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EUROPA, ONE OF JUPITER’S
four Galilean moons, is not the most wel-
coming place. On the surface, daytime
temperatures barely surpass −260 degrees
Fahrenheit (–160 degrees Celsius), and a
fractured, icy shell blankets the land-
scape. Giant geysers occasionally blast
water vapor 125 miles (200 kilometers)
above the surface — the equivalent of
about 20 stacked Mount Everests. If these
conditions weren’t enough to deter visi-
tors, intense radiation from Jupiter would
doom any living thing on the surface.
Yet Europa is considered one of the
best candidates to sustain life in the solar
system. Despite its extreme conditions,
the moon hits the trifecta of require-
ments for life as we know it: water,energy, and chemical building blocks.
Although Europa is just one-quarter the
diameter of our planet, it harbors a sub-
surface ocean twice the volume of the
oceans on Earth. This aquatic environ-
ment, which has been stable for billions
of years, may be a reservoir for life —
and scientists want to find out whether
it lurks beneath the surface.Subsurface oceans
Life on Earth originated at sea, so it’s
not a stretch to imagine life on Europa
starting in a similar environment. The
moon’s subzero temperatures prohibit
surface oceans like ours, but scientists in
the 1970s discovered an icy shell covering
Europa. Studies of the surface ice show itis largely water ice with a smattering of
related compounds like hydrogen perox-
ide, carbon dioxide, and sulfur dioxide.
While the shell’s thickness is uncertain
— best estimates range up to dozens of
miles — scientists are certain that a liq-
uid ocean circulates beneath.
Voyager 1 and 2 took the first close-up
images of Europa when they flew through
the Jupiter system in 1979. The images
showed a relatively smooth surface with
few craters or mountains, but scratched
with bands and ridges. The lack of large
impact craters, which build up as meteor-
ites strike a planetary body over millions
or billions of years, meant that some pro-
cess was erasing them. Separated ridges,
where it looked as if icy material had
gushed up between the walls, also sug-
gested a geologically active world.
Scientists observed long linear features
that they determined could be created if
the surface was disconnected from the
moon’s interior — for example, with a
liquid ocean sandwiched between them.
At about 485 million miles (780 mil-
lion km) from the Sun, Europa does not
receive enough heat to keep an ocean
liquid. But it has its own heat source:
Jupiter. As Europa travels in an eccentric
orbit around its host planet, differences
in the force of gravity from one side of
the moon to the other squish and squeeze
it. This friction is enough to heat up the
moon’s solid interior in a process known
as tidal heating. The heated rocky ocean
f loor could then maintain a liquid ocean
and induce circulation below the crust of
ice. If the ice shell is sufficiently thin in
places, the heated water might even seepJupiter’s icy moon is
a great candidate for
habitability. But life’s
best chance to exist
is beneath the crust.
EUROPA BY MARA JOHNSON-GROH
THE GALILEO SPACECRAFT captured Europa in great detail. This shot shows the largest portion
of the moon imaged at the highest resolution, offering a close-in view of several features, including regions
that researchers believe formed when the icy crust melted and refroze. Although the image has slightly
enhanced color, it largely approximates how the moon would look to the naked eye. NASA/JPL-CALTECH/SETI INSTITUTE