Study of a dust storm properties from the Mars Express OMEGA and PFS data

International audienceSuspended dust is one of the key elements of the Martian climate, and information about its properties is crucial for modeling and data analysis. Our study is aiming at providing new constraints on a local dust storm: dust particle properties, storm extent and its effect on the atmospheric temperatures. We base our study on the data of OMEGA (Observatoire pour la mineralogie, l'eau, la glace et l'activité, [1]) and Planetary Fourier Spectrometer (PFS,[2]) e aboard the Mars Express satellite. We have studied two orbits only 3 sols apart, 1201 and 1212, which map nearly the same geographical area, but in which the atmospheric state is very different. Orbit 1201 is clear and the dust optical depth is small: it is used to retrieve the surface albedo and the surface pressure. Orbit 1212 instead shows a very dusty region around the equator. We use the data from the two orbits to retrieve the dust properties from the orbit 1212. We retrieved the surface albedo from orbit 1201, and the surface pressure retrieved from OMEGA observations on orbit 1201 was provided by A. Spiga and F. Forget [3-4]. For the retrieval we used, however, the surface pressure from the Mars Climate Database, since the retrieved values and the database were in good agreement. We fit the single-scattering albedo of dust using the spectrum of the highest observed reflectance on this orbit (implying quasi-infinite dust optical depth), after which we retrieve the dust optical depth from all of the observed dust storm spectra. The fits are made for the wavelength range 1-4 micron. We have also made an Independent Component Analysis (ICA) to compare the orbits and show that the dust cloud is distinguishable from the surface features. The signal of the dust storm (from the 2.7 micron absorption band) is very clear and independent of the other components. The optical properties retrieved show some discrepancies with the commonly used Ockert-Bell [5] values. The retrieved high dust optical depths can be perhaps related to the local and seemingly vigorous nature of the storm. Comparison of the temperature profiles from models and PFS observations suggest that the dust is limited to lower altitudes. References: [1] Bibring J.-P. et al.(2004), ESA SP-1240, 37-49. [2] Formisano V. et al.(2005) Planet. Space Sci., 53, 963-974. [3] Forget F. et al.(2007), JGR, 112, E08S15. [4] Spiga A. et al.(2007) JGR, 112, E08S16. [5] Ockert-Bell M. E. et al.(1997) JGR, 102, 9039-9050. Acknowledgements: We wish to thank Aymeric Spiga and Francois Forget for the surface pressure data. AM wishes to thank the Association Franco-Finlandais pour le Reserche Scientifique et Technique, Wihuri foundation and Ehrnrooth foundation for financial support. Also funding from the Kordelin foundation and from the graduate school of astronomy and space physics is acknowledged. AM is presently working in Service d'aeronomie in Verrieres-le-buisson, France, funded by a post-doc grant from CNES, which is gratefully acknowledged