Abstract
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.