NASA's Mars Atmosphere and Volatile Evolution mission (MAVEN) is the first spacecraft specifically designed to study the upper atmosphere of Mars. MAVEN's goal is to determine how Mars lost its thick early atmosphere, and with it, its once hospitable climate.
While previous Mars orbiters have peered down at the planet's surface, MAVEN is spending part of its time gazing at the stars, observing the Martian atmosphere through a series of stellar occultations. As Mars rolls beneath MAVEN, due to the spacecraft's own orbital motion, background stars rise and set behind the planet. Their light dims as it passes through the tenuous atmosphere, with specific gases absorbing specific wavelengths. MAVEN uses its Imaging Ultraviolet Spectrograph to break apart this light and see which wavelengths are absorbed, allowing it to determine atmospheric composition at varying altitudes.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 184.108.40.206.0