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UC Berkeley paleontologist and geologist Jere Lipps spent a good deal of his career planning expeditions to Antarctica to search for life under the ice, which more or less qualifies him for planning expeditions to Jupiter's icy moon Europa.
After Mars, Europa is the number two pick of astrobiologists for places in our solar system that could harbor life. Two of Saturn's satellites are runners-up: Titan, because of its carbon-rich atmosphere; and icy Enceladus, where cryovolcanoes shoot geysers of water vapor into space. But Europa, an iceball slightly smaller than our Moon, is the clear favorite astrobiologists believe it contains a deep, salty ocean below its frozen crust. Both the ice and the ocean can make fine habitats, Lipps believes.
You might think ice is dead. Au contraire. "Life loves ice," Lipps explains. So long as they don't freeze and can find food, organisms can flourish, as Lipps observed during his years studying the habitats of the microorganisms, crustaceans, fish, and sea sponges that make their home around Antarctica's Ross Ice Shelf.
But, he cautions, there are differences between our own ice-covered oceans and Europa's. For example, Europa doesn't get much sunlight, so there's probably not much photosynthesis. Even creatures that live in Earth's deep seas often depend on a food chain that begins with photosynthetic creatures. Lipps believes that Europa may have other nutrient sources, such as chemicals that well up from the ocean floor.
Europa's surface certainly is more hostile than Earth's it bathes in Jovian radiation, and its surface temperature is estimated at minus 260 degrees Fahrenheit. But the outer few meters of the ice likely shelter everything below, Lipps says, and while the surface is frigid, the liquid water below can't get any colder than it gets on our planet. The tides may generate enough heat to keep Europa's oceans from freezing solid, and scientists have posited other heat sources that could help sustain life, such as radiation and thermal vents in the ocean floor.
If there's any life on Europa, it's trapped beneath a giant slab of ice, and it can't come to the surface without being frozen or irradiated. So how to detect it? Lipps explains that, because ice is constantly on the move, you can do paleontology with it just as easily as you can with rock. On Europa, ice sheets thrust above or below one another, smash together, and crack apart, pushing things once buried to the surface. The tidal action of Jupiter also opens rifts in the ice, which let materials well up from underneath and then freeze along the edges of the cracks. That these cracks have a distinct reddish-brown discoloration delights astrobiologists they wonder if this is residue formed by layers of dead bacteria.
The discolorations also are the sort of thing cameras can spot. For the last several years Lipps has been working with Lockheed Martin on a concept for a telescope called MIDAS (Multiple Instrument Distributed Aperture Sensor) that could fly by Europa and take pictures with a resolution of up to two centimeters from a distance of one hundred kilometers. Lipps' job is to tell them where to point it. "My search strategy for Europa is exactly what I would do if I was going out into the Grand Canyon," he says to look for broken, exposed areas and places where chaos in the terrain might have mixed up the layers and tossed up a residue of biological material. This includes not only the discolored ridges, but impact craters from meteorites, cliffs formed by clashing ice sheets, and places where warmer water from below rose to, and then flattened, the surface ice.
MIDAS is still in the planning stage, and no one knows when, or if, it might fly. Until recently, most scientists interested in Europa had pinned their hopes on NASA's planned Jupiter Icy Moons Orbiter, but the project was scrapped in 2005. Other probes that could penetrate Europa's ice or chip off bits of it to sample are still in the theoretical stage.
That hasn't dampened anyone's enthusiasm for Europa. "The chemicals are there," Lipps says. "There are energy sources there. There's water there, and those three things are what you need for life generally as we know it on Earth. So that's a good sign, that's why we want to go there."
Of course, there's a difference between having the right conditions for life and actually creating it, cautions NASA Ames astrobiologist Lynn Rothschild. She's an expert in what kind of conditions organisms can put up with: She has explored Yellowstone hot springs and Kenyan lakes, finding creatures that live in hot acid baths. She's been to the Bolivian Altiplano to learn about critters that can withstand intense UV exposure. Recently she and her research associate Dana Rogov have been studying salinity-loving microbes that live in the South Bay's Cargill salt flats. These extremophiles have taught her that the kinds of habitats that can support life are much more diverse than we once thought. "The chemical and physical conditions for life, we're increasingly realizing, are not even uncommon in our own solar system," she says. "One can't help but believe that there are literally millions of places where life could form."
That said, she points out, "We have yet to make life. We can't even take a cell that's dead and bring it back to life, and we've got all the building blocks right there." There's a weird gap in human knowledge we know what conditions life needs, and once it exists, Rothschild says, "we have very well-described evolutionary theory to understand how to go from microbes all the way to an elephant or a human or a sequoia." But the crucial step in between the animation of life's raw materials eludes us.
"Maybe we just haven't added the secret ingredient yet or we don't have the right proportions," she continues. "But say that I'm wrong, that there is some incredibly difficult step between having the right chemistry and physics and actually setting something that's alive in motion then it may not be common at all."
She weighs her expectations for what we might find in our solar system with similar caution. Unlike McKay, Rothschild would be delighted to find any life at all, even if it's our evolutionary cousin. (After billions of years of separation, it has probably developed some interesting kinks.) And if it's an independent genesis that hasn't, as the Rare Earthers predict, proceeded much further than pond scum, that's okay, too. "I think if people are sitting around waiting for Europa to come out and shake hands, that's really quite silly," she says. "To me, life is what's exciting. The rest is just details."
In fact, the most exciting part of finding life in our own solar system may boil down to sheer math. Our star is one of two hundred billion stars in our galaxy, the Milky Way, which is one of hundreds of billions of galaxies in the universe. If life originated twice around a single, unremarkable star, then really, the sky's the limit. "The difference between one data point and two data points is more than just double," McKay says. "If we found that life started twice right here in our own solar system, then that to me would be scientifically convincing evidence that life is widespread throughout the universe."
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