organic material biological. It’s probably
the most important atom after carbon,”
McKay says. And last, an energy source.
“Something to eat. Sunlight if you live
near the surface. Chemical energy if not.”
As scientists learn more about how
each moon satisfies these requirements,
mission preferences are continually
reshaped.
“Five years ago, Europa’s star was
pretty high, Titan’s was rising and we
barely knew that Enceladus would be
important and on this all-star list,”
remarks Stern, who says he would be
surprised if none of these moons hosted
alien life. “I don’t think it matters which
of these three worlds we find it on. Wherever we find life, it will open up a very
rich scientific vein.”
Water wonder
Some scientists think that vein runs
beneath Europa’s icy crust.
Four centuries ago, in 1610, Galileo
Galilei peered through a telescope in
Padua, Italy, and spied four moons orbiting Jupiter. Included among them was
the water world now known as Europa.
At about 3,120 kilometers in diameter,
Europa is the runt of the Galilean satellites, slightly smaller than Earth’s moon.
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reprOcessed by ted stryk; Nasa, JpL, space scieNce iNstitute; Nasa, JpL, space scieNce iNstitute
Yet this runt has been astrobiology’s
it-moon since scientists found evidence
more than a decade ago for a deep, liquid water ocean underlying its icy surface. Constructed somewhat like a candy
cordial with a smooth chocolate outer
layer, liquid interior and crunchy core,
Europa has a roughly 10-kilometer-thick
crust of ice with what scientists believe
is a 160-kilometer-deep ocean sloshing
beneath.
The ocean probably sweeps up minerals from the moon’s mantle, minerals
necessary for the building blocks of life
to form, says planetary scientist Robert
Pappalardo of NASA’s Jet Propulsion
Laboratory in Pasadena, Calif.
“If we’ve learned anything about life
on Earth, it’s that wherever you find
water, you find life,” says planetary scientist Kevin Hand, also of JPL. “So we
follow the water, and Europa is where
the water is.”
Moony trio chris mckay of Nasa’s ames research center says three
outer solar system moons (below, not to scale) are prime candidates for alien
life because they fulfill some of the key criteria for habitability: liquid water,
organics (aka carbon-containing compounds), nitrogen and an energy source.
Water
OrgaNics
NitrOgeN
eNergy
Europa
Water is abundant on europa in
the form of an ocean sloshing
beneath the icy crust. scientists
don’t yet know whether nitrogen
or organics are present, but
some think hydrothermal vents
might erupt from the seafloor
and power life.
Titan
the surface of titan sits
beneath a thick, nitrogen-containing atmosphere and
is soaked in organic hydrocarbons, which could serve as
an energy source. Whether a
water ocean lurks deep underground is not yet known.
Enceladus
Water, organics and nitrogen
pour into space from the satellite’s geysers, which require
an energy source for fuel.
still, scientists aren’t sure
whether liquid water has been
around long enough for life to
evolve on the tiny moon.
In addition to that salty ocean, scientists suspect that Europa has a tenuous
oxygen atmosphere, produced when
charged particles from Jupiter’s magnetosphere strike the moon’s icy surface.
Though these processes make the surface inhospitable to life, oxygen could
now be saturating the Europan sea and
powering life-forms swimming in the
abyss, says planetary scientist Richard
Greenberg of the University of Arizona
in Tucson.
Greenberg recently estimated how
long it would take for oxygen to sink
through the ice into the Europan soup,
once the top few centimeters were
saturated. He reported last year in
Astrobiology that it could be an almost
2-billion-year journey from the surface
to the inner salty sea. That time lag following the moon’s formation could give
aquatic life a chance to develop while
shielded from potentially damaging oxygen, just as life on Earth developed free
of oxygen for its first billion years.
“Life could get going during those
2 billion years, and it could form protective structures like cells,” Greenberg
says. Now, it’s possible for microorganisms—and even more complex life-forms — to use that oxygen as energy,
he says.
Hand thinks simple life-forms are
more probable, because sulfur compounds appear to be abundant on
Europa and microbial life can exist quite
happily on sulfur-based fuel.
Punching through the icy rind to
access either type of organism would
be a tricky task for an unmanned vehicle of the kind that might visit Europa.
But some scientists think that the
moon’s juicy innards might be spurting
through cracks, carrying some signature
of life beneath (if not the life itself ). The
luminous white surface is smeared with
dark red, possibly sulfur-rich deposits
that could be by-products of microbial
metabolism, Pappalardo says.
The surface ice at Borup Fiord Pass,
on Ellesmere Island in the Canadian
High Arctic, has similar sulfur streaks.
In Geobiology in July, Pappalardo, JPL’s
Damhnait Gleeson and colleagues identified a community of microbes living at
the deposits, suggesting that the smears
are the result of sulfur-oxidizing metabolism by the microbes.
Now, the team is working on identifying markers that would distinguish biological activity from mineral processes
that could also produce the discolorations—and thinking about ways to
remotely detect such differences, since
Europa’s harsh conditions mean a spacecraft couldn’t stick around there for long.
