430 Encyclopedia of the Solar System
7. Outstanding Questions and
Future Exploration
On September 21, 2003, theGalileomission came to the
end after 14 years in space, when the spacecraft disinte-
grated in the dense atmosphere of Jupiter. The demise of
the spacecraft was planned, since its onboard propellant
was nearly depleted and it was considered prudent to avoid
any chance of impact with Jupiter’s moon Europa in the fu-
ture, which could have happened if the spacecraft had been
left in orbit around Jupiter. AlthoughGalileo’s mission sig-
nificantly advanced our knowledge of Io, the failure of the
high-gain antenna to open (and subsequent low data rates)
prevented all but a very small part of Io’s surface to be im-
aged at high resolution. Future exploration by spacecraft is
needed to reveal Io’s surface in detail at a variety of differ-
ent wavelengths and to answer many outstanding questions.
The geometry ofGalileo’s orbit around Jupiter resulted in
lack of coverage at high resolution of Io’s Jupiter-facing side,
which should be a priority for future missions to observe.
However, even parts of the surface previously imaged are
likely to change because of the dynamic nature of Io, so
new missions will always reveal new features. At the time of
writing, there are no missions to Io planned, but there are
plans in place to observe Io during the Jupiter flyby of the
New Horizonsspacecraft in early 2007, while the spacecraft
is on its way to Pluto and the Kuiper Belt. Although many of
the outstanding questions may only be answered by a ded-
icated mission to Io, significant advances are possible from
missions of opportunity, such asNew Horizons, and from
ground-based and space telescope–based programs. Io has
been successfully observed by theHubble Space Telescope
and can potentially be observed from the Spitzer Infrared
Telescope Facility as well as from future orbiting telescopes.
Ground-based observations using Adaptive Optics, such as
those by Imke de Pater and Frack Marchis, have been a
major step forward in the study of Io because the spatial
resolution of these observations can now rival some of those
obtained fromGalileo.
One of the most significant questions raised byGalileo
concerns the nature of Io’s high-temperature volcanism. If
ultramafic compositions are involved, as is the current fa-
vored hypothesis to explain the very high temperatures, it
is difficult to explain how the magma composition would
have stayed ultramafic throughout Io’s history because dif-
ferentiation would have been expected, leading to evolved
types of magmas such as those we find in present-day Earth.
It is possible that the current style of volcanic activity is a
geologically recent phenomenon (i.e., Io has only recently
attained its resonant orbit with resulting tidal heating) or
that the response of Io’s lithosphere-mantle to tidal heating
has prevented extreme differentiation. Perhaps the magmas
are not ultramafic, but are basaltic, possibly superheated
during ascent. Compositional measurements of Io’s fresh
magmas would be invaluable for future missions to obtain.
Another intriguing question considers what thevolatiles
in the magma are dissolved in. The presence of explosive
volcanism on Io is evident from the plumes and dark de-
posits that are thought to be ash and magma fragments. On
Earth, the most common volatile is water, on Io sulfur and
sulfur dioxide have been detected in the plumes, but are
there other compounds?
Other aspects of Io’s geology are also intriguing. How are
the mountains formed? Nearly half of Io’s mountains are lo-
cated adjacent to volcanic centers (paterae), but they do not
appear to be part of the volcanic system. What can that re-
lationship tell us about Io’s crust? Questions also abound
about Io’s atmosphere and the interaction between Io and
the jovian magnetosphere, particularly the recognition of
the flux tube that allows the transfer of charged particles
between the two bodies. What are the sources of the atoms,
neutrals, and ions that are released into the plasma torus and
magnetosphere, and what physical processes allow them to
escape? These are just some of the key questions that have
developed about Io afterGalileoand that require further
analysis of existing data sets, and probably further data ob-
tained by new missions.Bibliography
Bagenal, F., McKinnon, W., and Dowling, T., eds. (2004).
“Jupiter: Planet, Satellites and Magnetosphere.” Cambridge Univ.
Press. Cambridge, UK
Geissler, P. E. (2003). Volcanic activity on Io during the Galileo
era.Annu. Rev. Earth Planet. Sci. 31 , 175–211.
Johnson, T. V. (1999). Io. In “The New Solar System,” 4th Ed.
(J. K. Beatty, C. C. Petersen, and A. Chaikin, eds.), pp. 241–252.
Kargel, J. S., et al. (2003). Extreme volcanism on Io: Latest
insights at the end of theGalileoera.EOS 84 , no. 33.
Kivelson, M.G., Khurana, K. K., Russell, C. T., and Walker, R.
J. (2001). Magnetic signature of a polar pass over Io.EOS 82.
Lopes, R., and Gregg, T. K., eds. (2004). “Volcanic Worlds: Ex-
ploring the Solar System Volcanoes.” Praxis Publishing Company
(Springer-Verlag).
Lopes, R., and Spencer, J. R., eds. (In press). “Io after Galileo.”
Praxis Publishing Company (Springer-Verlag).
Lopes-Gautier, R. (1999). Volcanism on Io. In “Encyclopedia
of Volcanoes” (H. Sigurdsson et. al., eds.), pp. 709–726. Academic
Press. San Diego
Lopes, R., and Williams, D. (2005). Io afterGalileo.Rept.
Progr. Phys. 68 , 303–340.
McEwen, A. S., Lopes-Gautier, R., Keszthelyi, L., and Kieffer,
S.W. (2000). Extreme volcanism on Jupiter’s moon Io. In “En-
vironmental Effects on Volcanic Eruptions: From Deep Oceans
to Deep Space” (J. Zimbelman and T. Gregg, eds.), pp.179–204.
Plenum. New York
Morrison, D., ed. (1982). “Satellites of Jupiter.” Univ. Arizona
Press, Tucson.
Nelson, R. M. (1997): Io. In “Encyclopedia of Planetary Sci-
ences” (J. H. Shirley and R. W. Fairbridge, eds.), pp. 345–351.
Chapman and Hall.
Spencer, J. R., and Schneider, N. M. (1996). Io on the eve of
theGalileomission.Annu. Rev. Earth Planet. Sci. 24 , 125–190.