135 GRASS GIS has prooven to be a solid system for handling and analysis of surface data of features on Earth. Since GRASS is Free Software, the freedom to use, distribute, study and modify it allowed to implement it into an efficient working environment for supporting planetary science research activities. The work has been focused on the development of procedures to handle reference systems used in planetary mapping as well as providing planetary bodies' ellipsoidal parameters into GRASS, following international research institutions conventions and recommendations. The data from planetary science archives can then be organized in GRASS maps in their correct projection and reference system. The practical use of GRASS in planetary mapping is presented with an analysis of geologic structures of planet Mars, using multispectral imagery and topographic data from different NASA missions and from ESA's Mars Express. FOSS4G2006 - Free And Open Source Software for Geoinformatics Session 11 : GRASS-DEV Alessandro Frigeri afrigeri@unipg.it Costanzo Federico geof@unipg.it Cristina Pauselli pauselli@unipg.it Giorgio Minelli gminelli@unipg.it Alessandro Frigeri afrigeri@unipg.it <MaKaC.conference.ContributionType object at 0xb3de856c>