Investigating Europa's Habitability with the Europa Clipper

The habitability of Europa is a property within a system, which is driven by a multitude of physical and chemical processes and is defined by many interdependent parameters, so that its full characterization requires collaborative investigation. To explore Europa as an integrated system to yield a complete picture of its habitability, the Europa Clipper mission has three primary science objectives: (1) characterize the ice shell and ocean including their heterogeneity, properties, and the nature of surface-ice-ocean exchange; (2) characterize Europa's composition including any non-ice materials on the surface and in the atmosphere, and any carbon-containing compounds; and (3) characterize Europa's geology including surface features and localities of high science interest. The mission will also address several cross-cutting science topics including the search for any current or recent activity in the form of thermal anomalies and plumes, performing geodetic and radiation measurements, and assessing high-resolution, co-located observations at select sites to provide reconnaissance for a potential future landed mission. Synthesizing the mission's science measurements, as well as incorporating remote observations by Earth-based observatories, the James Webb Space Telescope, and other space-based resources, to constrain Europa's habitability, is a complex task and is guided by the mission's Habitability Assessment Board (HAB).

Exploring the Interior of Europa with the Europa Clipper

The Galileo mission to Jupiter revealed that Europa is an ocean world. The Galileo magnetometer experiment in particular provided strong evidence for a salty subsurface ocean beneath the ice shell, likely in contact with the rocky core. Within the ice shell and ocean, a number of tectonic and geodynamic processes may operate today or have operated at some point in the past, including solid ice convection, diapirism, subsumption, and interstitial lake formation. The science objectives of the Europa Clipper mission include the characterization of Europa's interior; confirmation of the presence of a subsurface ocean; identification of constraints on the depth to this ocean, and on its salinity and thickness; and determination of processes of material exchange between the surface, ice shell, and ocean. Three broad categories of investigation are planned to interrogate different aspects of the subsurface structure and properties of the ice shell and ocean: magnetic induction, subsurface radar sounding, and tidal deformation. These investigations are supplemented by several auxiliary measurements. Alone, each of these investigations will reveal unique information. Together, the synergy between these investigations will expose the secrets of the Europan interior in unprecedented detail, an essential step in evaluating the habitability of this ocean world.

Serpentinite and the search for life beyond Earth

Hydrogen from serpentinization is a source of chemical energy for some life forms on Earth. It is a potential fuel for life in the subsurface of Mars and in the icy ocean worlds in the outer solar system. Serpentinization is also implicated in life's origin. Planetary exploration offers a way to investigate such theories by characterizing and ultimately searching for life in geochemical settings that no longer exist on Earth. At present, much of the current context of serpentinization on other worlds relies on inference from modelling and studies on Earth. While there is evidence from orbital spectral imaging and martian meteorites that serpentinization has occurred on Mars, the extent and duration of that activity has not been constrained. Similarly, ongoing serpentinization might explain hydrogen found in the ocean of Saturn's tiny moon Enceladus, but this raises questions about how long such activity has persisted. Titan's hydrocarbon-rich atmosphere may derive from ancient or present-day serpentinization at the bottom of its ocean. In Europa, volcanism or serpentinization may provide hydrogen as a redox couple to oxygen generated at the moon's surface. We assess the potential extent of serpentinization in the solar system's wet and rocky worlds, assuming that microfracturing from thermal expansion anisotropy sets an upper limit on the percolation depth of surface water into the rocky interiors. In this bulk geophysical model, planetary cooling from radiogenic decay implies the infiltration of water to greater depths through time, continuing to the present. The serpentinization of this newly exposed rock is assessed as a significant source of global hydrogen. Comparing the computed hydrogen and surface-generated oxygen delivered to Europa's ocean reveals redox fluxes similar to Earth's. Planned robotic exploration missions to other worlds can aid in understanding the planetary context of serpentinization, testing the predictions herein. This article is part of a discussion meeting issue 'Serpentinite in the Earth System'.