The Mystery of the Missing Methane

Hugh Pickens writes "Astrobiology Magazine reports that NASA's Spitzer Space Telescope has discovered something odd about GJ 436b, a planet about the size of Neptune located 33 light-years away, circling the star Gliese 436. The mystery? GJ 436b lacks methane, an ingredient common to many of the planets in our solar system. Methane is present on our life-bearing planet, manufactured primarily by microbes living in cows, and all of the giant planets in our solar system have methane too, despite their lack of cows. Spitzer was able to detect the faint glow of GJ 436b by watching it slip behind its star, an event called a secondary eclipse. As the planet disappears, the total light observed from the star system drops, and the diference is then measured to find the brightness of the planet at various wavelengths. Eventually, a larger space telescope could use the same kind of technique to search smaller, Earth-like worlds for methane and other chemical signs of life, such as water, oxygen and carbon dioxide. Adam Showman, a planetary scientist at the University of Arizona, says the 'provocative result' raises questions about the evolution of this planet, as well as the possibility that its atmosphere might represent an entirely new class of atmospheres that has never been explored."

Why So Much Focus on Cows?

[eldavojohn]

Methane is present on our life-bearing planet, manufactured primarily by microbes living in cows, and all of the giant planets in our solar system have methane too, despite their lack of cows.

Why are cows focused on so much when it comes to methane? The only study I can find lists livestock making up only 19% as a source of atmospheric methane [wikipedia.org]. That's little more than our industrial energy sector production and about half as much as our wetlands produce. From NOAA [noaa.gov]:

Rapidly growing industrialization in Asia and rising wetland emissions in the Arctic and tropics are the most likely causes of the recent methane increase, said scientist Ed Dlugokencky from NOAA's Earth System Research Laboratory.

Re:Assume much?

[Anonymous Coward]

Actually it was six observation at six different infrared wavelenghts over a six month period.

Re:Obvious solution

[reverseengineer]

I found a nice little illustration of the effect temperature has on the equilibrium of this reaction here. [davidson.edu] The calculation is actually for the related reaction using carbon (as coke) instead of methane, but the equilibrium constants are about equal for the temperatures discussed here. At atmospheric conditions on Earth, the equilibrium can be considered as shifted completely to the left. Virtually no carbon monoxide is produced from this reaction at temperatures less than about 600K. At a temperature of 956.7K, the levels of carbon and carbon monoxide are equal, and at higher temperatures, carbon monoxide is On GJ 436b, with a temperature of 800K, the equilibrium should still strongly disfavor CO production, and the calculation suggests that there should be around 13.6 times as much carbon (or methane in the case of GJ 436b) as there is carbon monoxide.

However, the researchers determined that "GJ 436b's atmosphere is abundant in CO and deficient in methane (CH4) by a factor of ~7,000. [ucf.edu]" The only way the planet could have gotten an atmosphere like that through this reaction equilibrium alone is if its temperature is really around 2000K instead of 800K. The researchers therefore argue that it's far more likely that some other mechanism is disrupting this equilibrium, like polymerization of methane that pulls it out of the system. In their Nature paper, they include a a chart [nature.com] of the atmospheric ratios of gas giants, both in our solar system and exoplanets; nothing else known has a CH4/CO ratio like that seen for GJ 436b.