Mem. S.A.It. Suppl. Vol. 75, 282 c  SAIt 2007 Memorie della Supplementi Relation between albedo changes and dust redeposition on Syrtis Major region of Mars A. Zinzi 1,2 , E. Palomba 2 , and M. D’Amore 2 1 Universit` a degli Studi, L’Aquila 2 Istituto di Fisica dello Spazio Interplanetario, Roma Abstract. Syrtis Major is a near equatorial dark region of Mars and its composition is known to be basaltic. It is also known that it is a region undergoing high albedo variations. In this study we analyzed MGS TES ancillary data (i.e. surface albedo, thermal inertia and atmospheric dust opacity) in order to relate these variations to different surface conditions. Looking at these data (spanning two separated biennia - i.e. 1999-2000 and 2003-2004) we noticed that dust opacity raises with albedo, but thermal inertia does not decrease with albedo, as expected if albedo variations was due to dust redeposition. So we found out that TES albedo is not the pure surface albedo, but it is linearly related with dust opacity. Key words. Mars – TES – Albedo – Syrtis Major – Thermal Inertia – Dust opacity 1. Introduction Several authors focussed their works on Syrtis Major, a nearly equatorial dark region of Mars, and so its composition is well know (Bandfield et al. 2000). One of the main characteristics of this region is its high albedo variation, probably due to the effect of dust redeposition. In particular Mustard and Cooper (Mustard and Cooper 2005) compared data from Phobos-2 ISM and Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES), two different instruments that orbited Mars in different epochs. Their attention was focussed primarily on the albedo features and so they found out that over all the regions examined both by TES and ISM the ratio between their respectively albedo is about 0.78. This value of the ratio is nearly Send offprint requests to: azinzi@aquila.infn.it constant over all the regions, apart from Syrtis Major, where they found a value of about 0.8 for Eastern Syrtis Major and 1.15 for Western Syrtis Major. They imputed the non unity value of the ratio to the different way to compute albedo between ISM and TES and tried to link the difference in albedo in Syrtis Major to difference in soil composition. They found a slightly different VIS/NIR spec- trum in West Syrtis Major respect to Eastern Syrtis, but this difference is not reproduced in the TIR spectrum and so they did not find a plausible reason for the albedo changing. The present work is devoted to finding an answer to their question and to do so we analyzed ancillary TES data, such as albedo, thermal inertia and atmospheric dust opacity.