You might expect the atmosphere of Earth to be a lot better mapped than that of any other planet. However, the wind patterns of Mars turn out to be much simpler than those of Earth, and combined with a bit of convenient luck, this means we now have a picture of the prevailing currents in the upper Martian atmosphere that is more advanced than anything for our home.
The atmosphere thins with altitude, and Mars has so little air to begin with that its upper layers, known as the thermosphere, are not far off a vacuum. Nevertheless, not only are there gas particles, mainly carbon dioxide, 120-300 kilometers (75-180 miles) above the Martian surface, but they move to redistribute heat from the Sun, both from the day side to night and the equator to the poles.
The Martian thermosphere is so thin the MAVEN satellite can dive through it without burning up. Dr Mehdi Benna of the Goddard Space Flight Center used the Neutral Gas and Ion Mass Spectrometer (NGIMS) instrument to sample the gas molecules on these passages, determining their speed and direction of travel. Benna reports in Science the winds vary with the seasons, but are quite stable from one year to the next. The work may help explain how Mars lost most of its atmosphere, and whether it was ever habitable.
"The refreshing thing is that the patterns that we observed in the upper atmosphere match globally what one would predict from models," Benna said in a statement. "The physics works." Nevertheless, the results contained some surprises, notably the size of short-term variations, and how influential mountains and valleys are even far overhead.
"[Topography] creates waves – ripple effects that flow up to the upper atmosphere," Benna said. “On Earth, we see the same kind of waves, but not at such high altitudes. That was the big surprise, that these can go up to 280 kilometers high."
The map results from Benna's realization that if NGIMS was swung backward and forward “like a windscreen wiper” on MAVEN's passage through the thermosphere it could detect passing particles' speed. Not surprisingly, he encountered resistance from the NASA team in charge of MAVEN – if something went wrong it was not like a technician could pop over to Mars to fix it. Nevertheless, the value of the potential observations overcame the desire to preserve the satellite at all costs, and MAVEN spent two days a month for almost three years sampling the movement, as well as composition, of Mars' atmosphere.
Benna initially wanted to apply his idea to the study of Earth's upper atmosphere, but no suitable instrument was in orbit at the time. Although the greater atmospheric complexity will add some challenges to an Earth mission, the viability of the project has been demonstrated and lessons learned on Mars are likely to be useful here.