In the mid-1960s, the United States Army Corps of Engineers carved a giant cave deep into permafrost in an Alaskan hillside so that scientists could do experiments inside ground that had been frozen for millennia. Entering the 110- meter Cold Region Research and Engineering Laboratory Permafrost Tunnel, as the facility is now known, is like “stepping right back into the Pleistocene,” says astrobiologist Richard Hoover. Frozen, long-dead tree roots and grasses dangle from the ceiling, and the bones of an extinct species of buffalo poke out of the walls.
But Hoover doesn’t visit the tunnel to scope out the remains of such flora and fauna. He’s there to search for living aliens—not extraterrestrials per se, but creatures so unusual that they can survive the tunnel’s harsh cold and dry environment. Similar conditions could exist on other planets, so finding living things soldiering on deep inside the permafrost could be a sign that life exists beyond Earth, says Hoover, who’s based at NASA’s National Space Science and Technology Center in Huntsville, Ala.
Until someone detects the first genuine extraterrestrial, it’s impossible to say whether any organisms reside beyond our friendly planet. But mounting evidence suggests that Earth might not be the only oasis in the cosmos, or even in our solar system.
To speed the search for extraterrestrial life, researchers are using extreme conditions on Earth to develop a flotilla of detection devices to tease out signs of life in unlikely places. By sending these machines on scouting missions to frozen ice caps, deep oceans, and subterranean environments on our planet and someday others, researchers may eventually find that we’re not alone in the universe after all.
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Extraterrestrials have long been a mainstay of science fiction, ranging from the terrifying Martians in War of the Worlds to the ugly but adorable ET. For many scientists, however, the real possibility of life on other planets is more exciting than anything fiction can offer.
Some researchers have considered more-exotic possibilities, but a carbon-based biochemistry similar to that of life on Earth is most likely, says Douglas Hudgins, an astronomer at NASA Ames Research Center in Moffett Field, Calif. “The variety of chemical compounds you can make based on carbon is vastly bigger than [you can make from] any other element in the periodic table,” he notes.
In any case, understanding familiar forms could now set the stage for recognizing stranger life in the future, he adds.
Researchers have long known that tiny fragments of one planet, such as meteors, can crash onto another planet’s surface. This gives life the potential to jump from place to place in our solar system. And recent research suggests that the ingredients necessary for life as we know it to evolve from scratch are present in many places besides Earth. The ingredients include liquid solvents, such as water; a variety of elements, such as nitrogen and phosphorus, for constructing organic molecules; and compounds that store energy.
According to Bruce M. Jakosky, director of the Center for Astrobiology at the University of Colorado in Boulder, the planet Mars and Jupiter’s moon Europa offer tantalizing combinations of these ingredients right in our celestial neighborhood.
As for the possibilities farther away, a study that Hudgins and his colleagues published in the Oct. 10, 2005 Astrophysical Journal suggests that life’s building blocks are probably scattered heavily throughout the universe.
By comparing the patterns of infrared radiation emitted by gas clouds, stars, and other celestial objects with the characteristic infrared signals of molecules in the lab, astronomers can identify the celestial compounds. Almost everywhere that astronomers have applied this technique during the past 20 years or so, they’ve found polycyclic aromatic hydrocarbons (PAHs), rings of carbon and hydrogen that have linked to form a variety of complex organic molecules. “If you’re interested in life, then you’re interested in organic carbon,” says Hudgins.
However, he notes, scientists searching for extraterrestrial life haven’t been interested in most PAHs because the molecules aren’t known to play important roles in organisms on Earth.
But while investigating the infrared signals put out by interstellar PAHs, Hudgins and other researchers recognized an anomaly. The signal didn’t entirely match the one given off by PAHs in the lab. When Hudgins and his team swapped one of the lab-made molecules’ carbons for a nitrogen atom, the new infrared signal matched the cosmic one.
The scientists were excited by this finding because the resulting molecules, known as polycyclic aromatic nitrogen heterocycles (PANHs), function as the heart of many larger molecules important to life. For example, PANHs serve at the core of chlorophyll, the green molecule on which photosynthesis depends, and of hemoglobin, the molecule that holds oxygen in red blood cells.
“The vast majority of what we’ve always called PAHs [in space] are in fact PANHs,” Hudgins asserts. PANHs arise primarily in the cool, dense interstellar clouds out of which stars and planets eventually form. Although organic molecules could not survive on the hot surface of a young planet, some could get trapped in small rocky fragments that could crash onto planets much later, when their surfaces had cooled to friendlier temperatures.
“What this means is that if you sprinkle these molecules on a nice, hospitable planet, they can be used as a raw material for developing life,” Hudgins says.
Right at home
On the crucial question of what constitutes hospitable conditions, scientists have recently had to revise their views. Previously, many researchers thought that the conditions necessary for life were so narrow that they existed only in places with certain favorable conditions on Earth. However, the study of this planet’s extremophiles—organisms that can withstand conditions beyond those that people can survive—is extending life’s known limits. And if organisms can inhabit a wider range of environments on Earth than had been suspected, the prospects for living organisms to develop on other planets look brighter.
For example, Hoover found colonies of bacteria when he sampled a layer of golden-brown sludge from the bottom of a frozen Pleistocene pond that was cut open to make the permafrost tunnel near Fox, Alaska. To his astonishment, as the ice containing the microbes thawed under a microscope, Hoover saw the rod-shaped cells start swimming around—picking up where they’d left off 32,000 years ago. “I knew immediately that we had living Pleistocene bacteria,” says Hoover.
The previously unknown species, which he and his colleagues named Carnobacterium pleistocenum, is a good model for life-forms that he and other researchers suspect could exist in a similar suspended state in Mars’ polar ice caps or in the ice crust on Europa, he adds.
In a very different part of the environmental spectrum, Raina Maier and her colleagues at the University of Arizona in Tucson recently found living bacteria in the most arid soils of the Atacama Desert of northern Chile. Some places there go thousands of years without receiving a drop of rain. Scientists frequently use the Atacama as an analog for the dry midsection of Mars.
Although previous research had suggested that some parts of this desert are sterile, when Maier and her team dug a little deeper in the soil, they found DNA signatures for several species of microbes. They even coaxed a few of these species to grow in the lab, the team reported in 2004.
These microorganisms “may be dormant for very long periods of time, but they’re still there, waiting patiently for … some rain, some organic matter that blows in that they could eat, or other good conditions,” Maier says. Then, the bacteria spring back to life.
It’s possible that microbes could be waiting in similar conditions under the dry surface of Mars or of planets outside our solar system, Maier says.
Look lively, now
Scientists won’t be building people-staffed labs on other planets anytime soon. Instead, some researchers are developing systems to detect life remotely in many environments.
Hans E. F. Amundsen of the University of Oslo and his colleagues have grouped together four instruments that they assert can spot a single microbe in an otherwise-barren area—the biological equivalent of finding a needle in a haystack.
Their system searches for different cellular materials: bacterial genes, components of bacterial cell walls, the energy-storing molecule called adenosine triphosphate (ATP), and various proteins. A hit from all four instruments is an almost certain indication of life, says Amundsen.
“Whenever you get lucky enough to send something to Mars, you need an answer you want to be certain of,” he says. “We wanted to get the same answer with several different techniques.”
The researchers recently tested their system in Svalbard, Norway, an Arctic island that seems to have geological conditions similar to those of the polar ice caps on Mars. When they ran ice samples from the island’s frozen volcanoes through their suite of instruments, the researchers detected rare microorganisms living in an almost dormant state.
Right now, the instruments are far too bulky and fragile to attach to a rover, says Amundsen. But the researchers expect further revisions to eventually miniaturize the machines and make them more robust, putting them in the running for a future trip to Mars.
Another system, being developed by Adam Schultz at Oregon State University and his colleagues, could set the stage to look for evidence of life in extraterrestrial oceans, such as those that might exist on Jupiter’s moon Europa.
“Europa is an icy world on the surface, but we have very strong evidence that below its surface is a very deep ocean,” says Schultz. “We’re almost certain that something exists there that’s equivalent to a seafloor hydrothermal system on Earth.”
Hydrothermal environments in Earth’s oceans, such as the 50,000-kilometer-long string of underwater volcanoes that forms the midocean ridge system, can be virtual smorgasbords of life. Often, researchers can see dense colonies of bacteria rising from cracks in the seafloor like “upward-flowing snow,” Schultz notes.
He and his colleagues are designing their machine to isolate water seeping from the ocean floor and to test its temperature and chemical composition. These factors could reveal whether that environment is conducive to life.
The water sample would then be shuttled to a holding chamber to be maintained at a constant pressure. That’s critical, says Schultz, because many microbes that live on the seafloor perish at sea level atmospheric pressure.
While the machine itself wouldn’t be flown on a future planetary mission, says Schultz, “it is a working test bed for sensors and devices that would be appropriate for just those sorts of missions.”
A third system, designed by David Wettergreen and Alan Waggoner of Carnegie–Mellon University in Pittsburgh and their colleagues would add a new life-detecting component to rovers similar to the ones currently surveying Mars. The system relies on four fluorescent dyes that attach to carbohydrates, proteins, DNA, or fatty molecules called lipids. These four materials are often present in life on Earth.
The machine would spray a target area with the four dyes. Then, using a bright xenon flash to make the dyes fluoresce, a camera would take four pictures through each of four filters tuned to the wavelengths emitted by each dye.
The rover would then beam the images to a computer, which would compare all the photos. In tests in Chile’s Atacama desert, three of the dyes identified microbes and other life as expected. “We like to see all four signals to confirm life,” says Waggoner. He and his team hope to have a perfected version ready to go for one of the next rover missions to Mars.
How will scientists make sure that the extraterrestrials aren’t just Earth life that hitched a ride on a previous mission? And if the search for life is ultimately successful, what will researchers do with the newfound aliens? Both questions fall into the realm of John Rummel, NASA’s planetary protection officer.
Rummel admits that his job title sometimes prompts snickers when he meets people for the first time. “They’d much rather have me chasing down aliens in the street à la Men in Black, but it’s really a different kind of job. You don’t get a badge or a gun, so you can’t retire early,” he jokes.
Instead, he and a group of other scientists design and enforce protocols to protect other planets from becoming accidentally colonized with Earth’s microbes. They also create guidelines to shield our planet from any dangerous microbes that may someday be brought back from other planets.
Planners for the recent Galileo mission to Jupiter, for example, had originally intended to leave the spacecraft orbiting the giant planet once its job was done. But Rummel and his team insisted that the craft be destroyed by plunging it into Jupiter’s dense, hot atmosphere. Otherwise, it might accidentally crash into Europa, where terrestrial organisms could conceivably prosper.
For spacecraft intended to land on some other body, sterilization protocols depend on the destination. Most scientists consider the surface of the moon to be extremely inhospitable to life, so moon missions require little more than a basic cleaning.
For missions to Mars, the guidelines are variable. Journeys to the arid regions there require that rovers get a thorough cleaning but stop short of sterilization because Earth microbes aren’t considered hardy enough to survive conditions on the planet. However, the protocols mandate full sterilization for any missions carrying life-detection equipment or going to places where there might be liquid water. Rummel and other researchers are exploring a variety of chemical and heat methods for cleaning and sterilization.
Rummel says that containment is the key to protecting Earth from possible alien germs brought back from future sampling missions. Current guidelines recommend that researchers treat extraterrestrial samples as highly dangerous until proved safe.
Nobody yet knows whether those safeguards are necessary. Until scientists come up with hard evidence of life beyond Earth, Rummel says, the existence of life elsewhere ultimately comes down to belief. “I have faith in a view of the universe that doesn’t make us the only fluke,” he says.