Surface pressure measurements help to achieve a better understanding of the main dynamical phenomena occurring in the atmosphere of a planet. The use of the Mars Express OMEGA visible and near‐IR imaging spectrometer allows us to tentatively perform an unprecedented remote sensing measurement of Martian surface pressure. OMEGA reflectances in the CO2 absorption band at 2 μm are used to retrieve a hydrostatic estimation of surface pressure (see companion paper by Forget et al. (2007)) with a precision sufficient to draw maps of this field and thus analyze meteorological events in the Martian atmosphere. Prior to any meteorological analysis, OMEGA observations have to pass quality controls on insolation and albedo conditions, atmosphere dust opacity, and occurrence of water ice clouds and frosts. For the selected observations, registration shifts with the MOLA reference are corrected. “Sea‐level” surface pressure reduction is then carried out in order to remove the topographical component of the surface pressure field. Three main phenomena are observed in the resulting OMEGA surface pressure maps: horizontal pressure gradients, atmospheric oscillations, and pressure perturbations in the vicinity of topographical obstacles. The observed pressure oscillations are identified as possible signatures of phenomena such as inertia‐gravity waves or convective rolls. The pressure perturbations detected around the Martian hills and craters may be the signatures of complex interactions between an incoming flow and topographical obstacles. Highly idealized mesoscale simulations using the WRF model enable a preliminary study of these complex interactions, but more realistic mesoscale simulations are necessary. The maps provide valuable insights for future synoptic and mesoscale modeling, which will in turn help in the interpretation of observations.

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 further 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).