6 February 2013 ESSC Brown Bag/Climate Dynamics Seminar
11:15 am, 529 Walker

Warming early Earth and Mars with H2

James Kasting, Ramses Ramirez, Ravi Kopparapu, and Michael Zugger

            H2 and other symmetric, diatomic molecules such as N2 or O2 are not normally considered to be greenhouse gases, as these molecules have no permanent electric dipole moment (like H2O) and no way of bending or stretching to create one (like CO2); hence, they do not couple strongly with electromagnetic radiation. But collisions with other molecules can nevertheless stimulate all three of these molecules to absorb radiation and use it to excite their pure rotational energy levels. N2 and O2 are relatively heavy molecules, and so their collision-induced absorption (CIA) occurs well out in the far infrared. But H2 is much lighter, causing its rotational energy levels to be more widely spaced, and so at room temperature H2 can absorb in the 8-12 mm “window region” where H2O and CO2 absorb poorly. Consequently, H2 could have been an effective agent in keeping Earth and Mars warm early in their histories when the Sun was less bright. It can, for example, explain how Mars could have been warm enough to form the valley networks that are seen on Mars’ surface today. Methanogens consume H2, though, and convert it into CH4; consequently, on Earth, H2 probably fell to low levels soon after life originated. So, the faint young Sun story for the Archean Eon may not be greatly affected by this phenomenon.