National Geographic : 2014 Jul
36 national geographic • july 2014 largest moon, has rivers, lakes, and rain. But Titan’s meteorological cycle is based on liquid hydrocarbons such as methane and ethane, not water. Something might be alive there, but what it would be like is very hard to guess. Mars is far more Earthlike, and far closer, than any of these distant moons. The search for life has driven virtually every mission to the red planet. The NASA rover Curiosity is currently exploring Gale crater, where a huge lake sat billions of years ago and where it’s now clear that the chemical environment would have been hospitable to mi- crobes, if they existed. A cave in Mexico isn’t Mars, of course, and a lake in northern Alaska isn’t Europa. But it’s the search for extraterrestrial life that has taken JPL astrobiologist Kevin Hand and the other members of his team, including John Leichty, to Sukok Lake, 20 miles from Barrow, Alaska. The same quest has lured Penelope Boston and her colleagues multiple times to the poisonous Cueva de Villa Luz, a cave near Tapijulapa in Mexico. Both sites let the researchers test new techniques for searching for life in environments that are at least broadly similar to what space probes might encounter. In particular, they’re look- ing for biosignatures—visual or chemical clues that signal the presence of life, past or present, in places where scientists won’t have the luxury of doing sophisticated laboratory experiments. Take the Mexican cave. Orbiting spacecraft have shown that caves do exist on Mars, and they’re just the sorts of places where microbes might have taken refuge when the planet lost its atmosphere and surface water some three billion years ago. Such Martian cave dwellers would have had to survive on an energy source other than sunlight—like the dripping ooze that has Boston so enchanted. The scientists refer to these unlove- ly droplets as “snottites.” One of thousands in the cave, varying in length from a fraction of an inch to a couple of feet, it does look uncannily like mucus. It’s actually a biofilm, a community of mi- crobes bound together in a viscous, gooey blob. The snottite microbes are chemotrophs, Boston explains. “ They oxidize hydrogen sulfide—that’s their only energy source—and they produce this goo as part of their lifestyle.” Snottites are just one of the microbial commu- nities that exist here. Boston, of the New Mexico Institute of Mining and Technology and the Na- tional Cave and Karst Research Institute, says that all told there are about a dozen communities of microbes in the cave. “Each one has a very distinct physical appearance. Each one is tapping into different nutrient systems.” One of these communities is especially in- triguing to Boston and her colleagues. It doesn’t form drips or blobs but instead makes patterns on the cave walls, including spots, lines, and even networks of lines that look almost like hi- eroglyphics. Astrobiologists have come to call these patterns biovermiculations, or bioverms for short, from the word “vermiculation,” mean- ing decorated with “irregular patterns of lines, as though made by worm tracks.” 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 cam- eras 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 Based on the discoveries of larger planets, scientists calculated that more than a fifth of stars like our sun harbor Earthlike planets. n Society Grant Penelope Boston’s research was funded in part by your Society membership.