TDLAS a laser diode sensor for the in situ monitoring of H 2O, CO 2 and their isotopes in the Martian atmosphere

Abstract

Within the framework of Pasteur-Exomars, we have proposed to measure in situ water vapor (H 2O, HDO, H 2 18O, H 2 17O) and carbon dioxide (CO 2, 13C 16O 2, 16O 12C 18O, 16O 12C 17O) isotopes and also CO, CH 4 and N 2O by absorption spectroscopy using near infrared laser diodes. The Service d’Aéronomie has a relevant experience in trace-gas detection with laser diodes. We have developed, with the support of the CNES and the CNRS, the SDLA diode laser spectrometer to measure in situ H 2O (at 1.39 μm), CH 4 (1.65 μm) and CO 2 (at 1.60 μm) in the middle atmosphere from stratospheric balloons. The concentrations are obtained with a precision error of a few percent and with a high temporal resolution that ranges from 10 ms to 1 s. The developed laser probing technique should be also highly efficient to study the Martian atmosphere as there are much higher amounts of water vapor and carbon dioxide in the Martian atmosphere than in the lower stratosphere (H 2O: 200 ppmv at 6 hPa on Mars, 5 ppmv at 10 hPa in the low stratosphere (LS); CO 2 95% on Mars, 360 ppmv in the LS). Hence, we propose to adapt the laser probing technique to the Martian atmosphere. The main objectives are to determine water vapor and carbon dioxide fluxes and to study boundary layer properties. The sensor will provide in situ daily, diurnally resolved measurements of near-surface H 2O and CO 2 concentrations over seasonal time scales. The additional isotopic measurements will provide quantitative constraints on the evolution of atmospheric composition and on the history of water on Mars.