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.
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Yet the team's results suggest that it may be as important to follow the liquid, rather than just liquid water.Skip to next paragraph
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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.