Stellar Occultations at UV Wavelengths by the SPICAM Instrument: Retrieval and Analysis of Martian Haze Profiles
Résumé
Observations made by the SPICAM ultraviolet spectrometer on board the Mars Express orbiter are presented. We focus on several hundreds of atmospheric profiles which have been collected over 3/4 of a Martian year by making use of the stellar occultation technique. The typical structure of the Martian haze possesses at least one discrete layer (60% of all cases) standing over an extended portion wherein opacity continuously increases down to the surface. Differences of morphology are, however, noted between profiles observed near the equator and profiles collected elsewhere. The Martian haze exhibits a pronounced seasonal signal manifested by variations of the maximum elevation at which particles are observed. For reasons related to both convective activity and changes in the hygropause level, cold regions display a much lower hazetop than warm regions. Using UV spectrometry data, we put constraints on haze microphysical properties. Vertical variations of particle size are keyed to variations of opacity; e.g., an increase of particle size is systematically observed near extinction peaks. This is the likely consequence of cloud formation which results into a local increase of particle cross section. Despite marked differences of aerosol profiles between low and high latitudes, haze properties above 60 km remain invariant, possibly reflecting the long-term presence of a background submicronic particle population. Several profiles have been analyzed in more detail to extract properties of detached cloud layers lofted above 40 km. Their optical depth ranges between 0.01 and 0.1 in the visible. Estimation of cloud particle size is technically restricted because of SPICAM wavelength sampling, but it generally yields a minimum radius value of about 0.3 μm, while several estimates are consistent with a robust 0.1-0.2 μm. This crystal size, significantly smaller than the 1 to 4 μm associated with recently classified type I and II clouds, suggests that a different class of clouds, henceforth type III clouds, can be extracted from our data. Observations made in the southern winter polar night indicate a very distinct aerosol behavior where particles are less abundant (τ < 0.1), confined to lower heights (vertical profile consistent with a Conrath parameter exceeding 0.04) and made of particles having a radius on the order of 0.1 μm. This shows that the Martian polar night is a region with a very clean atmosphere and with a distinct type of aerosols.
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