Methane, CO2 Earth's Greenhouse Gases In The Archean
Methane and carbon dioxide, not ammonia, were the greenhouse gases that compensated for our less energetic sun during the pre-oxygen Archean, according to Penn State geoscientists.
"We are looking at what the Earth's atmosphere was like prior to 2.3 billion years ago, before the rise of oxygen," said James F. Kasting, professor of geosciences and meteorology. "We believe it is likely that methane was the major component of the atmosphere then, and the major greenhouse gas."
Kasting and geosciences graduate student Alexander A. Pavlov, looked at a variety of ways to estimate the methane concentrations in the Archean atmosphere. Methane is produced by methanogenic bacteria, organisms that create methane from organic material or hydrogen and carbon dioxide.
"One way to estimate methane in the Archean atmosphere is to take today's methane production by organics and lower today's oxygen to that of the Archean, arriving at about 1,000 times higher than today's methane value," says Kasting. "However, other methods of estimation are better."
Looking at the way methanogenic bacteria produce methane, the researchers note that given abundant nutrients, these bacteria will convert hydrogen until insufficient energy exists to continue. According to thermodynamic analysis, this means that 90 to 95 percent of the hydrogen would be converted to methane.
"This analysis also produces a methane level in the Archean of 1,000 times today's level," Kasting told attendees at the Earth Systems Processes Conference Tuesday (June 26) in Edinburgh, Scotland. The conference is sponsored by the Geological Society of America and the Geological Society of London.
According to Kasting, this level of methane would compensate for the sun during the Archean, which produced only 80 percent of the energy that it does today. Previous suggestions for greenhouse gases to compensate for the sun's lower energy include carbon dioxide and ammonia. In the Archean, while Cyanobacteria, bacteria capable of photosynthesis similar to algae, are producing some oxygen, the oxygen is quickly reduced. Ammonia, which some, including Carl Sagan, thought a likely greenhouse gas for this stage in Earth's history, could not have served that purpose, according to Kasting and Pavlov.
"The problem with ammonia is that it will not persist in the atmosphere because sunlight easily breaks it apart into nitrogen and hydrogen," says Kasting. "In 1997, Sagan and others proposed that smog like that on Saturn's moon Titan could have protected the ammonia from photolysis."
According to the researchers, the sun's action on methane could have produced polyacetylenes, which made up the hydrocarbon smog. Previous analysis looked at this smog as if it were uniform with very small particles. Kasting and Pavlov used a photochemical model that calculated particle sizes in the atmosphere, suggesting particles from half a micron to a micron in the smog.
The depth of the smog layer is limited because, if the haze becomes too thick, rather than creating a greenhouse effect that traps heat within the Earth's atmosphere, the thick smog will reflect the sun's energy and cool the Earth.
"The haze could not have gotten too thick because it would have set up an anti-greenhouse effect if it did," says Kasting.
Particle size is important because it affects the ratio of ultraviolet light absorption compared to the visible light absorption. With large particles, the ultraviolet optical depth is only about two to three times the visible optical depth.
The researchers calculated how optically thick the smog layer could be and still act as a greenhouse layer. From the optical thickness, they calculated the ultraviolet thickness, which they found was not thick enough to shield the ammonia from photolysis.
"The fact that ammonia would not persist in the atmosphere under these smog conditions is unimportant from the point of view of the faint young sun paradox, because sufficient methane exists to produce a greenhouse effect," Kasting says. - By A'ndrea Elyse Messer
[Contact: James F. Kasting, Alexander A. Pavlov, A'ndrea Elyse Messer]
28-Jun-2001 |
