Identification of a new band system of isotopic CO2 near 3.3 μm: Implications for remote sensing of biomarker gases on Mars

We present the discovery of a new vibrational band system of isotopic CO2 (carbon dioxide) near 3.3 μm, with multiple strong P, Q and R lines in the prime spectral region used to search for Mars CH4 (methane). The band system was discovered on Mars using high-resolution spectrometers (λ/δλ>40,000λ/δλ>40,000, CSHELL and NIRSPEC) at telescopes (NASA-IRTF and Keck-2) atop Mauna Kea, HI. The observed line intensities and frequencies agree very well with values predicted by a vibrational band model that we developed using known parameters for the molecular levels involved. Using this model, we synthesized spectra for different observing conditions (from Space and ground-based telescopes) and for different spectral resolving powers (5000 to 40,00040,000). Although the total atmospheric burden on Mars is more than 150 times smaller than on Earth, the greater mixing ratio of CO2 ensures that its column abundance on Mars is almost 20 times greater than on Earth. Thus, weak telluric CO2 band systems appear much stronger on Mars. Many molecules of possible biological and geothermal interest have strong signatures at these wavelengths, in particular hydrocarbons owing to their strong ro-vibrational CH stretching modes. For example, the new isotopic CO2 band-system encompasses lines of CH4, C2H6 (ethane), CH3OH (methanol) and H2O (water). Implications for previous and future searches of biomarker gases are presented.