<font color="yellow">The reason being, of course, that methane wouldn't last long in an atmosphere with abundant oxygen!</font><br /><br />Photoreduction of CO2 on metal oxide dust catalysts (TiO2, for example) can replenish trace levels of methane.<br /><br />Now, if it were a water world (no metal oxides dust) and had methane in the atmosphere, this would just about eliminate the photoreduction mechanism.... <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center"><em><font color="#0000ff">- - - - - - - - - - - - - - - - - - - - - -</font></em> </div><div class="Discussion_UserSignature" align="center"><font color="#0000ff"><em>I really, really, really miss the "first unread post" function.</em></font> </div> </div>