Methane could be forming in Earth's mantle, US scientists have shown. The result suggests that untapped and unexpected reserves of natural gas and oil may exist deep beneath the planet's surface.

Fossil fuels such as oil and natural gas are organic materials made up of carbon and hydrogen. The consensus view is that all commercially viable petroleum and natural gas is made by biological processes - although methane can also be made in small amounts within volcanoes. In fact, the recent detection of methane in Mars's atmosphere has been interpreted as evidence either of ongoing volcanic activity or of life (see 'The search for life on Mars').

Researchers have suggested before that non-biological processes might also be creating stable reserves of fuel within the much more extreme conditions of Earth's mantle, but the idea remained controversial.

Now Henry Scott at Indiana University, South Bend, and his colleagues have made methane in their lab, and shown that it is stable at the intense temperatures and pressures found in Earth's mantle, 100-300 kilometres beneath the surface. They achieved the feat by squeezing together water, iron (II) oxide and calcite between two flattened diamond tips at 50,000-110,000 times the pressure at sea level, and heating the mixture to 1,500 °C.

"Our results show that hydrocarbons can form from non-biological material and be stable, deep within the Earth," says Scott. Because the mantle makes up 80% of the planet's volume, "much more carbon in the Earth could be in the form of hydrocarbons than we previously imagined", he adds. The team's work will appear in Proceedings of the National Academy of Sciences1.

Drill deep
The team next plans to investigate whether oil can be formed under similar conditions. But the methane result has already increased speculation that oil and natural gas reserves may exist in the mantle, ready to be tapped into.

However, it's not clear how commercially significant these potential stores of methane would actually be, says Alan Rankin, a geologist at Kingston University, London.

"You probably won't be able to drill for it, because it's far too deep," he explains. Oil wells usually only drill down to between 5 and 8 kilometres below Earth's surface, into relatively low-pressure regions.

Rankin adds that the best way to obtain methane generated in the mantle would be to look for pockets in the crust where rising gas has become trapped. But he predicts that the amount of gas is unlikely to be significant compared with the oil and gas reserves we already know about. "By the time the gas has reached the upper crust, most of it will have dispersed," he says.

Alien fuel source
The mechanism for methane production might also be at work on other planets. "A hydrocarbon-rich body such as Saturn's moon Titan could produce hydrocarbons throughout its interior," says Scott. "Even near its centre, the pressure is within our experimental range."

Methane could also be present in Mars's mantle, says team member Laurence Fried at the Lawrence Livermore National Laboratory, California. This might explain recent observations of methane in Mars's atmosphere, without any need for martian creatures. But the result could still be good news for those hoping to find life on the red planet. "Methane could be used as a fuel source by life forms," says Fried.