LOSS OF H AND O FROM MARS - IMPLICATIONS FOR THE PLANETARY WATER INVENTORY

The evolution of the Mars atmosphere, with regard to water, is influen ced by non-thermal escape of oxygen atoms. In our evaluation of the va rious escape processes we recognized that the widely quoted escape rat es for oxygen atoms originating from dissociative recombination (exosp heric O) are too high by an order of magnitude. Thus the originally po stulated coupling of H and O escape in the ratio of 2:1 can no longer be maintained. This has important implications for the stability of th e Martian CO2 atmosphere. This study takes into account temperature de pendent photodissociation coefficients, optical depths, ozone chemistr y, hydrogen and oxygen escape fluxes for conditions of an ancient dens e Mars atmosphere. Since the only source of hydrogen is water, the cal culated hydrogen escape flux represents an upper limit for an equivale nt water loss to space. Under this constraint, we find much lower oxyg en escape rates originating from water than current estimates, implyin g a loss of H2O to space corresponding to an equivalent depth less tha n or equal to 5 meter over the last 3.5 billion years. Our results cas t serious doubt on previous estimates that as much as 50-80 meters of H2O were lost to space during this period.