The Project
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.
2. Goals
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:
G1.
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.
G2.
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).
G3.
Definition of
the required characteristics for a community unified model (main
physical phenomena to be included, acceptable assumptions and
approximations).
G4.
Assessment of
possible future experimental work required to constrain the
models.
G5.
Definition of
suitable observation strategies for future missions namely
JUICE and Europa Clipper to discriminate between the existing
exosphere models.
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