 CARBON AND OXYGEN STABLE ISOTOPE MEASUREMENTS OF MARTIAN ATMOSPHERIC CO2 BY THE PHOENIX LANDER. P. B. Niles1, W.V. Boynton2, J. H. Hoffman3, D. W. Ming1, D. Hamara2, and the Phoenix Science Team, 1Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, TX 77058; 2Department of Planetary Sciences, University of Arizona, Tucson, AZ, USA; 3Physics Department, University of Texas, Dallas, TX, USA. (paul.b.niles@nasa.gov)  Introduction: Precise stable isotope measurements of the CO2 in the martian atmosphere have the potential to provide important constraints for our understanding of the history of volatiles, the carbon cycle, current atmospheric processes, and the degree of water/rock interaction on Mars [1]. The isotopic composition of the martian atmosphere has been measured using a number of different methods (Table 1), however a precise value (<1%) has yet to be achieved. Given the elevated δ13C values measured in carbonates in martian meteorites [2-4] it has been proposed that the martian atmosphere was enriched in 13C [8]. This was supported by measurements of trapped CO2 gas in EETA 79001[2] which showed elevated δ13C values (Table 1). More recently, Earth-based spectroscopic measurements of the martian atmosphere have measured the martian CO2 to be depleted in 13C relative to CO2 in the terrestrial atmosphere[7, 9-11].  The Thermal and Evolved Gas Analyzer (TEGA) instrument on the Mars Phoenix Lander [12] included a magnetic-sector mass spectrometer (EGA) [13] which had the goal of measuring the isotopic composition of martian atmospheric CO2 to within 0.5%. The mass spectrometer is a miniature instrument intended to measure both the martian atmosphere as well as gases evolved from heating martian soils.  Analytical Methods: Mars atmospheric CO2 was compared to CO2 contained in a calibration gas that the instrument carried to Mars, details of analytical methods were described previously [14]. The peak rates were averaged over an analysis period of 1 to 3 minutes for masses 44, 45 and 46 collected by TEGA's channel 4 CEM. These averages were corrected for the deadtime of the pre-amplifier discriminator (71 ns). The background measured in the instrument prior to ingesting gas was subtracted, backgrounds were typically below 5% of the signal, but in some cases were as high as 30% when the instrument had been run multiple days in a row. A correction for 17O in mass 45 was also applied using the terrestrial value for standard mean ocean water (VSMOW) of 0.0003799. Uncertainties Table 1. Reported values for isotopic composition of martian atmospheric CO2 including results from this study. (VPDB: Vienna PeeDee Belemnite; VSMOW: Vienna Standard Mean Ocean Water) Carbon 13C/12C uncertainty δ13C (VPDB) (‰) uncertainty (‰) Reference SNC-Trapped Gas (EETA79001) 0.0116 0.0001 36 10 Carr et al. (1985) Viking - Neutral MS 0.0111 0.0007 -10 58 Nier and McElroy (1977) Viking - GC MS 0.0112 0.0006 0 50 Owen et al. (1977) Atm-Spectroscopy 0.0104 0.0007 -73 58 Schrey et al. (1986) Atm-Spectroscopy 0.0106 0.0017 -60 150 Krasnopolsky et al. (1996) Atm-Spectroscopy 0.0112 0.0012 0 110 Encrenaz et al. (2005) Atm-Spectroscopy 0.0110 0.0002 -22 20 Krasnopolsky et al. (2007) Phoenix MS 0.01120 0.00005 -2.5 4.3 This study Oxygen 18O/16O uncertainty δ18O (VSMOW) (‰) uncertainty (‰) Reference Viking-Neutral MS 0.00204 0.00011 18 55 Nier and McElroy (1977) Viking-GC MS 0.00201 0.00010 0 50 Owen et al. (1977) Atm-Spectroscopy - CO2 0.00193 0.00024 -40 120 Schrey et al. (1986) Atm-Spectroscopy - CO2 0.00204 0.00004 18 18 Krasnopolsky et al. (2007) Phoenix MS 0.002061 0.000011 28.0 5.6 This study Figure 1. Carbon-Oxygen crossplot. Red triangle shows Phoenix measurement as compared to Viking[5, 6] and Earth based spectroscopy [7]. The green points show comparison of TEGA analysis of calibration gas on Mars and a laboratory analysis. The difference between these points can be used to correct for any instrumental fractionation effects.  are calculated as 2-sigma based on the standard deviation of 48 atmospheric measurements and 39 calibration gas measurements. Calculated uncertainties do not include potential systematic errors resulting from instrumental non-linearities.  Results: The results are plotted in Figure 1 and listed in Table 1. Carbon isotope ratios of martian atmospheric CO2 (-2.5‰) are similar to terrestrial atmospheric CO2 (-7‰) while the oxygen isotopic composition (+28.0‰) is significantly lighter than terrestrial atmospheric CO2 (+41‰). The results reported here are within the uncertainty of most previous isotopic measurements of martian CO2 with the exception of [2]. Discussion: The results reported here confirm that the carbon isotopic composition of martian atmospheric CO2 is not enriched in 13C and are in agreement with Krasnopolsky et al. [7]. This result is in contradiction with measurements made on trapped gases in martian meteorites [2]. This may be explained if CO2 trapped in impact melt glasses incorporated significant amounts of 13C-rich CO2 generated from carbonate minerals at the impact site.  Carbonates in martian meteorites are typically very enriched in 13C and have δ13CVPDB values near +40‰ [2-4] with some as high as +65‰ [15]. One important exception to this is the younger EETA 79001 which contains carbonates with δ13C value near +10‰[16]. The heavy isotopic values in the older meteorite carbonates are not consistent with formation from the atmospheric CO2 measured by Phoenix, however younger carbonates in EETA 79001 are consistent with formation from this reservoir (Fig. 2). In addition, measurements of the Δ17O composition of the carbonates in Nakhlites and ALH84001 suggest they incorporate a significant atmospheric component[17]. One scenario that could possibly explain these conflicting data is an early atmospheric carbon reservoir that had been enriched in 13C through early hydrodynamic escape[18]. Mantle degassing combined with carbonate formation through martian history lowered the δ13C of the atmospheric CO2 to its present value. In this scenario ancient 13C-rich carbonates found in ALH 84001 and the Nakhlites were formed from the early 13C-rich atmosphere while younger carbonates found in EETA 79001 formed from the modern atmosphere. The lack of large amounts of carbonates in the martian crust and low estimates of mantle degassing [19] could suggest that a heavy CO2 greenhouse was not present early in martian history. The oxygen isotopic composition of the atmospheric CO2 is heavy when compared to the silicates on Mars but is lighter than terrestrial atmospheric CO2. Similar to terrestrial CO2, it is likely that isotopic composition of martian atmospheric CO2 is dominated by exchange with liquid water. If this equilibration took place at an average temperature of 0°C, the δ18O of liquid water would be close to -17‰. This is consistent with a measurements of H2O(v) depleted in 18O by Bjoraker [20] using Earth based spectroscopy.  Conclusions: The TEGA instrument on the Phoenix spacecraft has made a direct measurement of the isotopic composition of martian atmospheric CO2 on the surface of Mars to a precision better than 0.5%. Martian atmospheric CO2 has a δ13C of -2.5±4.3‰ and a δ18O of +28.0± 5.6‰. 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Ancient martian carbonates field encompasses analyses of carbonates in ALH 84001 and Nakhlites[2-4]. Ancient Martian Carbonates 
