Whether the second criterion for life,
organic material, exists on Europa is a
big question mark. But another candidate moon, Titan, is soaked in it.
Dutch astronomer Christiaan Huygens
glimpsed Titan orbiting Jupiter’s ringed
neighbor, Saturn, in 1655. As its name
implies, Titan is big — roughly 5,150 kilometers in diameter, larger than Earth’s
moon or the planet Mercury. Images of
Titan at first just showed a blurry, orange-blue edge, the fluffy footprint of its thick,
nitrogen-rich atmosphere.
It wasn’t until the appropriately
named Huygens probe descended to
Titan’s surface in January 2005 that
astronomers could directly glimpse the
veiled moon’s surface. And it kind of
looked like home, on a hazy day.
Aside from Earth, Titan is the only
body in the solar system known to have
stable liquids on its surface, in the form
of streams and seas. The landscape
looks so similar that some researchers think Titan could be a gold mine for
clues about Earth’s early history. “We
can learn about the evolution of organic
chemistry, which is presumably part of
the process that led to life on Earth,”
says planetary scientist Ralph Lorenz of
Johns Hopkins University.
Titan’s surface lakes are not bodies
of water, though. Instead, they’re filled
with methane and ethane, hydrocarbons
that behave differently from water molecules. Normally gases on Earth, these
compounds are liquid at Titan’s surface
temperature of –180° Celsius.
And it rains on Titan. Dense clouds seasonally sprinkle the moon with methane,
scientists reported in March in Science,
raining down upon a surface characterized by modest mountains, dunes resembling asphalt and rocks made of water-ice.
“It’s an organic chemist’s dream, but
not necessarily a biologist’s dream,”
Hand says.
Still, Titan could host its own brand of
surface inhabitants based on hydrocarbons, with biochemistries very different
from those on Earth. “Titan is where you
go if you’re looking for weird life,” he says.
A Titan mission in the planning stages
would look for at least the beginnings of
any such life. After splashing down in a
100,000-square-kilometer hydrocarbon
lake called Ligeia Mare, the Titan Mare
Explorer would seek out large, complex
assemblages of organic molecules, compounds that might form into single amino
acids or proteinlike structures.
Sailing the methane lakes of Titan captures “a little bit of the romanticism of
ships exploring Earth’s ancient seas,” says
astrobiologist Dirk Schulze-Makuch of
Washington State University in Pullman.
Of the chance of finding life on Titan,
“I would say it’s probably 50-50,” he says.
Scientists don’t know enough about
how life evolved on Earth to rule out the
possibility of a hydrocarbon-based life-form, though attempting to detect such
critters could be a challenge. “We have to
be very open-minded and creative here,”
Schulze-Makuch says. “If life exists, how
it looks and how it interacts with the environment, that could all be very different.”
While some scientists suggest Titan
Sulfur streaks Red streaks visible on
Europa’s surface (bottom) may be akin to yellow
streaks currently under study in the Canadian
High Arctic (top). The Arctic streaks are rich in
sulfur and may be produced by a community
of microbes living nearby. Scientists are now
looking for remote markers of such life.
might host a deep, water ocean with
aqueous ammonia tens of kilometers
below the crust, such a pocket of liquid
can’t be sailed and any more-familiar
forms of life residing there would be basically inaccessible to any space probes.
Superfecta satellite
The salty water reservoir stirring
beneath Enceladus’ surface, though, is
known to shoot samples into the void
around Saturn.
In 1789, more than a century after
Titan’s discovery, Saturn’s tiny moon
Enceladus first glimmered through
William Herschel’s lens. But no one realized it was a giant water fountain until the
Cassini spacecraft dropped in for a visit,
snapping some astonishing pictures.
In 2005, Cassini swung by the Satur-
nian moon—only 500 kilometers
across — and observed plumes of mate-
rial erupting from its south pole. When
Cassini imaging team leader Carolyn
Porco first saw the jets, she says, she felt
“a strong sense of kinship with those
long ago who first set eyes on the gey-
sering turmoil of Yellowstone.”
The plumes are enormous geysers
spewing icy water and salty particles
hundreds of kilometers into space. The
moon-spit is loaded with organic mate-
rial and not only forms one of Saturn’s
fainter rings, the E ring, but also rains
down on the planet. Enceladus is the
only moon in the solar system known to
influence its planet’s chemistry. Its con-
tribution solves the mystery of where
the water in Saturn’s upper atmosphere
comes from, scientists reported in the
August 2011 Astronomy & Astrophysics.
“The plumes are, in my opinion, the
most spectacular dynamic phenomenon
we’ve discovered at Saturn, and one with
the most profound implications,” says
Porco, of the Space Science Institute in
Boulder, Colo.
Now, scientists think the reservoir
beneath Enceladus’ icy crust feeds the
plumes, as suggested by Frank Postberg
of Heidelberg University and colleagues
in the June 30 Nature. If such a reservoir
is the source of the plumes, liquid water
must exist beneath the crust.
FROM TOP: D. GLEESON, NASA, JPL; NASA, JPL, UNIV. OF ARIZONA, UNIV. OF COLORADO
www.sciencenews.org
