ESA JUICE RED BOOK — PART 6

Europa’s surface is notable for its very low density of impact craters (only 16 craters with diameters of 3–27 km could be identified), suggesting a young surface age (e.g., Greeley et al., 2004; Schenk et al., 2004). Europa’s surface can be subdivided into bright (bluish colour) plains, featuring numerous parallel ridges in a wide range of orientations, and darker, brownish mottled terrain (Lucchitta and Soderblom, 1982; Greeley et al., 2004; Prockter et al., 2010; Stephan et al., 2012)

Linear ridges are the most widespread landforms on Europa; the most common of which are double ridges, consisting of a pair of ridges with a medial trough. They are thought to have originated through a variety of mechanisms, including, e.g. tectonism, cryovolcanism, or diapirism, and require either the presence of liquid water in the shallow subsurface, or warm mobile ice underlain by an ocean at depth (Greeley et al., 2004, and references therein, Schmidt et al., 2011)

Europa also has a tenuous mainly O2 atmosphere (Hall et al., 1995) produced by intense radiation bombardment (though occasional venting cannot be ruled out). Na and K have also been measured from ground-based observations (Coustenis et al., 2010 and references therein). O2 is seen on Europa’s surface (Hand et al., 2006). The evidence for trapped O2 indicates that the radiolytically-produced O2 may be supplied to the subsurface ocean, where it could be a source of energy for life (Chyba, 2000)

An important objective at Europa will be to search for and study present and/or recently active processes. Their manifestation can be found both on the moon’s surface and in its atmosphere (plumes, geysers) and related ionosphere

The recent evidence for a large (~200-km high) plume of water vapour provided by Hubble Space Telescope (HST) imaging of far-UV emissions from excited H and O water dissociation products (Roth et al., 2014) indicates active processes at Europa and may allow accessibility to a subsurface liquid reservoir. The existence of such water vapor plumes, if confirmed in future observations, has far-reaching implications for the future exploration of Europa’s potentially habitable environment, similar to those discussed for Enceladus and its plumes (Spencer and Nimmo, 2013)

The source processes that control the variability and strength of plume activity on Europa are currently unclear, but rely on the geophysical mechanisms for activity that JUICE is already tailored to investigate thoroughly