Forecasters saw it coming a few days in advance: Winds were increasing, dust was kicking up across the landscape, and visibility was dropping. Flights would have to be canceled...another flight delay in a year full of them.
Except this time, the disruption was on Mars. The flight delay was for NASA’s Ingenuity helicopter, which in January 2022 became the first aircraft grounded by a dust storm on another planet (image above).
Both Mars and Earth are regularly battered by dust storms of various sizes. And although there are some similarities between the events—now including flight cancellations—there are also some key differences.
On Earth, one third of the land area is covered in sand and dust, whereas the rest is anchored by plant life, ice, water, and human settlements. The largest dust storms arise from vast deserts such as the Sahara (shown below), the Gobi, and the Arabian Peninsula. They also can kick up in the Outback, Patagonia, the southwestern U.S. and Mexico, or regions experiencing severe drought. According to scientists’ estimates, 20 to 40 million tons of dust are floating in Earth’s atmosphere on any given day, and anywhere from 1 to 3 billion tons of dust are lofted and deposited back to Earth each year (equivalent to 10,000 to 30,000 fully loaded aircraft carriers).
On Mars, dust devils and storms can arise just about anywhere because the planet is dry and dusty nearly everywhere we have looked. Billions of years of weathering—probably from ancient water and more recently by perpetual sand-blasting from wind-driven storms—have created a planet covered in grit and dust. The particles are very small and slightly electrostatic, so they stick to surfaces the way foam packing peanuts do on Earth.
On both Earth and Mars, dust storms can severely degrade air quality while clogging engines and gears. On Earth, planes can be grounded by airborne dust because it can damage jet engines and scour windshields, while also reducing visibility for pilots. On Mars, a major dust storm in June 2018 ended the mission of NASA’s 15-year-old Opportunity rover. So much dust was aloft for so long that the rover’s solar panels could not harvest enough sunlight to recharge the batteries.
Earthly dust is typically picked up by weather fronts and storms, with differences in temperatures, atmospheric pressure, and moisture content accelerating the air flow across the landscape. Strong winds move across dry lands and pick up loose grains of mineral dust, sometimes lofting it hundreds to thousands of feet into the atmosphere and carrying it tens to thousands of miles downwind. (The photo below, shot from the International Space Station, shows a storm over North Africa in 2014.) But the dust does not rise above the tropopause—the natural pressure barrier between the troposphere and the stratosphere. Even if dust gets that high, gravity and precipitation bring it back down to the surface. All of this usually occurs in a matter of hours to days.
Mars, however, has no tropopause. NASA aerosol scientist Ralph Kahn, who studied planetary atmospheres before focusing on Earth, explained that without this lid-like barrier (created by stratospheric ozone on Earth) and without precipitation to bring it down, Martian dust can rise higher into the atmosphere.
Martian winds mostly arise from differences in the way the surface is heated by sunlight between hemispheres, between day and night, and between elevated and depressed surfaces. These variations in heating lead to rising plumes of air on local scales (making dust devils) and spur the horizontal flow of winds on regional scales.
Though probably not as potent as the stormiest winds on Earth, the winds on Mars are strong enough to spin up everything from dust devils (shown in the animations below) to vast, sunlight-blocking plumes. Contrary to some movie fiction, the storms are unlikely to be potent enough to blow over vehicles and people. This is because the Martian atmosphere is thinner—about 1 percent of the density (pressure) of Earth’s atmosphere at the surface—so similar winds do not exert as much force on the Red Planet.
“Dust is the key to the Martian climate,” said Mark Lemmon, a planetary scientist from the Space Science Institute who studies aerosols and has been involved with NASA’s Mars program going back to the days of Pathfinder. “Dust is moved by the weather, but then the dust sustains that weather. It stores and moves energy around Mars the way water can move energy around on Earth.”
Once it is aloft, Mars dust tends to stay in the air a long time—sometimes for weeks to months. Gravity is weaker—about one third of what it is on Earth—and the thinner atmosphere means there is less drag. And once it is started, a dust storm on Mars can sometimes feed itself. Dust in the Martian atmosphere absorbs sunlight, which warms the air around it. This accentuates the heating differences between the surface and the air and between one part of the surface and another—both of which can cause more vertical and horizontal air flow.
“The impacts are cumulative and dramatic,” Lemmon said, “so a lot of dust accumulates in the sky.” The image above from the Mars Reconnaissance Orbiter shows a thick plume of dust over Utopia Planitia in November 2007.
Every year, Mars has a few moderately big dust storms that cover continent-sized areas and last for several weeks. And every so often, for reasons scientists cannot yet explain, Martian dust storms can build upon themselves until they engulf the entire planet. (The 2018 global dust event is shown below.) These massive, planetary-scale storms seem to occur about every three to four Mars years (six to eight Earth years). “There is nothing comparable on Earth,” Kahn noted.
One of the ways to gauge the intensity of dust storms on Earth is through aerosol optical depth (AOD), a measure of how much light is absorbed or reflected by particles of dust, natural aerosols, or pollutants in the air. According to Kahn, a clear day on Earth has an AOD of 0.1 to 0.2 in mid-visible light. When the AOD is 1.0, the intensity of sunlight at the surface drops by two thirds. A really thick smoke or dust plume might have an AOD of 7.0 or higher, which is why daytime can start to feel like night when the sky is full of particles from major wildfires or dust storms.
According to Lemmon, when the Opportunity rover stopped working during a global dust storm in 2018, the AOD on Mars was somewhere between 9 and 11. Similar global dust events also occurred in 2007 and 2001.
Dust storms typically grow largest on Mars in the southern hemisphere summer (northern hemisphere winter). While seasons on Earth are entirely a function of the tilt of our planet’s axis, Mars (which also tilts on an axis) is significantly closer to the Sun during its southern hemisphere summer. This leads to more heating of the surface and more dust getting lofted into the atmosphere than at other times of the year.
With southern summer now setting in, we might expect more flight delays in the coming months on Mars.
NASA Earth Observatory images by Lauren Dauphin, using VIIRS data from NASA EOSDIS LANCE, GIBS/Worldview, and the Joint Polar Satellite System (JPSS). Mars Reconnaissance Orbiter images by NASA/JPL-Caltech/MSSS and Perseverance Mars rover animations by NASA/JPL-Caltech/SSI. Astronaut photograph ISS040-E-90343 was acquired on September 8, 2014, with a Nikon D3S digital camera using an 80 millimeter lens, and is provided by the ISS Crew Earth Observations Facility and the Earth Science and Remote Sensing Unit, Johnson Space Center. Story by Michael Carlowicz, with reporting from Katy Mersmann, NASA Goddard Space Flight Center.