Precision of Wind Measurements on Mars Using a Microwave Limb Sounder

While aspects such as the temperature and composition of the Martian atmosphere are relatively well known thanks to observations from numerous space missions, Martian atmospheric dynamics remain poorly constrained due to the scarcity of direct wind measurements. The use of microwave limb sounders to retrieve winds has been proposed in the past, but no such instrument has yet flown to Mars. The precision of the wind measurements by such instruments has been studied before for a handful of cases, but a thorough study taking into account current knowledge on the variability of the Martian atmosphere has not yet been conducted. In this study, we propose a cross-correlation method to retrieve winds from the Doppler shift of various emission lines as measured by a generic microwave limb sounder, and characterize the variability of the precision of the retrieved winds during a whole Mars year assuming a given instrumental configuration.

With the assumed instrument setup, and by combining CO isotopes with differing line intensities, wind measurements with a precision better than 10 m/s can be obtained at tangent altitudes between 30 km and 140 km using 2 seconds of integration time (see Fig. 1), and between 20 km and 150 km when increasing the integration time up to 10 seconds. These results are consistent during most seasons, local times, and geographical locations in the planet. Exceptions with degraded precision can be found in atmosheric layers exhibiting strong wind shear, which limits the measurements that the cross-correlation method is capable of performing in case of strong vertical gradient. The small computational burden of the proposed method, combined with its applicability to microwave limb sounder measurements, enables full characterization of the spatial and temporal atmospheric variability without dependence on external sources. Moreover, our methodology can be easily adapted to other instrumental configurations, and used as a fast operational method for wind measurement of potential future instruments.