Observations of seasonal and permanent ice caps on Mars during three years by the OMEGA Vis/NIR imaging spectrometer on board Mars Express.

Abstract : The OMEGA Vis/NIR imaging spectrometer on-board Mars Express has been operating in Mars orbit since January 2004. The spectral range of OMEGA (0.35 to 5.1 μm) makes it possible to identify H2O and CO2 ices and frosts from their characteristic signatures in the near IR. Constraints on grain size and the volume fraction of dust and ice inclusions can be obtained from the relative depth of the absorption bands. Dust in the atmosphere, at the surface of the ice or as a contaminant within the ice results in a lower albedo and band strengths. The role of ice free aerosols can be assessed from the radiance factor at wavelengths corresponding to saturated ice bands. These observations provide important constraints for the CO2 and H2O seasonal cycles on Mars. During these three years of observations, OMEGA performed comprehensive investigations on the recess of the South seasonal cap in 2005 (M year 27), and the recess of the North seasonal cap in 2004 and 2006 (M years 27 and 28). These observations were performed with IFOV's ranging from ~ 10 km down to 1 km. They showed that the North seasonal cap is spectrally dominated by H2O ice after mid-spring, in sharp contrast to the South seasonal cap, dominated by CO2 ice. The South permanent cap was observed in February 2004 and January 2006. The North permanent cap was observed in late 2004 and early 2005. We will report on new observations of the North permanent cap starting in early November, 2006.