1. Scientific rationale,
Jupiter's moon Europa possesses a surface-boundary layer atmosphere that is often referred to as an exosphere since it is characterized by a quasi-collisionless gas (Johnson et al., 2004; Plainaki et al., 2010). Europa's neutral environment consists mainly of H2O, released through surface ion sputtering (Johnson et al., 2004; Shematovich et al., 2005; Cassidy et al., 2007), of O2 and H2, produced through chemical reactions among different products of H2O radiolytic decomposition (Johnson, 1990; Shematovich et al., 2005; Cassidy et al., 2010, Plainaki et al., 2012), and of some minor species (e.g. Na or K (Brown and Hill, 1996; Brown, 2001; Leblanc et al., 2002) and water group species like OH, O and H (Smyth and Marconi, 2006)). The nature of Europa's exosphere (source and loss processes) and its spatial and temporal variability is a complex field of active ongoing research dealing with the study of neutral-plasma interactions coupled over a significant range of space and time scales.
The existing observations of Europa's exosphere have provided important constraints for assessing its generation and loss rates. However, a direct measurement of the main exospheric species (H2O, O2, H2) has not been performed yet since the limited available observations are just proxies of these bulk constituents (e.g. OI UV emission served as a proxy for O2). Moreover, the HST observations always cover only the moon's illuminated hemisphere hence no view of the night hemisphere is available from the Earth's orbit. The morphology of the UV observations of OI emissions at 1304 Å and 1356 Å at Europa, attributed primarily to electron impact dissociative excitation of O2, provided evidence either of the existence of inhomogeneous neutral gas abundance across the surface (Cassidy et al., 2007; McGrath et al., 2009) or of a highly variable plasma environment (Saur et al., 2011), or both.
Numerous modeling efforts have been made in order to support the one or the other scenario. Cassidy et al. (2007) supported that intrinsic or solar illumination may change the Europa's ice properties that control its albedo, porosity and sputtering. Saur et al. (2011) showed that spatial variations of Jupiter's magnetospheric electron density and temperature influence the electron impact dissociation process responsible for the oxygen UV emissions. Plainaki et al. (2012; 2013) demonstrated that the spatial distribution of Europa's exosphere is explicitly time-variable due to the time-varying relative orientations of solar illumination and the incident plasma direction, both factors driving the O2 release efficiency from the surface. Recently, a transient endogenic H2O exosphere source, consistent with two 200-km-high plumes of water vapor, was discovered through the analysis of HI Lyman-α 1215.67 Å, OI 1304 Å, and OI 1356 Å data obtained with HST/STIS (Roth et al., 2014). Whereas a clear influence of the magnetospheric fluctuations on the aurora morphology was identified in the study by Roth et al., 2014, the effects on the emission morphology of the plasma variability and the inhomogeneous neutral environment were not disentangled.
Despite the numerous modeling efforts, our knowledge of a. the overall radiation-induced physical and chemical processes that eject molecules from the icy surface to Europa's exosphere and b. the exchange of material between body-surfaces and Jupiter’s magnetosphere, is still poor. In lack of an adequate number of in situ observations, the existence of a wide variety of models based on different scenarios (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) and considerations (e.g. homogeneous (or not) source/loss rates) has resulted in a yet fragmentary understanding of Europa's exosphere physics. 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. The inhomogeneity of the exosphere sources is another debated topic. While in many models the initial exosphere source/loss rates are assumed to be spatially homogeneous (e.g. Smyth and Marconi, 2006), in other approaches a sputtering and radiolysis rate (leading to H2O, O2 and H2 release) dependent both on the moon's surface temperature and plasma impact has been implemented (e.g., Plainaki et al., 2012; 2013). As a third approach, Saur et al. (1998) modeled the plasma action on Europa's neutral environment, assuming an O2 atmosphere with slight hemispheric asymmetries determined by the ion flux variation at the moon rather than anisotropies in the surface release processes. In summary, the existence of several models based on very different approaches eventually imposes the need for an overall revision for the determination of a largely accepted unified model of Europa's exosphere. The availability to the science community of such a model is particularly urgent in view of the planning of the future JUICE mission (Grasset et al., 2013) observations. Namely, the study of the transient plumes (Roth et al., 2014) – with their potential implication on the nature of the moon's inner ocean - will have as mandatory prerequisite an accurate characterization of the exospheric background.
Understanding the details of the radiation-induced release mechanisms at the icy surface of Europa, the exchange of material between the moon and the magnetosphere of Jupiter, the exosphere dynamics and the dependence of its overall morphology on external parameters (e.g. plasma, moon-illumination) is of particular importance, since it is related with ongoing and novel research in different-discipline areas (e.g. MHD, ENAs, plasma-neutral interactions). The Science Goals of the proposed study can be summarized in the following points:
<![if !supportLists]>G1. <![endif]>Review of the available observations (in situ and telescope data), search for potential synergies between different datasets and assessment of related variability. This review will be based on data published in literature or public data available on the NASA PDS, HST and/or other archives.
<![if !supportLists]>G2. <![endif]>Analytical comparison of all existing models of Europa's exosphere and determination of the main improvements required to current models; in particular, definition of required improvements for numerical techniques (e.g. hybrid MHD and DSMC modeling or 3-D DSMC modeling).
<![if !supportLists]>G3. <![endif]>Definition of the required characteristics for a community unified model (main physical phenomena to be included, acceptable assumptions and approximations).
<![if !supportLists]>G4. <![endif]>Assessment of possible future experimental work required to constrain the models.
<![if !supportLists]>G5. <![endif]>Definition of suitable observation strategies for future missions namely JUICE and Europa Clipper to discriminate between the existing exosphere models.
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