Artist's concept of the Aquarius/SAC-D observatory, a collaborative international space mission between NASA and Argentina's space agency, Comision Nacional de Actividades Espaciales (CONAE), with participation by Brazil, Canada, France and Italy. Aquarius, the NASA-built primary instrument on CONAE's SAC-D observatory, will provide NASA's first space-based global measurements of salinity -- the concentration of dissolved salt -- at the ocean surface. Salinity is a key missing variable in satellite observations of Earth that will help scientists better understand the links between ocean circulation, the global cycling of freshwater and climate.