LEAD: September 21st, 2015, marks the one-year anniversary of NASA's MAVEN spacecraft circling Mars.
1. MAVEN's goal is to determine how Mars lost its thick early atmosphere, and with it, its once hospitable climate.
2. The spacecraft's Imaging Ultraviolet Spectrograph measures how the light from background stars dims as the starlight passes through different layers of the Martian atmosphere. This tells scientists about the atmosphere’s chemical makeup and its structure.
3. The vertical distributions of oxygen, hydrogen, and carbon dioxide are important clues to Mars’ climate history.
TAG: MAVEN is the first spacecraft specifically designed to study the upper atmosphere of Mars.