By Joel Achenbach
Washington Post Staff Writer
Monday, February 23, 2009
Europa vs. Titan. They're two moons in the outer solar system, both circling gas giants but otherwise as alien from each other as alien can be. One orbits Jupiter and is a crusty iceball with signs of a very deep subsurface ocean. The other orbits Saturn and has a thick atmosphere, dramatic weather, lakes of liquid hydrocarbons, methane rain, and sand dunes of organic material the color of coffee grounds.
Both have long been celebrated in film and fiction: Europa has a key role in the movie "2010," the iffy sequel to the classic "2001: A Space Odyssey"; Titan pops up in Kurt Vonnegut's "The Sirens of Titan" as the home of a marooned space traveler from the planet Tralfamadore.
In real life, both are prime targets in the search for life beyond Earth. The problem is that it is neither simple nor cheap to send a probe to these distant worlds. NASA has faced a bureaucratic quandary: Which moon should have priority?
For many months and years, two scientific camps polished their proposals, each hoping that its moon would get official sanction as NASA's next "flagship" mission to the outer solar system. The answer finally arrived last week: Europa, and by extension the whole Jupiter system, will be first.
The mission is still very much in the preliminary phase and would not launch until 2020, with arrival at the Jupiter system in late 2025. NASA hastened to say that this was not a setback for Titan and the Saturn system, which will remain high on the list for a future mission, but officials said the Europa plan was more technically feasible at the moment. NASA will team up with the European Space Agency, which will have its own probe at the same time, focused on another Jupiter moon, Ganymede.
Was Europa the right call? Depends on what you're looking for.
Previous robotic probes -- particularly the Pioneer, Voyager, Galileo and Cassini missions of the past four decades -- have brought the outer solar system into closer view, with each dazzling new image inciting yet more questions about these exotic worlds. But four centuries after Galileo first spotted the moons of Jupiter, they've still been observed only in flyby missions. The same goes for the moons of Saturn. The next step is to park a spacecraft in orbit around one of the moons and scrutinize it with all the instruments we can muster.
Europa, with its liquid water, is more likely than Titan to have life as we know it, though Titan might possibly have exotic chemical processes that would represent life-as-we-don't-know-it.
"Does life require liquid water as the liquid medium, or are other liquids possible hosts for, if not life as we know it, some kind of organized chemistry?" asks Jonathan Lunine, a planetary scientist at the University of Arizona and a member of the team pushing for a Titan mission. "You'd be testing the limits of what the word 'life' really means in the cosmos."
Titan is certainly the more dynamic world, far more Earth-like than Europa, and chockablock with the kind of carbon-based molecules that fascinate organic chemists.
"Titan is a broader and richer scientific target," said Ralph Lorenz, a planetary scientist at Johns Hopkins University. "It's a world that has very familiar processes occurring under very exotic conditions. . . . It's a wonderful laboratory for exploring how planets work."
Another Saturn moon has also elbowed its way into the conversation: Enceladus. The NASA probe Cassini, still orbiting Saturn, discovered that Enceladus has geysers of frozen water shooting from its southern hemisphere. Scientists would love to get a closer look.
But Europa had major advantages in the contest with Titan. From a bureaucratic standpoint, it was simply Europa's turn. Europa was supposed to be the target of a mission that was canceled nearly a decade ago, and it was the top priority in a 2003 National Academy of Sciences review of possible planetary missions. Titan, meanwhile, has been studied recently by Cassini.
The Titan proposal was rather complicated, involving an orbiter, a lander that would splash down in a hydrocarbon lake, and a balloon that would cruise through the atmosphere taking snapshots. The Europa proposal calls for a single spacecraft that will go into orbit after a long tour of some of Jupiter's other moons.
The Russians, meanwhile, have expressed interest in putting a lander on Europa. But NASA has rejected that approach for now, pleading insufficient knowledge of the moon's surface.
"We could land on Europa, but it's very high-risk. The problem is we don't have very many good high-resolution photos. We don't have a good enough feel for what the surface looks like," said Karla Clark, NASA study leader for the Europa mission.
Reta Beebe, a planetary scientist at New Mexico State University, says the ideal spacecraft might be a "hopper," designed to land on the surface, then lift off briefly and essentially hop over various ice boulders and rifts that would probably make the surface impossible to explore with a rover.
"It's getting into science fiction," she cautioned.
Europa, Ganymede and Titan may all have subsurface oceans, but Europa's is closest to the surface. Whether there could be something swimming down there is purely speculative, but we know that life on Earth thrives in the most improbable of places, from hydrothermal vents in the blackest depths of the sea to lakes permanently paved with ice in Antarctica.
"Life itself, simple life, seems to be very, very durable, and very common everywhere," Beebe said. But of life on Europa and on other moons of the outer solar system, she added: "It's not going to be very easy to find. I don't expect people to find life on these bodies in my lifetime."