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Digging in dirt and rock is a big business. Oil and gas lie beneath Earth's surface in certain places, and these reservoirs are the planet's main sources of fuel. Oil and gas found near Earth's surface are often described as fossil fuels. Most scientists favor the idea that these hydrocarbon fuels were formed by the breakdown of ancient plants and animals. However, recent research also shows that methane gas can form in the crust when there are no living creatures around. Until now, all the digging has happened only in Earth's outer layer, called the crust. Oil and gas wells normally go no deeper than about 6 kilometers. A new study shows that natural gas, mainly methane, may also form in a much deeper layer called the mantle. This means that new sources of energy could lie at depths of 100 kilometers (62 miles) or more. Researchers from Indiana University South Bend wondered if this could also happen deeper down. So they did a lab experiment to simulate conditions in the mantle. They combined materials normally found at those depths. Then they put the mixture under extreme heat and pressure. The experiment produced tiny bubbles of methane gas, the scientists report. However, no one knows yet how much methane, if any, is actually present in the mantle. And, if it is present, whether any gas might seep up into the crust and emerge from spots on the ocean floor. The research could provide important clues about how life began on Earth. Some bacteria feed on methane. If methane were present in the mantle, it could support populations of microbes, allowing them to survive in such an extreme environment. It may also be worth looking for underground stores of methane on Mars and other planets when searching for signs of life.