Life on Saturn's moon Titan: Who needs water anyway?
The search for life on Saturn's moon Titan shows that organisms appear to thrive on far less water than conventional wisdom holds is needed to keep microbes active and alive.
Organisms with a persistent case of malodorous breath? Blood based on liquid methane? Life forms more like lichen than house cats?
But Dirk Schulze-Makuch suggests something simpler, if just as exotic: single-cell Titan residents, similar to the liquid-asphalt-loving species he and his colleagues have discovered in Pitch Lake, a natural pool of liquid asphalt on the island of Trinidad in the Caribbean.
These organisms thrive on liquid hydrocarbons. Lakes filled with liquid methane and other hydrocarbons cover up to 10 percent of Titan's surface at times -- part of what researchers term the moon's hydrocarbon cycle, similar to Earth's water cycle.
Temperatures there are far colder than those found at Pitch Lake, the researchers acknowledge. Still, they say, the discovery marks Pitch Lake as a useful stepping-off point for trying to understand the potential for life in what they call liquid-hydrocarbon environments on Titan.
Scientific sleuthing in tar pits
Compared with bacteria and other single-celled organisms found at natural pitch seeps such as the La Brea Tar Pits in Los Angeles, the organisms at Pitch Lake "were distinctly different from there," says Dr. Schulze-Makuch, an astrobiologist at Washington State University in Pullman, Wash. "About 30 percent of the species we detected are unknown organisms."
But the more profound implication of the discovery may lay in the observation that these organisms appear to thrive on far less water than conventional wisdom holds is needed to keep microbes active and alive, team members say.
In its Mars exploration program, NASA's mantra has been "follow the water." It's a bumper-sticker phrase that highlights the importance scientists have attached to understanding Mar's climate history. That, in turn, will yield important clues on whether the red planet once hosted -- or may still host -- at least simple forms of organic life.
Yet the team's results suggest that it may be as important to follow the liquid, rather than just liquid water.
From an astrobiology standpoint, "that's probably the importance of this," says Villanova University astronomer Edward Guinan, whose research focuses on studying extrasolar planets and the evolution of sun-like stars. Dr. Guinan is a member of the team making the discovery.
Pitch Lake is the world's largest surface reservoir of liquid asphalt. The hydrocarbon lake is some 250 feet deep and covers nearly 100 acres. This patch of Trinidad is the source for most of the asphalt makes its way north to repave roads and highways along the US East Coast.
It can be an awkward place to work. Tar and asphalt sticks to and stains everything, Dr. Guinan recalls. And it can be a tough lake to leave if, during an attempt to collect samples, one steps on a taffy-like spot in an otherwise stiff surface. "One of my shoes is still there," Guinan says.
The samples of liquid asphalt the team gathered ranged in temperature from around 90 degrees Fahrenheit to 132 degrees.
Gas bubbles were dominated by hydrocarbons including methane, ethane, propane and tiny amounts of butane. Isotope studies of the hydrocarbons suggested that they resulted from the activity of microbes.
But that also presented a puzzle.
The "water activity" in the samples -- a numerical measure of whether water is present in sufficient amounts to sustain chemical reactions in microbes -- was either at or well below the minimum conventional wisdom suggests is needed to sustain microbial life, particularly at the samples' temperatures.
How much water?
Previous research had hinted that external sources of water may not be all they are cracked up to be as a necessary ingredient for life. Five years ago, researchers at the University of Utah found that during the growth phase of E. coli bacteria, 70 percent of the water inside the cells was a byproduct of the cells' conversion of food to energy. It didn't come from the outside, through the cells' walls.
So Schulze-Makuch's team analyzed the samples and found tell-tale signs of life: RNA, amino acids, and fatty acids. Their assays indicated between 1 million and 10 million microbes were present in each gram of material. One gram is around three-hundredths of an ounce.
By analyzing the RNA they found, the group found that the microbes came from two broad biological "domains," bacteria and archaea -- also single-cell organisms. But unlike bacteria, archaea lack a cell nucleus.
The issue of unexpectedly low water activity still remains a question mark, Schulze-Makuch acknowledges. It is possible that the organisms whose genetic material they recovered could inhabit tiny reservoirs of water trapped in the asphalt samples.
Still, Pitch Lake, with its new organisms and hydrocarbon environment, "represents a unique opportunity to evaluate the critical limits for life in the universe," the team concludes, serving "as a useful analog for evaluating the potential for life in liquid hydrocarbon lakes" NASA's Cassini mission to Saturn has discovered on Titan.
The team has submitted its results to the journal Astrobiology, and has posted a draft on the website arXiv.