Europa is a puzzle. The sixth largest moon in our Solar System,
Europa confounds and intrigues scientists. Few bodies in the
Solar System have attracted as much scientific attention as
this moon of Jupiter because of its possible subsurface ocean
of water. The more we learn about this icy moon, the more
questions we have. Because the nature of science is to ask
questions, we cannot resist the mystery of Europa and its
potential for possessing an ocean.
The United States sent two spacecraft, Pioneer 10 and 11, to
Jupiter in the early 1970's. No one knew if a spacecraft
could survive a flyby of Jupiter, but the Pioneers did survive,
and they sent back valuable information for the next space
mission. However, the Pioneer photographs of Jupiter's largest
moons were fuzzy and dim. The twin Voyager spacecraft flew by
Jupiter and its moons in 1979, giving us our first close-up
view of Europa. Voyager pictures show pale-yellow icy plains
with red and brown mottled regions. Long cracks run for
thousands of kilometers over the surface. On Earth, these
cracks would indicate such features as tall mountains and deep
canyons. But none of these features are higher than a few
kilometers on Europa, making it one of the smoothest objects
in our Solar System.
Images from Galileo
If we look at the surface more closely, as we have with the
instruments on the Galileo spacecraft, we see some fascinating
features. Europa looks like broken glass that is repaired by
an icy glue oozing up from below. Low ridges, straight and
curved, crisscross the surface. Flows and fractures, pits
and frozen "puddles" - all hint at a unique geologic history.
Large circular features could be the sites of impacts or the
result of upwelling of material from beneath the surface.
Making sense of this chaotic landscape is a challenge to
Europa's Puzzling Surface
Despite the chaos of its surface, Europa is probably the kind
of puzzle that science can solve. Some of our questions are:
"How old is the surface? How were the cracks and other
features made? What is under the ice?" To answer them, we
collect data and make careful obervations, applying what we
know about geology, physics, and chemistry. Geologists figure
out the age of a surface by counting the impact craters formed
where comets, meteorites, and other debris hit the surface.
Comparison to Our Moon
Earth's Moon has young and old craters literally everywhere,
which tells us that it has been geologically inactive for
more than a billion years. Earth has been impacted at least
as many times as the Moon, but Earth's surface has been
smoothed by active geological processes such as plate
tectonics and volcanic flows, and by constant weathering.
Like our Moon, Jupiter's satellites Ganymede and Callisto
are heavily cratered Ð evidence of very old and inactive
surfaces. On Europa, however, only a few large craters have
been identified. Unless Europa has somehow avoided these
impacts, which is unlikely, relatively recent events must
have smoothed over the craters.
Geologic Action on Europa
Looking at the pictures from Galileo, we see evidence of
geologic action on Europa. Small blocks of crust float like
icebergs over an invisible sea. Some blocks are tilted, others
rotated out of place. Dark bands of ice and rock spread
outward from a central ridge. What is the cause for
this activity? In a gravitational tug of war of incredible
dimensions, Europa is pulled in different directions by Jupiter
and by the planet's other moons in a process called tidal
flexing. Over one Europan day, it stretches and compresses up
to several tens of meters. The outer surface of Europa is a
rigid sphere. Imagine Earth covered by a blanket of ice that
traps the oceans below. In the course of a day, these oceans
rise and fall. This is what happens to Europa. The flexing
of Europa's surface continues until the brittle crust cracks.
We don't know what happens when the crust fractures. The
process may be slow and steady, advancing only centimeters at
a time - or, it may cause ice volcanoes or geysers to erupt
violently, showering the surface with material from below.
Is There Life in the Oceans of Europa?
Another interesting possibility arises from this tidal flexing
of Europa. Heat generated by the expansion and contraction may
be enough to melt part of the crust underneath the surface,
creating lakes or oceans below. The possibility of liquid
water just below Europa's surface naturally leads to the
question of whether life could have evolved there. Scientists
have discovered marine life on Earth that thrives in the deep
ocean near hydrothermal vents. This discovery provides
us with a model for how similar organisms might survive on
Europa. However, liquid water is just one of life's key
ingredients. Many other factors, including organic material
and a continuous energy source, must be present. Even if
there is no ocean currently on Europa, one may have existed
in the past, perhaps leaving fossilized remains to be found
by a future mission.
General / Overview
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