Europa 447
Europa may have all three: water of the ocean, organic
compounds that have been delivered to the satellite, and
chemical energy from radiolysis and possibly chemosynthe-
sis. The evidence for liquid water within Europa is strong,
as discussed earlier, and Europa’s sub-ice ocean may have
a greater volume than that of all Earth’s surface water.
Cometary and asteroidal impactors have rained onto the
surfaces of the Galilean satellites throughout solar system
history. Just as Ganymede and Callisto have been darkened
by impactor material, similar material must have been deliv-
ered to Europa, where its young and bright surface implies
that much of this material is now incorporated into the ice
shell and ocean. Moreover, the original accretion of Europa
may have delivered carbon in the form of CO and CO 2.
Metabolic reactions within living cells depend upon
chemical reactions betweenoxidantsandreductants. For
animals, this depends on taking in oxygen, which is com-
bined with sugars to produce CO 2 and water. For plants,
CO 2 is combined with water to form sugars and oxygen.
In extreme environments on Earth, and possibly within
Europa, more exotic materials such as hydrogen sulfide
(H 2 S), formaldehyde (HCOH), methane (CH 4 ), or even
sulfuric acid (H 2 SO 4 ) can be key to metabolism. The key is
that chemical disequilibrium must exist, which organisms
then exploit to create the energy needed for life.
Whether Europa has sufficient chemical energy to sup-
port life is the most significant unknown in understand-
ing Europa’s potential for life. Irradiation of surface ice
can form molecules of oxygen and hydrogen, with most
of the hydrogen floating away but much of the oxygen and
other oxidants remaining behind, like a condensed out at-
mosphere frozen into the uppermost centimeters of ice. If
these oxidants can be delivered to the ice shell and ocean,
they may be able to power the chemical reactions necessary
for life. Some of these oxidants will be churned into the up-
per meter of ice by small impacts. Geological processes such
as chaos formation may be able to deliver near-surface ma-
terials to the ocean, but the means of surface-ocean com-
munication remain poorly understood. Some oxygen and
hydrogen is also produced within the ice shell and ocean
by radioactive decay of potassium, but this alone could not
provide much energy for life.
If Europa’s rocky mantle is tidally heated, then hy-
drothermal systems could exist on Europa’s ocean floor. On
Earth, hot chemical-laden water pours into the oceans, de-
livering organic materials and reductants into the water. If
hydrothermal systems exist at the bottom of Europa’s ocean,
and if oxidants are delivered from the ice shell above, then
the necessary chemical disequilibrium that could be used
by life exists.
Another important consideration is whether Europa’s in-
terior environment is stable enough through time, such that
if life ever developed it would still exist today. Europa’s
ocean may have persisted for aeons thanks to internal ra-
dioactive heating and the warming resulting from Jupiter’s
gravitational tug. However, the internal heating induced by
the Laplace resonance is not necessarily ancient, and (as
discussed earlier) the intensity of tidal heating may have
varied (perhaps cyclically) through time. It is an open ques-
tion whether chemical energy sources for life exist within
Europa and have been sufficiently stable to support life
through time. Even if life does not exist within Europa to-
day, it may have existed in the past.9. Future Exploration
The unique requirements for studying Europa—primarily
the harsh radiation environment around Jupiter, and the
fuel needed to get a spacecraft into orbit around the
satellite—make any mission there both technically and fi-
nancially challenging. Nevertheless, the possibilities for life
on this icy moon are sufficiently intriguing that such a mis-
sion has a high priority within the scientific community.
Key scientific questions remain to be answered, including
whether there is indeed a liquid water ocean, the character-
istics and composition of this ocean, the means of surface-
ocean exchange, and whether Europa can support life. A
spacecraft in orbit around Europa could make continu-
ous gravimetric and topographic measurements of the tidal
bulge and magnetic measurements of the conductive layer
below the ice shell. Ice-sounding radar would be able to
sense shallow water deposits including partial melt and may
be able to probe to the bottom of the ice if it is relatively
cold and thin. The only way to acquire an unambiguous
measurement of the thickness of Europa’s ice shell (short
of actually drilling through it) is to make seismic measure-
ments, by landing a seismometer on the surface.
The composition of Europa’s surface is not well known,
so high spectral and spatial compositional measurements
are also needed to understand Europa’s evolution and
surface processes. Experiments designed to determine
Europa’s potential for life are best made with a lander on the
surface, either a stationary scientific laboratory or a rover.
Spacecraft data has yielded tantalizing insights into Europa’s
history and evolution, but there is still much we do not know.
Further exploration is the only way we will learn Europa’s
deepest secrets.Bibliography
General
Greeley, R., Chyba, C. F., Head III, J. W., McCord, T. B.,
McKinnon, W. B., Pappalardo, R. T., and Figueredo, P. (2004).
Geology of Europa. In “Jupiter: The Planet, Satellites, and Magne-
tosphere” (F. Bagenal, ed.), pp. 329–363. Cambridge Univ. Press,
Cambridge, United Kingdom.