The atmospheric physics of the Galilean moons of Jupiter is one of the major interests of the international scientific community. The recent discovery of transient water plumes erupting off the surface of Jupiter’s moon Europa (Roth et al., 2014), with their potential implication on the nature of the moon's inner ocean, opened a new chapter in the study of Europa's neutral environment; in this context, the accurate characterization of the moon's exospheric background is imposed as a mandatory prerequisite.
Europa's surface-boundary layer atmosphere - often referred to as an exosphere since it is characterized by a quasi-collisionless gas (Johnson et al., 2004; Plainaki et al., 2010) - is a complex field of active ongoing research. Understanding the source and loss processes of Europa's neutral environment and its spatial and temporal variability requires knowledge of a vast series of neutral-plasma interactions coupled over a significant range of space and time scales. Although the existing observations of Europa's exosphere have provided important constraints for determining its generation and loss rates, a direct measurement of the main exospheric species (H2O, O2, H2) has not been performed yet; the limited available observations are just proxies of these bulk constituents (e.g. OI UV emission is a proxy for O2). Moreover, in lack of an adequate number of in situ observations, the existence of several models based on very different approaches (e.g. assuming either the collisional (Shematovich et al., 2005; Smyth and Marconi, 2006) or the collisionless (Cassidy et al., 2007; Plainaki et al., 2012) approximation) has resulted in a yet fragmentary understanding of Europa's exosphere physics. For example, whereas the collisionless approximation ignores the detailed chemistry between the exospheric constituents and the plasma/UV environment, the existing kinetic models (1-D or 2-D) do not consider different configurations between Jupiter, Europa and the Sun and the effect that they would have on the exosphere spatial distribution and the neutral escape rate. In view of the planning of the future JUICE mission observations, the need for an overall revision for the determination of a largely accepted unified model of Europa's exosphere is necessary.
In a coherent logical frame including theoretical, observational and space activities, we propose an ISSI group with representatives from the major disciplines (MHD, exosphere/ionosphere science and remote sensing) related to the Europa science. The main science goals of this 'larger view approach', intimately related to the mission of ISSI-Bern, are:
• to review the available observations (in situ and telescope data), to search for potential synergies between different datasets and to assess the related variability
• to compare all existing models of Europa's exosphere and to determine the main improvements required to current models
• to define the required characteristics for a community unified model (main physical phenomena to be included, acceptable assumptions and approximations)
• to assess possible future experimental work required to constrain the models
• to define suitable observation strategies for future missions namely JUICE and Europa Clipper
Each of these goals will result in a final document to be publicly distributed to the science community. Moreover, at least one paper in a peer reviewed journal on our current understanding of the exosphere of Europa and its spatial and temporal variability based on both observations and models, including also the potential effects of the water plumes, will be produced.