NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft has allowed researchers to map the winds that blow high above the red planet's surface, reports a new study, which measures the global circulation of Mars' upper atmosphere for the first time. The results inform our understanding of how Mars lost most of its ancient atmosphere and provide a useful comparison for understanding Earth's upper atmosphere. As in other Earth-like planets in the Solar System, the circulation of Mars' upper atmosphere (the thermosphere and ionosphere) is driven by both solar heating from above and ascending energy from the lower atmosphere below. The circulation in the upper atmosphere plays an important role in transporting and redistributing material and energy planet-wide. While these processes have been studied in Earth's atmosphere for decades, the nature of Mars' thermospheric winds remains largely uncharacterized. Mehdi Benna and colleagues report new data collected by MAVEN as it repeatedly dipped into the Martian upper atmosphere at altitudes of roughly 140-240 kilometers, which enabled them to measure the wind speeds in Mars' thermosphere and map the resulting atmospheric circulation. According to Benna et al., the circulation patterns are simpler than Earth's and stable over Mars' seasons, indicating the long-term stability of the planet's climate over the Martian year. Also, the winds in some locations closely follow the surface topography of the surface far below; these findings suggest the detection of "orographic gravity waves" - giant atmospheric waves produced when winds blow over large changes in surface height - in Mars' upper atmosphere, the authors say.
