It turns out that patterns like these aren't made only by microorganisms growing on cave walls. "It happens on a variety of different scales, usually in places where some resource is in short supply," says Keith Schubert, a Baylor University engineer who specializes in imaging systems and who came to Cueva de Villa Luz to set up cameras for long-term monitoring inside the cave. Grasses and trees in arid regions create bioverm patterns as well, says Schubert. So do soil crusts, which are communities of bacteria, mosses, and lichens that cover the ground in deserts.

If this hypothesis holds up—and it's still only a hypothesis—then Boston, Schubert, and other scientists who are documenting bioverms may have found something crucially important. Until now, many of the markers of life astrobiologists have looked for are gases, like oxygen, that are given off by organisms on Earth. But life that produces an oxygen biosignature may be only one kind among many.

"What excites me about bioverms," says Boston, "is that we've seen them at all these different scales and in all these wildly different environments, and yet the characters of the patterns are very similar." It's highly plausible, she and Schubert believe, that these patterns, based on simple rules of growth and competition for resources, could be literally a universal signature of life. In caves, moreover, even when the microbial communities die, they leave the patterns behind. If a rover should see something like this on the wall of a Martian cave, says Schubert, "it could direct you where to focus your attention."

At the opposite end of North America, the scientists and engineers shivering at Sukok Lake are on a similar mission. They're working at two different locations on the lake, one next to a cluster of three small tents the scientists have dubbed "Nasaville," and the other, with just a single tent, about a half mile away as the crow flies. Because methane gas bubbling from the lake bottom churns up the water, ice has a hard time forming in some places. To snowmobile from one camp to the other, the scientists have to take a curving, indirect route to avoid a potentially fatal dunking.

It was the methane that first drew the scientists to Sukok and other nearby Alaska lakes back in 2009. This common hydrocarbon gas is generated by microbes, known collectively as methanogens, that decompose organic matter, making it another potential biosignature astrobiologists could look for on other worlds. But methane also comes from volcanic eruptions and other nonbiological sources, and it forms naturally in the atmosphere of giant planets like Jupiter as well as on Saturn's moon Titan. So it's crucial that scientists be able to distinguish biological methane from its nonbiological cousin. If you're focused on ice-covered Europa, as Kevin Hand is, ice-covered, methane-rich Sukok Lake isn't a bad place to get your feet wet—as long as you don't do it literally.

Hand, a National Geographic emerging explorer, favors Europa over Mars as a place to do astrobiology, for one key reason. Suppose we do go to Mars, he says, and find living organisms in the subsurface that are DNA based, like life on Earth. That could mean that DNA is a universal molecule of life, which is certainly possible. But it could also mean that life on Earth and life on Mars share a common origin. We know for certain that rocks blasted off the surface of Mars by asteroid impacts have ended up on Earth. It's also likely that Earth rocks have traveled to Mars. If living microbes were trapped inside such spacefaring rocks and survived the journey, which is at least plausible, they could have seeded whichever planet they ended up on. "If life on Mars were found to be DNA based," says Hand, "I think we would have some confusion as to whether or not that was a separate origin of DNA." But Europa is vastly farther away. If life were found there, it would point to a second, independent origin—even if it were DNA based.

Europa certainly seems to have the basic ingredients for life. Liquid water is abundant, and the ocean floor may also have hydrothermal vents, similar to Earth's, that could provide nutrients for any life that might exist there. Up at the surface, comets periodically crash into Europa, depositing organic chemicals that might also serve as the building blocks of life. Particles from Jupiter's radiation belts split apart the hydrogen and oxygen that makes up the ice, forming a whole suite of molecules that living organisms could use to metabolize chemical nutrients from the vents.

The big unknown is how those chemicals could make it all the way down through the ice, which is probably 10 to 15 miles thick. The Voyager and Galileo missions made it clear, however, that the ice is riddled with cracks. Early in 2013 Hand and Caltech astronomer Mike Brown used the Keck II telescope to show that salts from Europa's ocean were likely making their way to the surface, possibly through some of those cracks. And late in 2013 another team of observers, using the Hubble Space Telescope, reported plumes of liquid water spraying from Europa's south pole. Europa's ice is evidently not impenetrable.

This makes the idea of sending a probe to orbit Europa all the more compelling. Unfortunately the orbiter mission the National Research Council evaluated in its 2011 report was deemed scientifically sound but, at $4.7 billion, too expensive. A JPL team led by Robert Pappalardo went back to the drawing board and reimagined the mission. Their Europa Clipper probe would orbit Jupiter, not Europa, which would require less propellant and save money, but it would make something like 45 flybys of the moon in an attempt to understand its surface and atmospheric chemistry, and indirectly the chemistry of the ocean.

All told, Pappalardo says, the redesigned mission should come in at under two billion dollars over its whole life span. If the mission concept goes forward, he says, "we envision a launch sometime in the early to mid 2020s." If that launch takes place aboard an ­Atlas V rocket, the trip to Europa will take about six years. "But it's also possible," he says, "that we could launch on the new SLS, the Space Launch System, that NASA is currently developing. It's a big rocket, and with that we could get there in 2.7 years."