used to improve estimates of their longevity (e.g., Chivers et al, 2020; Michaut & Manga, 2014) and habitability (Schmidt, 2020), as the composition of the fluid impacts both the freezing point of the fluid and biologically important properties such as water activity and chaotropicity (Oren, 2013; Pontefract et al, 2017; Pontefract et al, 2019). Additionally, coupling predictions of impurity entrainment with contemporary solution fractionation models will determine if certain ocean components are preferentially incorporated into or excluded from the shell, revealing the detailed chemical structure of icy worlds and identifying any enrichments or depletions of import (e.g., Vance et al, 2019; Zolotov et al, 2004). Determining the lifetime and composition of liquid water features within the ice shell is of profound importance in considering whether such reservoirs could be putative habitats, relevant for both planetary exploration and planetary protection (NRC, 2012; Schmidt, 2020).…”

“…It is important to note that “salinity,” here, refers to a bulk property of the fluid representative of the ion species present (i.e., utilizing a singular molecular diffusivity value). In reality, individual ion species diffusivities vary and when combined with complex ion‐ion interactions can lead to additional chemical processes (e.g., double diffusion, hydrate precipitation, and fractionation) that may alter the composition of the forming ice and brine (e.g., Vance et al, 2019). Our approach provides a first‐order estimate of salt and impurity entrainment in planetary ices and creates the potential for follow‐on research investigating the detailed thermochemistry of ices and brines in our solar system through the use of contemporary chemical modeling tools such as PHREEQC, the Gibbs SeaWater (GSW) Oceanographic Toolbox, and SeaFreeze (Journaux et al, 2020; McDougall & Barker, 2011; Neveu et al, 2017).…”

“…The conductive nature of Europa's ocean (Khurana et al, 1998; Kivelson et al, 2000), as well as spectral measurements (McCord et al, 1999), suggests the presence of dissolved salts, but nearly all of its intrinsic properties (thickness, composition, and structure) remain poorly constrained. Potential Europan ocean chemistries have been explored in a number of studies (Marion et al, 2005; McKinnon & Zolensky, 2003; Vance et al, 2019; Zolotov & Kargel, 2009; Zolotov & Shock, 2001). Here, we implement the chemistry proposed by Zolotov and Shock (2001), who assumed that Europa's ocean formed during its differentiation via partial aqueous extraction from bulk rock with the composition of CV carbonaceous chondrites (Table 2).…”

Entrainment and Dynamics of Ocean-derived Impurities within Europa's Ice Shell

“…It is important to note that "salinity," here, refers to a bulk property of the fluid representative of the ion species present (i.e., utilizing a singular molecular diffusivity value). In reality, individual ion species diffusivities vary and when combined with complex ion-ion interactions can lead to additional chemical processes (e.g., double diffusion, hydrate precipitation, and fractionation) that may alter the composition of the forming ice and brine (e.g., Vance et al, 2019). Our approach provides a first-order estimate of salt and impurity entrainment in planetary ices and creates the potential for follow-on research investigating the detailed thermochemistry of ices and brines in our solar system through the use of contemporary chemical modeling tools such as PHREEQC, the Gibbs SeaWater (GSW) Oceanographic Toolbox, and SeaFreeze (Journaux et al, 2020;McDougall & Barker, 2011;Neveu et al, 2017).…”

The Salty Secrets of Icy Ocean Worlds

“…The new work is important for beginning to quantify the possible salt content of the ice. While others have noted that the concentration of salts within the lithospheres of icy worlds is certainly non-zero (Vance et al 2019), the new work suggests an upper limit on the salt content. If Europa's ice froze rapidly in a manner similar to the formation of sea ice, regions near the top of the ice that formed first and quickest are likely to have held on to substantial amounts of salt, between 5-50% of the salinity of the original fluids.…”