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Feeling the Pressure: What Atmospheric Measurements tell us about Martian Weather
By Jeffrey Marlow

Here's a challenge for all aspiring mountaineers: start at sea level, and begin climbing, ascending 20 vertical feet every minute. Continue for four hours, reaching the elevation of Denver, no water breaks allowed.

Winded? I wouldn't blame you: you've just experienced the equivalent atmospheric pressure change that Curiosity records every martian day, about 15% of the total pressure.

What accounts for this enormous swing in atmospheric pressure? One key is the thin atmosphere itself: with fewer overall gas molecules to go around, a small change in their abundance at a certain location will have a relatively large effect on the pressure. The other contributing factor is a little more complicated.

MSL Deputy Project Scientist Ashwin Vasavada explains that on Mars, atmospheric pressure works a little differently. "It's a very unique aspect of Mars," he says. "Many other aspects of the weather behave similarly to Earth."

The pressure shifts are controlled by the local ground temperature: warmer rocks and dust mean higher atmospheric pressures. In the morning, the Sun's first rays begin to shake the surface awake, at the molecular scale anyway, causing faster vibrations and increasing the temperature. This creates a bubble of warmer air above the ground during the daylight hours. Winds at higher levels drive away from this bubble, leaving fewer molecules in the column of air above the ground. As the planet rotates, this "bubble of warm air translates around the planet, like a tide," Vasavada says. "And as that bubble translates around, there's atmospheric mass leaving that area, creating a spot of low pressure."

So far, Curiosity's weather has closely matched what the team had anticipated based on computerized weather models. But there are refinements to be made, as Vasavada explains. "The wind and pressure system is driven by the topography and the fact that we're at the bottom of a big hole in the ground." And while the large pressure swings are explained by the diurnal cycle, "we'd like to match the smaller fluctuations to the topographical features of Gale Crater."

Ready for the descent? After an hour break, it's time to start going back down in our experiential model of martian pressure. This time you've only got two hours. When you get there, enjoy your time at sea level and rest up: this same workout is on the schedule again tomorrow.