Are we alone? The hunt for ET.
• Starting today, a six-part series of talks filmed before a live audience at The Edge at Melbourne's Federation Square will be broadcast every day this week online at theage.com.au
Hosted by science columnist Peter Spinks, the series features a panel of four scientists who consider the possibility of habitable worlds in the universe, the chances of them harbouring life, what aliens, if they exist, might be like, and how we might one day communicate with them across space and time.
Scientists have found life in the most unusual places.
ET, the Steven Spielberg creation with emaciated torso and limbs, spindly fingers and bulging, hairless head, is one potential model for what alien life might look like. Another might be unlike any bipedal primate, having green metallic skin and purple tentacles for extracting chemicals from the atmosphere.
Whatever form extraterrestrials may take, most are probably no more advanced than the microbes in your gut or garden soil. Some might share features with the supremely resilient extremophiles found in strange places on earth, scientists now believe.
Able to withstand excruciating cold, heat, acidity, radioactivity or high salinity, a few of these earth-bound microbial oddballs flout the basic tenets of biology, such as the need for sunlight or oxygen.
Some bacteria, for instance, eke out a pitch-black existence in the deepest parts of the ocean, such as the 11-kilometre-deep Mariana Trench.
Residents of the coal-black Movile cave below southern Romania also require no light – or even oxygen – and use hydrogen sulphide from surrounding rocks to generate energy. Deep beneath the Mediterranean Ocean, the armour-encased, tentacled Loricifera, 250 micrometres long, also scavenge energy from hydrogen sulphide.
Larger organisms also flourish under extraordinary conditions. For example, nematodes – free-living worms of slender, cylindrical shape – thrive in the water-filled fissures of South African gold mines, more than three kilometres underground.
None are odder than tardigrades, also called water bears, of which more than 1000 species have been listed. The bizarre barrel-bodied beasties bear four pairs of stumpy legs, usually with minuscule claws, and range in length from 0.1 to 1.5 millimetres. Like most segmented invertebrates, they use their elongated, piercing mouthparts to eat plants or bacteria.
Tardigrades show up in moist environs: from lakes, ocean sediments, hot springs, mountain peaks and solid ice to films of surface water found on mosses, lichens, algae and plant litter, as well as in soil, ditches and stone walls. Thanks to their ability to enter a state of extreme hibernation, called a tun, they are regarded as one of the toughest creatures on earth.
Along with managing temperature extremes from as low as near absolute zero (minus 273.15 degrees Celsius) to as high as 151 degrees, tardigrades can survive without water for years, as well as pressures six times greater than those at the bottom of earth's deepest oceans. Many also withstand punishing radiation that would destroy other life forms.
The rugged animals have endured the hostile conditions of open space, once surviving for 10 days aboard a European Space Agency satellite, the Foton-M3 mission launched in September 2007. Neither of two species of dehydrated tardigrades aboard the satellite was affected by the vacuum of space. Some even withstood full exposure to the sun's ultraviolet rays, more than 1000 times stronger in space than on Earth, with a few going on to lay eggs.
Of all the extremophiles known to science, tardigrades are perhaps the likeliest to exist in an alien environment, says Mark Stevens of the South Australian Museum and Associate Professor at Adelaide University's School of Earth and Environmental Sciences.
"They certainly survive our freezers – and their tun resting stage is one of the most resistant to unfavourable conditions of the kind found in outer space."
Melbourne University astrophysicist Alan Duffy agrees. "I can't say if an ET-tardigrade would look like our version, but it would have to have similar capabilities, as the space environment is tough."
He is open to the prospect of life on one of Jupiter's moons, Europa – "as liquid water, elements for life and an energy source all seem to be present there".
If ET is out there, Dr Duffy notes, it most likely resembles these hardy creatures rather than more complex aquatic life-forms.
Monash University astrophysicist Rosemary Mardling also bets on Europa for alien life, although she qualifies: "Such life, if it exists, may or may not be in the form of extremophiles. Perhaps there are places on Europa's surface where there is only ice on top of the ocean, in which case some sunlight may filter through."
Studies of extremophiles on earth teach scientists how adaptable life can be, says Swinburne University astronomer Christopher Fluke. "From simple microbes to tardigrades, life can survive and flourish in unexpected places," he says. "That's good news for the search for life on other worlds."
Scientists are fast learning about the variety of other worlds that might host surface water in liquid form – thought by biologists to be essential for life.
"Exploring the ocean on an exoplanet around another star remains in the realm of science fiction, but perhaps we can begin our search closer to home," Associate Professor Fluke says. "If Europa has a planet-wide ocean beneath its icy surface, it could well host extraterrestrial versions of extremophiles. These creatures would need to withstand incredibly high pressures and get energy from a source other than from sunlight."
Nice and simple
Some scientists argue that ET, if it exists, is likely to start as a relatively simple microbe rather than resembling anything from earth. "Tardigrades are unlikely to have spontaneously evolved in other places," says Melbourne University astrophysicist Katherine Mack. "They might have tagged along on some spacecraft and be sitting on Mars or the moon or something, dormant and waiting for conditions to improve so they can run around again. But I think if we're looking for native extraterrestrial life, we're probably looking for simple microbes."
There could be more complex life-forms too, she admits. "But there's no reason to expect they'd look anything like tardigrades or anything else on earth."
Biologists are constantly surprised to find life in unexpected places: deep under the ocean; trapped in ice; in lakes thought to be inhospitable, Dr Mack reminds. "I don't rule out the possibility of finding life in a place like Saturn's moon Enceladus, where there is a sub-ice ocean that is salty like the oceans of earth, has organic compounds and is heated by the moon's tidal stretching."
Scientists do not know how life began, she adds. "But we know that once you have the basic ingredients, the main requirements for life are some kind of energy gradient, like a heat source, and the ability of molecules to move, as they can in water. The oceans of Enceladus seem to meet those criteria."
Tardigrades are related to arthropods, the animal kingdon's largest phylum, containing more than a million species. It encompasses animals with no backbones, segmented bodies, jointed limbs and external skeletons, such as insects, crustaceans, arachnids, centipedes and millipedes.
Studies suggest that tardigrades survive for ages. "There are reports of them being found in 100-year-old herbarium specimens – that is, dried plant material," says Associate Professor Stevens. "But current evidence suggests their survival limit is within the range of a decade."
One scientist revived specimens of the genus Macrobiotus after nearly seven years, and other researchers reported that tardigrades from the Antarctic survived eight years of storage. "No studies have investigated intervening periods and it's possible they survive anywhere between eight and 120 years," Associate Professor Stevens says.
The tardigrade's trick is to enter a stage known as "desiccant resistance", or cryptobiosis, a curious physiological state in which metabolic activity - the sum total of their biochemical processes - is reduced to an undetectable level. Such a state occurs in plant and animal groups that have adapted to extraordinarily dry conditions.
Associate Professor Stevens and colleagues are now studying tardigrades from Antarctic nunataks, areas of rock emerging above the surrounding ice sheets. "They are probably isolated and survived for over 10 million to 20 million years," he says.
His team brings back to the laboratory samples of soil frozen at minus 20 degrees. "So far, even years on, when sorting and thawing soils, we find tardigrades, nematodes and rotifers [minute aquatic animals using wheel-like organs for swimming and feeding] still alive. They are amazing creatures."
Learn more about nematodes in the Australian curriculum textbook Oxford Big Ideas Science 7 (Oxford University Press, 2012)
Follow the antics of tiny tardigrades at: http://tardigrades.bio.unc.edu/movies/
Explore their biological make-up at: www.iwu.edu/~tardisdp/tardigrade-facts.html
Check out the unmanned Foton-M3 mission at: www.esa.int/esaCP/SEMN5ZMPQ5F-FeatureWeek-0.html
Environmental Science: www.vcaa.vic.edu.au/Pages/vce/studies/envscience/envscindex.aspx
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