Fertile Frontiers

The solar system’s spotted bully and its ringed sidekick are holding some tantalizing treasures in their gravitational clutches. Circling Jupiter and Saturn are more than a hundred moons, including some of the most promising hosts for extraterrestrial life in the solar system.

But not every one of these moons is an equal opportunity extraterrestrial petri dish. Scientists are now debating which might be best for a life-seeking mission. Their attention is focused on a frozen trio: Titan, Enceladus and Europa.

For centuries, these satellites appeared in the sky as mere points of light. Now, the three moony musketeers have personalities. Enormous Titan is exotic, the home of hydrocarbon lakes and a thick atmosphere. Tiny Enceladus spits salty water into the void around Saturn. And deceptively placid, ice-crusted Europa probably hosts a sloshing ocean so deep it tickles the moon’s rocky mantle.

Scientists don’t expect to find Europan plesiosaurs or Titanian redwoods, of course. But some experts think these moons may be the best chance for turning up tiny, animated microbes — or at least their footprints.

“It’s worth noting that the three strongest candidates are all in the outer solar system,” says Alan Stern of the Southwest Research Institute in Boulder, Colo. Indeed, these far-flung worlds might even trump Earth’s nearest planetary neighbors. “The inner planets are not such good candidates. Venus, Mercury — not even on the list. Mars? Not as high as these three.”

Deciding which of the three worlds to visit isn’t exactly spawning a knock-down, drag-out fistfight, but some scientists have their favorites. Such preferences are based on characteristics thought to make a habitat friendly to familiar — or unfamiliar — flavors of life.

“When we search for life beyond Earth, I think there are four things we need to look for,” says Chris McKay of NASA’s Ames Research Center in Mountain View, Calif. First, he says, is liquid water. “That’s always the starting point.” Second, organic material, which means carbon-containing compounds. “Water is what life lives in; organic material is what it’s made of.” Third is nitrogen. “You can think of it as the secret ingredient. You’ve got to add that to make the 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.

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.”

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.

Otherworldly option

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 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 Saturnian moon — only 500 kilometers across — and observed plumes of material 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 geysering 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 material 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 contribution 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.

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, nitrogen 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 Jupiter Europa Orbiter, isn’t possible under the current funding climate. So if scientists can’t send one large mission to each moon, they’ll have to decide how else to distribute the work.

Stern suggests planning smaller trips, less than a billion dollars each (the Titan Mare Explorer falls into this cost category) that could begin to form a foundation for future visits.

“You can’t think about the exploration of any of these objects as a one-time trip,” says Carl Pilcher, director of NASA’s Astrobiology Institute in Mountain View, Calif. “Each mission, we develop a deeper understanding that helps us ask better-informed questions. It’s kind of like building a cathedral: It takes a hundred years or more, and each generation passes the torch to the next.”