Although nearly all methane on Earth has a biological origin, scientists have recently begun to appreciate how many ways abiogenic methane can be generated. The essential precondition for abiogenic methane, says Juske Horita of the Chemical Sciences Division at Oak Ridge National Laboratory in Tennessee, is the presence of molecular hydrogen (H2) and carbon dioxide (CO2).
"If you put CO2 and hydrogen together, thermodynamics dictates that it has to go to methane," says Horita.
Most abiogenic methane is generated by the "serpentinization" reaction, which forms the mineral serpentine. At mid-oceanic ridges, water heated by magma reacts with rocks like olivine, which contain high levels of the catalysts iron and magnesium. During serpentinization, hydrogen liberated from water reacts with carbon from carbon dioxide to form methane. The reaction creates heat and vast deposits of serpentine on the ocean floor.