Life and the Seas of Europa
Jupiter’s icy moon Europa has been of interest to astronomers for hundreds of years, and to planetary scientists since it was first imaged by the Pioneer spacecraft in the 1970s. Since then the NASA missions Voyager 1 and 2, Galileo and most recently New Horizons have all paid the moon a visit, and thanks to the remarkable high resolution imagery returned by Galileo in particular, it is possible that scientists may have uncovered the single most promising candidate for hosting astrobiology in our Solar System to date.
Europa is a moon with a difference. The smallest of the Galilean satellites, and the second most proximate to Jupiter, Europa has a surface topography matched by no known planet or moon. It is smooth and bright, and its young surface – only 40-90 million years old, is relatively devoid of significant impact craters when compared to its Jovian neighbours Ganymede and Callisto. The icy shell of Europa is pockmarked with a series of prominent crisscrossing surface features known as lineae, but also with domes, albedo features and surface ridges, all thought to be the geomorphological products of a subterranean ocean of liquid water churning slowly beneath the brittle crust. Jupiter and its extensive family of moons orbit the Sun at roughly 5 Astronomical Units (AU), a distance at which the light of our nearest star is faint and weak, and planetary surface temperatures are well below freezing. Nevertheless, it is thought that beneath Europa’s ~20km thick crust an ocean of salty liquid water exists, perhaps a hundred kilometres deep, warmed by the internal heat generated by Europa’s rapid transit around Jupiter, as well as from the gravitational interaction with its Laplace resonance partners, Ganymede and Io.
The Europan ocean represents a completely unique geochemical system and a potential astrobiological repository unlike any other in our Solar System. Cut off from light and the planetary surface, the circulation of Europa’s salty subsurface ocean is most likely driven by convection from below, where heat and reductant input originates directly from the upper mantle. Oxidation products such as oxygen and hydrogen peroxide may be formed via radioactive decay within the ocean and ice shell, but also produced and deposited extensively on the surface crust via ion irradiation. These products are likely to be the Europan ocean’s main source of oxidising power, eventually penetrating the ice/ocean barrier under conditions of preferential or partial melting of the surface crust, possibly due to a localised plume of upwelling warm water from below. Ubiquitous lineae, surface ridges and ‘chaotic’ terrain on the surface of Europa are put forward as evidence of this process occurring over timescales of 10 – 100 million years. Whether a discreet boundary exists at the surface crust/ocean interface or if an intermediate layer of slushy material separates the two zones is not immediately clear.