On Nov. 12, Philippine amateur astronomer Christopher Go photographed a bright, orange-colored splotch on Mars he suspected might be the start of a dust storm. He was right. Photos taken the very next night showed that the bright spot had quickly expanded south to fill the deep recesses of Valles Marineris, the largest canyon in the solar system. Overnight it had morphed from a local into a regional storm.
By Nov. 20 billowing clouds of dust covered much of the planet's western hemisphere, and the storm continues to expand its reach as I write. A quick look at the photos shows just how rapidly a dust storm can turn from a minor event into a big one. At this point we don't know if the tempest will plateau and then recede or evolve into a planet-wide storm like the one that happened in 2018. That summer, Mars might well have been called the Orange Planet. Air-born dust shrouded nearly the entire globe.
Solar heating is responsible for producing dust storms. Temperature differences from place to place create differences in air pressure. And changes in air pressure are what makes the wind flow. While Martian dust storms can occur just about anywhere and at any time, they're more prevalent during the southern hemisphere summer. Heat from the sun causes the air at the Martian surface to stir and rise, picking up dust and lifting it into the air. The dust is very fine, so once suspended in the thin atmosphere, it takes time to settle out.
Not only that, but once a puff of dust leaves the surface it absorbs sunlight and heats up further. When conditions are right that can drive a feedback loop. More heat creates more wind which lifts more dust and expands the cloud, producing ever-stronger and more widespread winds. The result: a self-perpetuating and expanding blanket of dust that billows across the landscape, gathering strength with each passing day, or Sol, as they're called on Mars.
Unlike the Earth with its oceans and seas that might help to knock down a dust storm, Mars has exactly zero bodies of liquid water. Lack of water both on the ground and in the atmosphere allows a storm grow with little resistance.
In this way, what may have started as a simple dust devil can evolve into a planet-encircling storm. Or separate smaller storms can merge into a single more powerful front. The time is ripe — it's now mid-summer in the southern hemisphere with large variations in temperature from day to night.
NASA's InSight lander, which includes a weather station, takes daily measurements of temperature, wind and pressure from its location on Elysium, a flat, smooth plain near the Martian equator. This past week it's recorded daily highs around 15° F and nighttime lows around -140° F (-11° to -93° C). Talk about extreme!
In the movie The Martian a powerful dust storm topples equipment and wreaks havoc at the Mars base. If you watched the movie and suspected there was some artistic license taken in that scene you were right. Winds can blow up to 60 mph (97 kph) on the Red Planet, but because the air is so thin (less than a thousandth the pressure of Earth's atmosphere) it would feel no different than a gentle, 8 mile (15 km) an hour breeze here on Earth.
Right now, U.S. observers can only view the hemisphere opposite the big storm, where the weather's still clear. That will change come early December when the dusty western hemisphere will face our direction. Who knows? Maybe the storm will grow big enough to make it over to our side before that happens. Amateur astronomers across the country are poised.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.