Dynamic controls on subsurface water chemistry and habitability on icy moons

Icy moons are natural satellites whose surfaces contain a substantial fraction of frozen materials, most notably water ice. They lie beyond the snow line and orbit gas or ice giant planets, such as Jupiter, Saturn, Uranus, and Neptune. Recent observations and theoretical studies have indicated that several icy moons host regional or global subsurface liquid water oceans that likely persist for 107–109 years. Moreover, various organic matter and bioessential nutrients have been detected on icy shells or within plumes from several icy moons, implying potentially habitable ocean chemistry. Consequently, ocean-bearing moons become tantalizing targets for the search for extraterrestrial habitable environments and potential biosignatures.

Chemical properties of subsurface oceans in icy moons

Water chemistry is a critical factor in assessing planetary habitability. Among the possible ocean-bearing icy moons, best constrained cases are Enceladus, Europa, Ganymede, and Titan. For example, Cassini's analyses of the ongoing plume from Enceladus and icy grains in Saturn's E ring suggested a major composition of Na+, Cl−, and CO32−/HCO3− with minor K+ and NH3/NH4+ in the subsurface ocean. Moreover, the presence of silica nanoparticles and molecular hydrogen further indicates ongoing hydrothermal activity at the ocean floor. Unlike Enceladus, current constraints on the subsurface ocean chemistry of other icy moons are heavily dependent on surface remote sensing and/or computational modeling, due to the lack of direct sampling of materials from the ocean. Nevertheless, their subsurface oceans are inferred to have a major composition of inorganic ions (e.g., Na+, Mg2+, Cl−, and/or NH4+) and common gases (e.g., CO2), similar to Enceladus.

However, different icy moons could have vastly different chemical conditions and availabilities of bioessential elements, affecting their habitability. For example, Enceladus’s seawater is proposed to be alkaline and reducing, with high levels of bioessential nutrients (e.g., ammonium and orthophosphate)3 and organic matter with various functional groups.2 Such chemistry may favor the stability of possible biomolecules and metabolism of any chemosynthetic microbes. Unlike Enceladus, the detection of various oxidized sulfur species on the surface of Europa suggests a possibly oxidizing and acidic ocean rich in sulfate,4 although these oxidants may also be surface radiation products. On icy moons of larger radius, such as Ganymede and Titan, high-pressure ice layer(s) would inhibit seawater alteration of the silicate seafloor,1 which plays key roles in supplying nutrients and hydrothermal energy on ocean worlds. Notably, all the aforementioned observations represent only a modern snapshot in their long history, but their habitability, particularly the potential for the origin of life, would have been heavily shaped by multiple dynamic factors discussed below.