Remote sensing of surface pressure on Mars with the Mars Express/OMEGA spectrometer: 1. Retrieval method

Observing and analyzing the variations of pressure on the surface of a planet is essential to understand the dynamics of its atmosphere. On Mars the absorption by atmospheric CO₂ of the solar light reflected on the surface allows us to measure the surface pressure by remote sensing. We use the imaging spectrometer OMEGA aboard Mars Express, which provides an excellent signal to noise ratio and the ability to produce maps of surface pressure with a resolution ranging from 400 m to a few kilometers. Surface pressure is measured by fitting spectra of the CO₂ absorption band centered at 2 μm. To process the hundreds of thousands of pixels present in each OMEGA image, we have developed a fast and accurate algorithm based on a line-by-line radiative transfer model which includes scattering and absorption by dust aerosols. In each pixel the temperature profile, the dust opacity, and the surface spectrum are carefully determined from the OMEGA data set or from other sources to maximize the accuracy of the retrieval. We estimate the 1-σ relative error to be around 7 Pa in bright regions and about 10 Pa in darker regions, with a possible systematic bias on the absolute pressure lower than 30 Pa (4%). The method is first tested by comparing an OMEGA pressure retrieval obtained over the Viking Lander 1 (VL1) landing site with in situ measurements recorded 30 years ago by the VL1 barometer. The retrievals are finther validated using a surface pressure predictor which combines the VL1 pressure records with the MOLA topography and meteorological pressure gradients simulated with a General Circulation Model. A good agreement is obtained. In particular, OMEGA is able to monitor the seasonal variations of the surface pressure in Isidis Planitia. Such a tool can be applied to detect meteorological phenomena, as described by Spiga et al. (2007).