Evidence for diurnally varying enrichment of heavy oxygen in Mars atmosphere

Terrestrial groundbased spectroscopy of Mars at subsolar latitude in October 2007 (southern summer, Mars year 28) reveals variability in the depth of a spectral absorption feature from a carbon dioxide isotopologue, (OCO)-O-16-C-12-O-18 ('628 CO2'), which correlates with increasing surface temperature in the early afternoon of the Martian sol. The correlation suggests a fractionation process that depletes isotopically heavy CO2 from the atmosphere by surface adsorption at night and restores atmospheric enrichment during the day through thermal desorption. Measurements reported here yield a depleted condition at noon, with O-18 depleted to -92 +/- 23 parts per thousand relative to a terrestrial isotopic standard (VSMOW, Vienna Standard Mean Ocean Water), increasing to +71 +/- 18 parts per thousand at 13:19 1ST, over a temperature increase from 266.9 K to 275.4 K. The average is +9 +/- 14 parts per thousand, consistent with results from landers and remote spectroscopy that averaged data collected over a broad range of daylight local times to obtain an average close to the terrestrial standard. Past measurements of CO2 isotopologues have been inconsistent with each other and in some cases inconsistent with predicted enrichment in heavy isotopes, with some measurements even slightly depleted in heavy isotopes. Local solar time of a measurement thus may skew this important constraint on estimating the density of the primordial atmosphere, although seasonal polar temperature variation also may have a substantial influence.