Moreover, scientists now believe a planet doesn't have to be the same size as Earth to be habitable. That's not all: It turns out that the range of temperatures and chemical environments where extremophilic organisms might be able to thrive is also greater than anyone at Drake's meeting could have imagined. In the s oceanographers such as National Geographic Explorer-in-Residence Robert Ballard discovered superheated gushers, known as hydrothermal vents, nourishing a rich ecosystem of bacteria.
Feasting on hydrogen sulfide and other chemicals dissolved in the water, these microbes in turn feed higher organisms. Scientists have also found life-forms that flourish in hot springs, in frigid lakes thousands of feet below the surface of the Antarctic ice sheet, in highly acidic or highly alkaline or extremely salty or radioactive locations, and even in minute cracks in solid rock a mile or more underground. The one factor that biologists argue is critical for life as we know it is water in liquid form—a powerful solvent capable of transporting dissolved nutrients to all parts of an organism.
In our own solar system we've known since the Mariner 9 Mars orbiter mission in that water once likely flowed freely on the red planet. So life might have existed there, at least in microbial form—and it's plausible that remnants of that life could still endure underground, where liquid water may linger. Jupiter's moon Europa also shows cracks in its relatively young, ice-covered surface—evidence that beneath the ice lies an ocean of liquid water.
At a half billion miles or so from the sun, Europa's water should be frozen solid.
Ecologists Study the Interactions of Organisms and Their Environment
But this moon is constantly flexing under the tidal push and pull of Jupiter and several of its other moons, generating heat that could keep the water below liquid. In theory, life could exist in that water too. In NASA's Cassini spacecraft spotted jets of water erupting from Saturn's moon Enceladus; subsequent measurements by the spacecraft reported in April of this year confirm an underground source of water on that moon as well.
Scientists still don't know how much water might be under Enceladus's icy shell, however, or whether it's been liquid long enough to permit life to exist. The surface of Titan, Saturn's 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.
Astrobiology - The Hunt for Life Beyond Earth
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 microbes, if they existed. A cave in Mexico isn't Mars, of course, and a lake in northern Alaska isn't Europa.
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. 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 unlovely droplets as "snottites. It's actually a biofilm, a community of microbes bound together in a viscous, gooey blob.
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The snottite microbes are chemotrophs, Boston explains. Snottites are just one of the microbial communities that exist here. Boston, of the New Mexico Institute of Mining and Technology and the National Cave and Karst Research Institute, says that all told there are about a dozen communities of microbes in the cave. Each one is tapping into different nutrient systems.
One of these communities is especially intriguing 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 hieroglyphics.
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Astrobiologists have come to call these patterns biovermiculations, or bioverms for short, from the word "vermiculation," meaning decorated with "irregular patterns of lines, as though made by worm tracks. Microbe-rich biofilms—referred to as snottites—drip from the sunless walls of Cueva de Villa Luz in Mexico.
Journey to the Centre of the Earth
Living off sulfur compounds, the microbes in turn feed midges that live in the cave. It turns out that patterns like these aren't made only by microorganisms growing on cave walls. 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. 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.
W hen I was a child, around the corner from my home in east London lived a man named William Lyttle.
Could There Be Life on Mars Today?
For decades Lyttle had been digging a series of tunnels underneath his house. He lived out the last few years of his life far above the ground in a high-rise block. The Mole Man of Hackney, Will Hunt argues in his winningly obsessive history of our relationship with underground places, might best be understood as an evolutionary throwback. As a species, he says, humankind has always been fascinated with what lies beneath the surface of the world. This obsession had practical origins — we descended underground to mine precious minerals, or to seek protection from enemies and predators — but it also served more spiritual purposes.
The ancient Greeks went into caves in search of visions and wisdom. At Lascaux and Chauvet the Magdalenians, sophisticated pre-modern peoples who lived in western Europe during the Palaeolithic era, drew extraordinary pictures on their walls. Hunt first became interested in such places when, as a teenager, a teacher told him about an abandoned railway tunnel that ran near his home in Providence, Rhode Island. He broke in with some friends and was entranced by the strangeness of the subterranean world he had uncovered, and the sense of transgression exploring it provided.
He has been going underground ever since. It began as a quest to understand my own preoccupation; but with each descent … a more universal story emerged. I saw that we — all of us, the human species — have always felt a quiet pull from the underground, that we are as connected to this realm as we are to our own shadows. The genre is based on and has in turn influenced the Hollow Earth theory.
The earliest works in the genre were Enlightenment -era philosophical or allegorical works, in which the underground setting was often largely incidental.
In the late 19th century, however, more pseudoscientific or proto-science-fictional motifs gained prevalence. Common themes have included a depiction of the underground world as more primitive than the surface, either culturally, technologically or biologically, or in some combination thereof.