Science News - October 8, 2011

Scientists have a few proposals for getting up close and
personal with any life that may be lurking on an outer solar
system moon. The Titan Mare Explorer, in the running with
three other Discovery class missions for approval in 2012,
would splash down in one of Titan’s hydrocarbon lakes and
sail the planet’s seas to learn more about its chemistry.
Another one of Saturn’s moons, Enceladus, could be the
future destination of a mission aptly named “LIFE” for “Life
Investigation For Enceladus.” During a ;yby costing less
than $500 million, the craft would collect samples from
jets spewing out of the orb’s interior
and return the samples for lab analysis. The
low price of the mission could be a major sell-
ing point, especially during today’s tight funding
environment. A trip planned to Europa, called the
Jupiter Europa Orbiter (shown), could get chopped
if scientists can’t ;nd a way to lower its $4.7 bil-
lion price tag. The orbiter would circle Europa to gain
insight into its potential for habitability, including mea-
suring the thickness of its icy crust. — Elizabeth Quill

Porco says salt concentrations in the plume hint at a reservoir in contact with rock, which means the same kinds of chemical reactions that take place on Earth (and can support life that does not rely on photosynthesis) might be occurring on the moon.

It’s also warm, at least zero degrees Celsius: “a balmy place for lots of organisms to thrive,” Porco says.

The plumes make the moon’s innards relatively easy to study, which is one reason Porco, McKay and many other scientists favor a mission to Enceladus.

“The samples are coming out for free,”
McKay says. “It’s like there’s a big sign
saying, ‘Free samples, take one.’ ”
Enceladus has water, organics, nitro-
gen and an energy source —the only
place in the solar system with all four
boxes checked, McKay says. “If I had a
little scooter and could fly anywhere,
the first place I would fly is the plume of
Enceladus,” he says.

A mission to Enceladus could be much more focused than a visit to, say, Europa, Porco says, since scientists know where all the action is. There are at least 70 jets bursting from fractures at the south pole, the most vigorous of which are associated with the hottest locales. A probe could land near the plume and collect the contents as they rain down, she says. “It could be snowing microbes on the south pole.”

But understanding of Enceladus’ geochemical processes is still young, and some scientists worry that they don’t know enough about the moon to send a mission there yet. Pappalardo, who studies Europa, raises the question of how long

a subsurface ocean might have existed — whether it’s a transient phenomenon or persistent enough to stick around and allow life to evolve under the ice.

While some models suggest a global ocean is unlikely to last long enough for life to develop, a small regional sea might, Porco says. “The only remaining question is: Is prebiotic chemistry, or perhaps even life, stirring beneath the south pole of Enceladus?” she says. “And the only way we’ll know the answer to that is by going back to Enceladus, properly equipped to find out.”

Making choices

Though scientists are putting together the pieces of these moony puzzles bit by bit, the pictures will be blurry as long as assembly is from afar. So far, researchers know that Europa is probably a water world, but might not have organics, nitrogen or a life-powering energy source. Titan is saturated with organic material and shrouded by nitrogen, but lacks liquid water on its surface. And Enceladus is superpowered, fulfilling all four of McKay’s criteria, but its subsurface pocket of water might be too young for life to have evolved.

Because there is still so much to find out, wherever Earth’s robotic emissaries arrive next, they will help sate the curiosity of a legion of scientists. There is no bad target.

“Asking me if I think we should send
probes to Enceladus or Titan instead
of Mars or Europa is like taking a kid to
Disneyland and telling them they can
only go on one ride,” says Nathan Strange,
a mission architect at NASA’s JPL who
has worked on mission design for all three
moons. “Why not let us go explore all of
these places?”
Charles Elachi, director of JPL, thinks
that’s possible. “From a technical point
of view, they’re within our engineering
capabilities of the next 10 years,” he says.
“I don’t have a favorite. As a scientist, the
ideal is to go to all three of them. But I
would add Mars, too.”
The limiting factor is funding, he
says. Sending large, multibillion-dollar
flagship missions to each of the moons,
such as a proposed craft called the Jupi-
ter Europa Orbiter, isn’t possible under
the current funding climate. So if scien-
tists can’t send one large mission to each
moon, they’ll have to decide how else to
distribute the work.