Flashes of lightning in volcanic ash clouds may have helped set the stage for life on Earth. Volcanic plumes were ideal crucibles for sparking stable nitrogen to form reactive compounds that led to the first organic molecules, chemists report in the 15 August Geophysical Research Letters.
Nitrogen is the most common element in air, but living cells cannot use its inert molecular form of two nitrogen atoms bound tightly together. Rather, some energy source must break the bond and free the nitrogen atoms to react with other elements. Today, bacteria "fix" nitrogen this way routinely. But before life arose, nitrogen fixation must have occurred via purely chemical means: That's because amino acids and nucleic acids, which both contain nitrogen, were likely the organic compounds that developed into Earth's first lifeforms. Some reactive nitrogen may have streaked to Earth aboard comets and space dust, but scientists believe most of it formed in the atmosphere. The leading candidates for triggers have been solar ultraviolet rays, lightning in thunderstorms, and superhot impact plumes from meteorites.
Volcanic lightning may supplant those as the main early source of fixed nitrogen, says atmospheric chemist Rafael Navarro-Gonzalez of the National University of Mexico in Mexico City. He and his colleagues, including Nobel laureate Mario Molina of the Massachusetts Institute of Technology, mimicked the highly charged volcanic plumes from that era with a lab mixture of water vapor, carbon dioxide, and other gases exhaled today by Hawaiian volcanoes, which tap primitive magma deep in Earth's mantle. Using intense microwave bursts, the scientists heated the gases to 10,000oC, about the same temperature as an ash cloud zapped by lightning. The result: huge amounts of nitric oxide (NO), which would have fallen to the early Earth as acid rain. In that form, says Navarro-Gonzalez, nitrogen was available to create the building blocks of life. "Ordinary lightning probably was not as important," he says, because continents--above which most lightning occurs--were much smaller 4 billion years ago.
"Their arguments are the state of our knowledge" of early Earth, says space scientist Christopher McKay of NASA's Ames Research Center at Moffett Field, California. A similar process may have occurred during volcanically active periods on Mars, McKay notes. The lack of legions of bacteria to recycle nitrogen back into the Martian atmosphere may explain why the red planet has no gaseous nitrogen today, he says.