Depletion of <SUP>13</SUP>C in CO in the Atmosphere of Mars Suggested by ExoMars-TGO/NOMAD Observations

The atmosphere of Mars is mainly composed by carbon dioxide (CO<SUB>2</SUB>). It has been predicted that photodissociation of CO<SUB>2</SUB> depletes <SUP>13</SUP>C in carbon monoxide (CO). We present the carbon <SUP>13</SUP>C/<SUP>12</SUP>C isotopic ratio in CO at 30-50 km altitude from the analysis of the solar occultation measurements taken by the instrument Nadir and Occultation for Mars Discovery on board the ExoMars Trace Gas Orbiter (ExoMars-TGO). We retrieve <SUP>12</SUP>C<SUP>16</SUP>O, <SUP>13</SUP>C<SUP>16</SUP>O, and <SUP>12</SUP>C<SUP>18</SUP>O volume mixing ratios from the spectra taken at 4112-4213 cm<SUP>-1</SUP>, where multiple CO isotope lines with similar intensities are available. The intensities of the <SUP>12</SUP>C<SUP>16</SUP>O lines in this spectral range are particularly sensitive to temperature, thus we derive the atmospheric temperature by retrieving CO<SUB>2</SUB> density with simultaneously measured spectra at 2966-2990 cm<SUP>-1</SUP>. The mean δ <SUP>13</SUP>C value obtained from the <SUP>13</SUP>C<SUP>16</SUP>O/<SUP>12</SUP>C<SUP>16</SUP>O ratios is -263‰, and the standard deviation and standard error of the mean are 132‰ and 4‰, respectively. The relatively large standard deviation is due to the strong temperature dependences in the <SUP>12</SUP>C<SUP>16</SUP>O lines. We also examine the <SUP>13</SUP>C<SUP>16</SUP>O/<SUP>12</SUP>C<SUP>18</SUP>O ratio, whose lines are less sensitive to temperature. The mean δ value obtained with <SUP>12</SUP>C<SUP>18</SUP>O instead of <SUP>12</SUP>C<SUP>16</SUP>O is -82‰ with smaller standard deviation, 60‰. These results suggest that CO is depleted in <SUP>13</SUP>C when compared to CO<SUB>2</SUB> in the Martian atmosphere as measured by the Curiosity rover. This depletion of <SUP>13</SUP>C in CO is consistent with the CO<SUB>2</SUB> photolysis-induced fractionation, which might support a CO-based photochemical origin of organics in Martian sediments.