AN ASSESSMENT OF GEOLOGICAL CONDITIONS AT ICY SATELLITE OCEAN FLOORS

Numerous icy satellites in the outer Solar System host known, or suspected, subsurface oceans. These bodies are of substantial interest because some plausibly host the components considered crucial for life—water, energy, and organics—at an ocean-floor, rock–water interface. Yet the geophysical conditions at these interfaces remain poorly understood. For example, although Terran mid-ocean ridge hydrothermal systems are often regarded as analogues for potential chemoautotrophic habitats on icy moons, the mechanical properties of these extraterrestrial ocean floors may be quite different. On Earth, plate motion leads to the development at spreading centers of shallow (1–3 km deep) magma lenses that represent localized heat sources, as well as extensional tectonic systems that maintain regions of high permeability otherwise sealed by mineral precipitation. Without plate tectonics, heat extraction is likely less efficient, and geological activity at the surface of silicate layers more limited. For bodies with differentiated silicate interiors, such as Europa, seafloor volcanism may have occurred, with pillow basalts and hyaloclastite breccias characterizing much of the physiography of these ocean floors; bodies with undifferentiated cores could feature serpentinized rock in direct contact with water. The ocean floors on Earth are saturated by seawater to at least the penetration depth of opening-mode fractures (i.e., joints). A first-order rock-mechanical assessment suggests that joints propagate, and thus seawater is present, to depths of up to a few kilometers into Europa’s silicate interior, whereas the much lower overburden pressure at diminutive Enceladus enables these fractures to penetrate to tens of kilometers. Such fractures would readily facilitate the circulation of seawater within the upper portions of these bodies’ rocky interiors, particularly at sites of elevated heat flow. In contrast, high-pressure ices blanket the silicate interiors of differentiated Ganymede and possibly Titan, and the great pressure (~3 GPa) at these interfaces likely prevents tidal or thermal stresses from driving tectonic deformation there. For these larger moons, then, there is probably little to no communication between their rocky cores and the subsurface oceans above.