Using NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, a team of planetary researchers have measured the speed and direction of winds in the Martian upper atmosphere between about 150 and 200 km (93-124 miles) above the surface. The results are published in two papers in the journal Science and the Journal of Geophysical Research: Planets.
Computer-generated visualization of the orbital paths (white dots) taken by NASA’s MAVEN spacecraft as it mapped winds (blue lines) in the Martian upper atmosphere; red lines coming from the white dots represent local wind speed and direction, measured by MAVEN’s Neutral Gas and Ion Mass Spectrometer instrument. Image credit: NASA’s Goddard Space Flight Center / MAVEN / SVS / Greg Shirah.
“The observed global circulation provides critical inputs needed to constrain global atmospheric models,” said Dr. Mehdi Benna, a scientist at NASA’s Goddard Space Flight Center and lead author of the Science paper.
“These are the same models that are used to extrapolate the state of the Martian climate into the distant past.”
“The winds observed in the Martian upper atmosphere are sometimes similar to what we see in global model simulations, but other times can be quite different,” added Dr. Kali Roeten, a researcher at the University of Michigan and lead author of the paper published in the Journal of Geophysical Research: Planets.
“These winds can also be highly variable on the timescale of hours, yet in other cases, are consistent throughout the observation period.”
The wind data used by the team were collected by MAVEN’s Neutral Gas and Ion Mass Spectrometer (NGIMS) between 2016 and 2018.
“NGIMS’ original purpose was to determine the structure and composition of the Martian atmosphere by measuring in it the amounts of ions and gases,” the scientists explained.
“However, although it was not originally designed to do so, in April 2016, we began using NGIMS to observe horizontal winds.”
“Pausing normal collection of data, scientists on Earth programmed the instrument to nod back and forth so that it could detect the direction of winds along its track.”
Some results were as the authors expected, and others were surprises.
“One surprise came when we analyzed the shorter-term variability of winds in the upper atmosphere, which was greater than anticipated,” they said.
“On Mars, the average circulation is steady, but if you take a snapshot at any given time, the winds are highly variable. More work is needed to determine why these contrasting patterns exist,” Dr. Benna added.
The second surprise was that the wind hundreds of kilometers above the planet’s surface still contained information about landforms below, like mountains, canyons, and basins.
“As the air mass flows over those features, it creates waves — ripple effects — that flow up to the upper atmosphere and can be detected by MAVEN and NGIMS,” Dr. 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 km (174 miles) high.”
This is the first detection of topography-induced gravity wave ripples in the thermosphere of any planet, even Earth.
The researchers have two hypotheses for why these waves last so long unchanged.
“For one, the atmosphere on Mars is much thinner than it is on Earth, so the waves can travel farther unimpeded, like ripples traveling farther in water than in molasses,” they said.
“Also, the average difference between geographic peaks and valleys is much greater on Mars than it is on Earth.”
“It’s not uncommon for mountains to be 20 km (12 miles) tall on Mars, whereas Mt. Everest is not quite 9 km (5.5 miles) tall, and most terrestrial mountains are much shorter.”
M. Benna et al. 2019. Global circulation of Mars’ upper atmosphere. Science 366 (6471): 1363-1366; doi: 10.1126/science.aax1553
Kali J. Roeten et al. MAVEN/NGIMS Thermospheric Neutral Wind Observations: Interpretation Using the M-GITM General Circulation Model. Journal of Geophysical Research: Planets, published online December 12, 2019; doi: 10.1029/2019JE005957