Stable isotope measurements of martian atmospheric CO₂ at the Phoenix landing site

Carbon dioxide is a primary component of the martian atmosphere and reacts readily with water and silicate rocks. Thus, the stable isotopic composition of CO₂ can reveal much about the history of volatiles on the planet. The Mars Phoenix spacecraft measurements of carbon isotopes [referenced to the Vienna Pee Dee belemnite (VPDB)] [δ¹³C_VPDB = -2.5 ± 4.3 per mil (‰)] and oxygen isotopes [referenced to the Vienna standard mean ocean water (VSMOW)] (δ¹⁸O_VSMOW = 31.0 ± 5.7‰), reported here, indicate that CO₂ is heavily influenced by modern volcanic degassing and equilibration with liquid water. When combined with data from the martian meteorites, a general model can be constructed that constrains the history of water, volcanism, atmospheric evolution, and weathering on Mars. This suggests that low-temperature water-rock interaction has been dominant throughout martian history, carbonate formation is active and ongoing, and recent volcanic degassing has played a substantial role in the composition of the modern atmosphere.