Massive dust storms periodically blanket the Red Planet—and we’re still not sure why.

Just over 50 years ago, NASA’s Mariner 9 arrived at Mars, becoming the first spacecraft to orbit another world. Earlier flyby missions had yielded brief glimpses of the Red Planet. This latest endeavor, however, would photograph the entire surface of an alien world.

But Mars would hold onto its mysteries for a while longer. Mariner 9’s first images showed a planet completely hidden by a bland murk of dust, except for the peaks of a handful of large volcanos. The spacecraft had arrived during the most severe dust storm seen on Mars in 70 years of telescopic observation.

Happily, after a few months, the dust cleared, and Mariner 9 began mapping the surface, discovering evidence of past water and the giant canyon system Valles Marineris. But the dust event would foreshadow one of the biggest challenges of exploring the Red Planet in the ensuing decades: Martian dust is everywhere—it periodically engulfs the entire planet and evidence suggests that some of it even streams into interplanetary space. And the dust sticks to everything, posing threats to current robotic explorers and—in the future—to humans during their long stays on Mars.

Yet we’re still not sure where the Martian dust comes from—and how it spreads. The dust storms are, in a word, baffling. “Global dust storms occur on average only three times per Mars decade,” says Claire Newman, a planetary scientist at Aeolis Research in Arizona. Her fascination is driven by “these tiny, simple particles getting into the atmosphere and having such profound effects on an entire planet.”

Mystery Storms

Figuring out why some Mars years have storms and most don’t is an enduring puzzle. “We have observed only three global storms since the period of continuous coverage began, and each was different,” says Newman. “Understanding these storms from such a limited dataset is kind of like trying to learn the rules of a complicated card game by watching only a few rounds with no explanations provided.”

The weather on Mars—other than the dust storms—is actually rather predictable. That’s because the winds are driven by solar heating, and the solar illumination on the Red Planet varies only slowly from day to day, and is almost identical year to year. As such, the winds on any given day are often nearly the same as those on the same day during the previous year.

So what is the persistent hidden variable amid these unchanging weather conditions that explains why some years have global dust storms and others don’t? On Earth, the main factor in year-to-year variations is the heat stored in the oceans, which we now monitor by satellite and by buoys, allowing us to predict, for example, whether the weather in the next few months will be affected by El Niño or La Niña.

But on Mars, where the atmosphere is too thin to retain heat, the most likely source of variability seems to be the distribution of dust on the surface. “If a global storm lifts lots of dust from one region and leaves it bare, then by the same time the following year there may still not be enough dust there to produce a local or regional dust storm, even if the winds in both years were identical,” says Newman.

An added challenge to predicting storms is feedback, when dust absorbs sunlight and heats the atmosphere locally. This, in turn, can strengthen the local winds, which then lift more dust and so on. Since the global storms begin as small regional storms that grow or merge, even very small changes in the wind can make all the difference between a year with a storm and one without.

Another feedback has been discovered recently by a trio of orbiting spacecraft sent by NASA and the European Space Agency (ESA) to observe the planet. Not only is Mars dusty because it is dry, but it may be dry because it is dusty. Some water remains on Mars in permafrost and ice deposits, and perhaps in deep groundwater. But researchers believe that in the past, much more water was present—the evidence is valleys carved by oceans and rivers. One possibility is that the water was lost into space, but how? The cold atmosphere on Mars freezes water vapor at low altitudes.

Data gathered by the three NASA-ESA orbiters might hold the answer. The spacecraft observed that a 2019 dust storm heated the air at higher altitudes, increasing the middle atmosphere’s water content tenfold. After the water reaches the upper atmosphere, it’s believed that the sun’s ultraviolet radiation breaks it down into oxygen and hydrogen—which, as the lightest element, is easily lost into space. Thus, the occurrence of dust storms may have been a crucial factor in drying the planet.

Ashes to Ashes, Dust to Dust

So where does all this dust come from? On Earth, dust is largely made by the grinding action of glaciers, although some is also produced in rivers and by the abrasion of blowing sand. Dust can blow around, sometimes over remarkable distances (iron in dust from the Sahara Desert may be important in fertilizing the Amazon rain forest), but generally it gets trapped when it falls into the oceans. The Earth’s biggest dust deposits, called “loess,” date from the Ice Ages, when there was more glacial dust production, sea levels were lower (thereby exposing sediment-rich coastal plains), and the cold dry atmosphere had less rainfall to rinse the dust out of the air.

Mars is colder and drier than even the Ice Age Earth. The relative absence of moisture means that snow does not accumulate rapidly to drive glaciers, so dust production is weak. On the other hand, there are no oceans on Mars to act as dust sinks, so any existing dust keeps swirling around the planet. 

The dust isn’t deposited uniformly, however. John Grant, a senior geologist at the National Air and Space Museum’s Center for Earth and Planetary Studies, remarks: “There are bright areas on the surface where [dust] often accumulates and darker areas where it appears there is typically relatively less. The regions around the polar caps are favored areas for accumulation in the current and recent past on Mars, but there appear to have been different or at least additional areas where accumulation was favored in the past.”

Measurements by the Mars Exploration Rovers (Spirit and Opportunity) of the elemental composition of Mars dust show it to be rich in sulfur and chlorine. While these are abundant elements in evaporites (minerals deposited by drying lakes), they are also abundant in volcanic ash. One region on Mars is particularly rich in these elements: the Medusa Fossae Formation (MFF). This is an extensive deposit of downwind material to the west of Mars’ major volcanos, and images reveal to geologists that the region is abundant in yardangs (wind-sculpted ridges that form in soft volcanic ash deposits). Another group of scientists, radar astronomers, call the region by another name: “Stealth.”  The MFF reflects very little of the energy from beams directed at it by radio telescopes on Earth, consistent with a low-density, fine-grain deposit tens or hundreds of meters thick. This evidence points to the MFF as being the single largest source of dust on Mars today.

Recently, hints have emerged that Mars might even breathe dusty traces throughout the solar system. The Juno spacecraft, now in orbit around Jupiter, has sensitive star-tracking cameras that can also detect debris shed when dust particles slam into the massive solar arrays that Juno needs to maintain enough power to operate so far from the sun. Scientists noticed that, on the way to Jupiter, the dust impacts detected by the star trackers were particularly abundant when the spacecraft was in the same orbital plane as Mars. In fact, the Juno team speculates dust from Mars, or possibly its moons, might be responsible for the zodiacal light, the faint glow sometimes seen in dark skies in the broad plane of the planets.

Part of the story might be that, as the floating dust absorbs sunlight, it heats the atmosphere locally and creates its own updraft. At the crests of Martian mountains, simulations have shown that these updrafts can converge to form “rocket storms” that shoot dust up to an altitude of 30 to 40 kilometers. In fact, Mariner’s images showed haze layers improbably high in the atmosphere—they might have been launched by these storms. But a mechanism to get dust from these altitudes all the way into space still remains a mystery.

Dust Deviled

The dust storms have taken a toll on the robots sent to explore the Red Planet. In 1997, the first NASA rover, Sojourner, lost about 0.25 percent of its solar output every sol (Mars day) due to dust settling on its solar panels. Sojourner’s experience with dust drove expectations for how long Spirit and Opportunity would last on the Red Planet. “Everything we had planned for involved a mission of 90 sols for each rover,” recalls Grant.

But every so often, the solar array output of the rovers recovered. Dust devils—whirlwinds caused by convective updrafts driven by solar heating—seem to have removed the dust from the rovers’ solar panels, enabling Spirit and Opportunity to function longer than anticipated.

Unfortunately, Spirit got stuck in a sand trap facing away from the sun and didn’t make it through its third Martian winter. Communications were lost after 2623 sols. Still, Opportunity kept operating—that is, until the dust storm of 2018. “Opportunity had been through a dust storm before, but the 2018 global event was much larger and there was a lot of growing concern as the sky darkened and rover power dropped,” says Grant. “I definitely kept my fingers crossed, and it was really sad when the end of mission was finally declared.” Opportunity had lasted an incredible 5352 sols before its dusty demise.

Just months afterward, the InSight lander arrived and began monitoring seismic activity and weather. The lander was equipped with the largest solar panels ever sent to Mars, designed to give it a lifetime of just over one Martian year (669 sols)—even without any cleaning events. Grant was optimistic that dust devils might still lend a hand now and then. “After the occasional solar panel dust clearing wind/dust devil events that enabled sufficient power for the rovers, I had similar hopes for dust-clearing events at InSight,” he says. “I felt it would just be a matter of time before there would be a large dust-clearing event at InSight that would bring back that ‘new lander smell.’”

But Mars had different plans. The output of InSight’s solar panels declined mercilessly and is less than a quarter of what it was when it arrived. This was not entirely unexpected, because orbital imaging of the Elysium plains prior to the mission showed that the formation rate of dust-devil tracks was about a tenth of that at Gusev crater (where Spirit had landed).

If the formation rate of dust-devil tracks—created by the removal of surface dust from the ground—is a reliable proxy for the removal of dust from solar panels, then a good array-cleaning event by a dust devil might occur at InSight only once in 10 Mars years or so.

This prediction, sadly, appears to have been borne out. As such, we have a new and important puzzle to confront as we plan future missions: What controls dust devil activity at different places on Mars? We have detected lots of vortices at Elysium using InSight’s sensitive pressure gauges, wind sensors, and even its seismometer since the low pressure in a whirlwind causes the ground to deform measurably. But we have not seen a single dust devil in the thousands of images from InSight’s cameras.

One clue might lie in the dust devil tracks. At Gusev, the tracks were often 50 meters or more in width, and many of those tracks were curly, suggesting that dust devils wandered around in calm conditions. By contrast, at Elysium, the tracks are predominantly narrow and very straight. This suggests the prevailing wind might be stronger, blowing the dust devils in a consistent direction, and perhaps somehow suppressing the formation of the largest, most intense dust devils.   

There are doubtless other factors at work, such as the effect of nearby topography on winds and the small-scale roughness of the terrain. But until we understand what places on Mars might see regular dust removal and which do not, we cannot be sure solar-powered landers can operate for much longer than a Mars year unless we build in prohibitive margins—or some means to clean dust from the panels.

A self-cleaning system is more difficult than you might think. Very fine dust can tenaciously cling to surfaces, in part because of electrostatic effects—a familiar effect to anyone who has had styrofoam packing peanuts stick to their clothes. As on the moon, dust settling into equipment and spacesuit seals will pose a significant challenge. When Apollo astronauts tried using a whisk brush to clear dust from the cooling radiators of the lunar roving vehicle, they had little success and the batteries ran the risk of overheating. Airblasts, high frequency vibrations, and grids of wires to convey dust electrostatically are ideas that have been proposed, but we don’t know if any of them will actually work under Mars conditions.

Dust Busters

Meanwhile, InSight lacks any such gizmos. I’m a planetary scientist at the Johns Hopkins Applied Physics Laboratory and, as the power situation worsened last fall, I suggested, not entirely in jest, that all we could possibly do was sprinkle some sand on the solar arrays—the idea being that sand is more easily moved by the wind than is dust, and perhaps it might scrub off some of the dust. This “sandblasting” idea was based on a theory that sought to explain how dust devils managed to kick up dust, despite the low wind speeds in the thin Martian atmosphere. Researchers suggested that the wind-blown sand was acting as the mechanism for lifting dust off the planet’s surface.

This theory got a boost when, in early 2021, NASA endorsed a plan that would address another problem: the Martian wind blowing against the lander’s seismometer tether was creating noise that was interfering with accurate seismic readings. The solution was to create a secondary windshield by burying the tether. Accordingly, InSight’s robotic arm, which had emplaced the seismometer on the ground two years before, was used to scoop up some dirt (mixed sand and dust) and pour it onto the tether. Strikingly, where some of that drizzled material hit the now-dusty metal windshield over the seismometer, the surface was gleaming, scrubbed clean of the dust.

This literally glaring demonstration of particle adhesion mechanics gave mission managers the rationale to attempt a sand scrub of the arrays. A team of engineers and scientists began to meet regularly to plan the operations. We developed new command sequences to scoop up dirt and sprinkle it, given the physical limitations on how the arm’s joints can move. We reviewed meteorological data on wind speed and direction at different times of day to determine when the sand would be best winnowed onto different parts of the array and gauged how the lander energy and human labor expended in these efforts compared with the uncertain benefit.

The task became urgent, as not only was the unremitting accumulation of dust reducing the efficiency of the solar arrays, but Mars was moving toward southern winter. Mars’ orbit around the sun is eccentric, such that the Mars-sun distance changes over the year and even at the equator, the amount of sunshine varies appreciably. Mars was approaching aphelion—its farthest distance from the sun—in fall 2021, when not only would the amount of sunlight available be at its lowest point, but the lower temperatures meant that InSight needed more energy each day to power electrical heaters to prevent critical components, like the battery, from getting too cold.

On sol 884 (May 22, 2021) we tried the sprinkle. Images showed that some dirt fell on the lander deck as expected—but a dark swath appeared over part of the solar panel where sand had drizzled sideways on top of the bright red dust, partly removing it. The engineering telemetry showed an instantaneous jump in the current from the solar panel—about four percent. This might not sound like much, but the increase enabled InSight to continue its seismic studies in the ensuing months, gathering data on the largest quakes ever observed on the Red Planet.

The sand-scrub, however, probably just postponed the inevitable: Earlier this year, in January, a dust storm caused power to drop, forcing the lander to hunker down in “safe mode” with its instruments off for a couple of weeks. InSight is back online, but how much longer it can last remains to be seen. One day, the dusty cold and dark will get too much for InSight, and its mission will end. In any case, next time you draw “clean me” with your disapproving finger on a dusty surface on Earth, consider that, for Martian explorers, dust is a matter of success or failure. 

Ralph Lorenz is a planetary scientist at the Johns Hopkins Applied Physics Lab. He is the author of 10 books, including Space Systems Failures, Exploring Planetary Climate, and Titan Unveiled.

Remote Repairs

The next time you get frustrated while fixing your car, spare a thought for engineers who have to repair vehicles billions of miles away.

When Martian dust covered the solar arrays of NASA’s InSight lander—threatening to end its mission—a team of scientists and engineers came up with a novel plan to “sand scrub” the dust (see main article). The dust-removal technique is the latest example of the outside-the-box thinking that engineers have relied on for decades to repair malfunctioning spacecraft that are millions—sometimes billions—of miles away.

Consider, for instance, Voyager 1 and 2. Despite their tremendous distances from Earth—more than 14 billion and 12 billion miles, respectively—engineers have been able to undertake repairs and perform crucial maintenance. In 2017, for example, engineers discovered that the primary attitude thrusters on both spacecraft had degraded to the point that they might not be able to keep themselves oriented toward Earth. To make sure the spacecraft could continue to maintain proper orientation, the team successfully fired another set of thrusters that had been used for trajectory-correction maneuvers during flybys of Jupiter and Saturn early in the mission (in the 1980s).

The Galileo probe, launched in 1989, was 18 months into its journey to Jupiter when disaster struck. NASA engineers instructed the spacecraft to unfurl its high-gain antenna. But it refused to unfold—which meant that 90 percent of the data collected wouldn’t reach Earth, since the spacecraft would instead have to rely on its slower, low-gain antenna. JPL engineers tried just about everything, such as spinning the spacecraft at up to 10 rpm to shake the antenna loose. Nothing worked, but NASA managed to salvage the mission by reprogramming Galileo’s onboard computer to compress data and images more efficiently, while also boosting the sensitivity of several Deep Space Network antennas on Earth to receive the probe’s signals.

In December 2016, the Curiosity rover developed a problem with its drill, which it had used to collect 15 rock samples during its trek across the Martian surface. The drill—which sits at the end of the mobile laboratory’s mechanical arm—is equipped with two stabilizer posts that touch the rock to steady the device while the drill’s feed mechanism moves the bit forward into the rock. But when the feed mechanism stopped responding to commands, drilling operations ceased until early 2018, when engineers developed an alternative technique called feed-extended drilling: The bit is kept in an extended position and is then advanced into the rock by the motion of the robotic arm rather than by the feed mechanism.

Mark Strauss is the managing editor at Air & Space Quarterly.

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This article first appeared in the Spring 2022 issue of Air & Space Quarterly. Read the full issue.