Occultation of stars in the UV: Study of the atmosphere of Mars

The stellar occultation technique permits effective sounding of a tenuous planetary atmosphere, such as the atmosphere of Mars. On the basis of measuring characteristics of the SPICAM Light UV spectrometer for the spectral range of 118–320 nm, resolution 0.9 nm (to be flown at Mars‐Express ESA mission), we have made some model simulations in order to estimate how the vertical profiles of CO 2 , temperature, O 3 , aerosols, and clouds can be determined by the stellar occultation method. From the atmospheric absorption imprinted progressively on the star spectrum, the line densities of CO 2 , O 3 , and dust are retrieved as a function of altitude. An inversion yields local densities of CO 2 (and therefore the temperature), ozone, and dust particle concentration. Several (3–5) occultations per orbit, preferably on the night side of the orbit, are foreseen, allowing good geographical coverage independently on the orbit. Observing a bright star (one out of ∼30 in the sky), atmospheric density will be determined in the altitude range of 10–170 km with an accuracy of 0.5–4%; temperature will be determined at 20–140 km with an accuracy of ∼3 K, and ozone can be retrieved with an accuracy of a few percent in a layer between 25 and 40 km or below, depending on its vertical distribution. Optically thin cloud structures can be studied, including night clouds, which cannot be observed in scattered light. Molecular oxygen can be measured with an accuracy of 10%, and a search of SO 2 will be performed. The results of this study can easily be applied to other instrumental characteristics.