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Mars' Atmosphere from Curiosity
The Sample Analysis at Mars (SAM) instrument on the Curiosity rover that landed on Mars in August last year is designed to study the chemical and isotopic composition of the martian atmosphere. Mahaffy et al. (p. 263) present volume-mixing ratios of Mars' five major atmospheric constituents (CO2, Ar, N2, O2, and CO) and isotope measurements of 40Ar/36Ar and C and O in CO2, based on data from one of SAM's instruments, obtained between 31 August and 21 November 2012. Webster et al. (p. 260) used data from another of SAM's instruments obtained around the same period to determine isotope ratios of H, C, and O in atmospheric CO2 and H2O. Agreement between the isotopic ratios measured by SAM with those of martian meteorites, measured in laboratories on Earth, confirms the origin of these meteorites and implies that the current atmospheric reservoirs of CO2 and H2O were largely established after the period of early atmospheric loss some 4 billion years ago.
Stable isotope ratios of H, C, and O are powerful indicators of a wide variety of planetary geophysical processes, and for Mars they reveal the record of loss of its atmosphere and subsequent interactions with its surface such as carbonate formation. We report in situ measurements of the isotopic ratios of D/H and 18O/16O in water and 13C/12C, 18O/16O, 17O/16O, and 13C18O/12C16O in carbon dioxide, made in the martian atmosphere at Gale Crater from the Curiosity rover using the Sample Analysis at Mars (SAM)’s tunable laser spectrometer (TLS). Comparison between our measurements in the modern atmosphere and those of martian meteorites such as ALH 84001 implies that the martian reservoirs of CO2 and H2O were largely established ~4 billion years ago, but that atmospheric loss or surface interaction may be still ongoing.