A new test that produced methane under conditions mimicking the deep interiors of Earth and Mars lends support to an idea that the gas could theoretically support unseen colonies of microbes on both worlds. And the study hints at the possibility of a potential vast supply of petroleum products.
While the lab work doesn't reveal what's really down there, it has nudged a controversial theory about what's under our feet one step closer to the mainstream.
The research was led by Henry Scott of Indiana University at South Bend and was published online last week by the National Academy of Sciences.
Methane, the main component of natural gas, is a hydrocarbon along with other petroleum products like oil and coal. All are the remains of dinosaurs, plants and other things long dead.
At least that's the conventional thinking, based on the knowledge that petroleum products are laced with biological molecules.
But other scientists have offered a radical alternative. One of them, the late Thomas Gold, of Cornell University, theorized in the 1990s that petroleum products are instead created from gaseous hydrocarbons, like methane, that have been inside Earth since it formed 4.5 billion years ago. Gold figured the gas migrates from the mantle toward the surface through cracks in the planet's crust. Some is trapped, and some is chemically altered into oil and coal.
The lab work shows that part of Gold's theory is plausible. But how does Gold explain the biological molecules found in oil?
His still-controversial argument was that microbes live down there and eat the oil and other hydrocarbons, using them as energy sources instead of the Sun.
Gold proposed there was as much life below, in terms of mass, as on the surface.
Several discoveries have shown that life can indeed thrive in seemingly inhospitable conditions. In 2002, scientists reported finding organisms 660 feet (200 meters) below Idaho. They eat hydrogen and belch methane. Other microbes are known to use methane for energy. And life has been found to endure temperatures of 212 degrees Fahrenheit (100 Celsius) around hot springs under the sea. Gold's creatures would only need to endure 248 degrees Fahrenheit (120 Celsius) to live a few miles down, he calculated.
Scott's research team squeezed iron oxide, water and calcite to intense pressure and applied the sort of heat common to Earth's mantle -- up to 2,700 degrees Fahrenheit (1,500 degrees Celsius). Methane formed in a simple chemical reaction.
"Our experiments and calculations show that a simple hydrocarbon, methane, is stable at reasonable conditions for the upper mantle," Scott told SPACE.com. "As a result, if there are primordial hydrocarbons [in the mantle] they should be stable, and other carbon sources could indeed react to form methane."
Petroleum companies generally drill down no more than 5 miles (8 kilometers). The lab results show methane could be stable well down into the mantle, even below 62 miles (100 kilometers), said study member Laurence Fried of the Lawrence Livermore National Laboratory. That raises the possibility for a whole lot of heating fuel.
"Due to the huge volume of the mantle -- about 20 times that of the currently exploited surface -- the reserves could potentially be larger than those at the surface," Fried said.
"We have not, however, shown that such reserves actually exist," Fried cautioned. "Furthermore, I am not aware of a present-day technology for exploiting the reserves."
Nonetheless, the study is important, said Freeman Dyson, professor emeritus at the Institute for Advanced Study at Princeton.
"Not because it settles the question whether the origin of natural gas and petroleum is organic or inorganic," Dyson said, "but because it gives us tools to attack the question experimentally. If the answer turns out to be inorganic, this has huge implications for the ecology and economy of our planet as well as for the chemistry of other planets."
The meaning for life
The most immediate effect of the study may be on the thinking of astrobiologists.
In March, scientists announced the European Space Agency's Mars Express probe had found persistent traces of methane in the atmosphere of Mars. Methane can be produced in volcanic eruptions, but since there's no apparent volcanic activity on Mars, the methane was taken by some scientists to be a tantalizing sign of possible microbial activity.
The new lab work clouds that view, revealing that atmospheric methane may not be the useful "biomarker" some scientists had hoped for in the search for life at Mars or beyond our solar system.
"It reaffirms that the presence of methane on other planets -- such as the recent findings on methane in the Martian atmosphere -- is not necessarily indicative of life," said University of Toronto geologist and chemist Barbara Sherwood Lollar, who was not involved in either study.
The lab research is also a reminder that ET could be very hard to find, thriving inside a planet rather than on the surface. Chemically produced methane far underground could act as fuel "which deep subsurface microbial communities may access for energy and growth," Sherwood Lollar said in an e-mail interview.
The search goes on
Sherwood Lollar and others are currently studying the possibility of deeply entrenched life here on Earth, with funding from NASA's Astrobiology Institute. The work is intended to help the space agency "design strategies and techniques for sampling the Martian subsurface," she said.
Thomas Gold would have been pleased to learn of the new finding. He died June 22 at age 84.
Gold's idea of a huge subterranean ecosystem isn't the only highly debated thought he expressed. Gold teamed with Fred Hoyle and Hermann Bondi to lay out the steady state theory of the universe, in which there is no beginning or end. Most theorists now adhere to the Big Bang model instead. In 1955, Gold sparked controversy by suggesting the Moon's surface is covered with a fine rock powder. The Apollo 11 crew brought some of that powder back and proved him right in 1969.
Until now, Gold's theory of "The Deep, Hot Biosphere" (the title of a 1992 paper and a 1999 book) has remained controversial.
"This is exactly what he has been predicting for a long time," said Carvel Gold, his wife, of the methane study, "and it is sad that the news came just three months too late for him to enjoy it."
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