WATER IN THE MARTIAN ATMOSPHERE AS RELIC OF RECENT IMPACTS

The processes leading to hydrogen escape from the martian atmosphere discriminate sufficiently between H and D to give a 3 or 4-fold enrichment of deuterium in the upper atmosphere, but not the 5.2 times enrichment observed. If the present complement of water is a remnant of abundant early water lost from the planet, enrichment would be much higher. Steady sublimation of ice slightly enriched in D would explain the present datum. Alternatively, I propose episodic inputs into the atmosphere on a 10(5)yr scale, from crater ejecta of impacts in relic lakes and permafrost, as consistent with current cratering records. The last big cratering event (>2km diameter) that reset the D:H in mobile ice and atmospheric H2O would have occurred 4x10(4)yr ago.