In order for Europa to maintain an ocean that is oxygenated like Earth’s, oxidants need to propagate downward from Europa’s icy surface8,9. One probable mechanism to achieve this is downward surface material transport through geologic processes. Hand et al.8 demonstrated that Europa could maintain an oxygenated ocean if downward cycling occurs only once every 30–70 Myr, and remain at least partially oxygenated with cycling every 500 Myr.

Whether or not Europa’s surface has been continually renewed throughout its history, or if some catastrophic process globally erased an ancient surface in relatively recent geologic times, are significant open questions that reflect our limited understanding of Europa’s icy shell. Currently, we understand that the shell is cold and stiff near the surface, producing fractures and faults in response to geologic stresses (Fig. 1). Where the ice shell is warm near the ocean interface, it may undergo solid-state ice convection (localized ice masses rising and falling buoyantly within the subsurface), a process akin to mantle convection on Earth.

Fig. 1: Cutaway views of Europa’s water ice shell and saltwater ocean. We show the scale of (a) the individual layers in relation to each other, and (b) an artistic schematic (artist: Jeff Nentrup) of different tectonic and geodynamic processes that might be at play within the ice shell today or in the past. Each of these processes may play a unique role in defining Europa’s habitability. Full size image

Europa’s prominent bands are tabular extensional zones of ridges and grooves that transect the surrounding terrain, inferred to have opened through mechanisms analogous to mid-ocean ridge spreading on Earth10 (Fig. 1). Where the brittle ice is thick and strong, the ice rifts apart to form faulted bands that disrupt the original terrain11. Where the brittle ice is thin and weak, warm interior ice and incorporated ocean material may be exposed at the surface11.

In contrast to globally observed extensional processes, observations indicative of geologic mechanisms that remove old surface material are rare. A key discovery in this area was made by Kattenhorn and Prockter12, who found that an area of Europa’s surface ~100 km wide has been removed through an apparent subduction-like process. However, because subducted icy plates may quickly equilibrate with the shell’s interior12 (Fig. 1), Europa likely lacks the endogenic potential required to continuously renew its surface13.

Whether the surface is continually or discretely renewed, Europa’s tectonic processes are not the only modes of geologic transport acting on the body today. Another prominent and potentially active terrain on Europa, known as chaos, suggests the collapse of icy material to produce a mangled icy substrate on which fragments of the original surface have been translated. Leading hypotheses invoke convective transport of solid material14 or drainage of subsurface lakes15. While there is not yet consensus on the mechanisms of chaos formation, the terrain likely contributes to the downward transport of surface material.

Major difficulties remain in predicting the volume and frequency that material from the surface is conveyed to the interior ocean. If Europa’s surface was continually renewed by consistent endogenic processes, one would expect there to be an indication of repeated or cyclic geologic surface deformation. Instead, it appears that Europa has moved from global surface deformation, recorded in its ridged plains, to local formation of chaos and ridges16. This may indicate a globally perturbed body moving back toward equilibrium, expending the energy required for continued global deformation. With no definitive observations of near-surface melt and the rarity of overt convergent tectonics, future studies and observations of possible geologic transport mechanisms are critical to understanding Europa’s habitability.