somebody said atmosperic science ?
well guys you got an enviromental engineering / air quality major in the house
to answer your question :methane is actually a very stable little molecule,due to it´s molecular structure ( i can go in to detail if you like .. ), and in earth´s troposphere ( that´s the layer of the atmosphere that goes from the ground to about 8- 10 km ) it has a half-life of several years before it is deposited or destroyed.
in the higher layers of the atmosphere however, methane is broken down by UV light ( photolysis ) and ofte reacts with certain natural occuring chemicals to produce water ( which is a major concern of climate scientists since methane and water are natural greenhouse gases and the methane level is constantly rising due to animal husbandry )
on mars however, photolysis probably occurs even in the lower layers of the atmosphere (no ozone layer to hold back the hard UV fraction ) and destroys any methane molecule. when you add the fact that methane is highly volatile and can easily escape into space due to mars low gravity it is easy to see that any methane they find is probably not very old. (my gues is maybe weeks...it depends on the concentration of methane and certain physical parameters of the mars atmosphere i don´t know about )
the real amazing thing is that i know no anorganic reaction that produces methane. as far as my books say, methane is always a product of microbiological activity in the absence of oxygen and the presence of water
so as long as they don´t find a way how mars possibly could produce methane - yes ladys and gentlemen ,pop the champagne ,because even so itis tiny and has only one cell, we have found life on mars
