By Environment Correspondent Alister Doyle

OSLO (Reuters) - Scientists have produced hydrogen by accelerating a natural process found in rocks deep below the Earth's surface, a short-cut that may herald the wider use of what is a clean fuel, a study showed on Sunday.

Used in rockets and in battery-like fuel cells, hydrogen is being widely researched as a non-polluting fuel, but its use is so far hampered by high costs. A few hydrogen vehicles are already on the roads, such as the Honda FXC Clarity and Mercedes-Benz F-Cell, and more are planned.

Researchers in France said aluminum oxide speeded up a process by which hydrogen is produced naturally when water meets olivine, a common type of rock, under the high temperatures and pressures found at great depths.

"We have overcome a preliminary step for a carbon-free energy production," lead researcher Muriel Andreani of the University Claude Bernard Lyon 1 in France told Reuters.

The addition of aluminum oxide accelerated the natural process by between 7 and 50 times, using temperatures of between 200 and 300 degrees Celsius (400-570 Fahrenheit) at a pressure equivalent to twice the depth of the deepest ocean.

In the process, olivine turns into the mineral serpentine and water splits into its components, hydrogen and oxygen.

Currently, the most widely used technology for producing hydrogen - separating it from natural gas - requires far higher temperatures of 700 degrees Celsius (1,300 Fahrenheit) and releases heat-trapping carbon dioxide as a by-product.

Using lower temperatures would save energy and money.

WATER VAPOUR

Fuel cells, which meld hydrogen with oxygen in the air to yield electricity, emit only water. That makes them attractive as a way to cut greenhouse gas emissions and air pollution.

Far more research is needed to see if the French findings could be increased to a commercial scale, said Jesse Ausubel of the Rockefeller University in New York.

"Scaling this up to meet global energy needs in a carbon-free way would probably require 50 years," he said in a statement. "But a growing market for hydrogen in fuel cells could help pull the process into the market."

The findings will be presented to the American Geophysical Union, meeting in San Francisco from December 9-13, after an initial report in the journal American Mineralogist in October.

The work is part of the Deep Carbon Observatory (DCO), a 10-year project due for completion in 2019 involving 1,000 researchers in 40 nations.

Among puzzling DCO findings, experts said, microbes living in tiny fissures in deep rocks where hydrogen forms naturally, often continents apart, seem to be related to one another in what may be a "deep subterranean microbe network".

Matt Schrenk of Michigan State University said life extended to at least 4 or 5 km (2.5-3 miles) deep under land, and it was unclear how similar microbes had spread to places as far apart as South Africa, North America and Japan.

"It is easy to understand how birds or fish might be similar oceans apart, but it challenges the imagination to think of nearly identical microbes 16,000 km apart from each other in the cracks of hard rock at extreme depths," he said.

The DCO project is trying to combine chemistry, biology, geology and even astrophysics. "It is a path to discovery between new areas of science," Robert Hazen, head of the DCO at the Carnegie Institution of Washington, told Reuters.

(Editing by Mark Heinrich)