When you peer across the rusted Martian surface through photos, you would be forgiven for thinking
that Mars is a place of stillness. Flat, dusty landscapes seem to stretch out in all directions,
with nothing but scatterings of rock and the faint whistling of the wind to keep at bay the
almost oppressive solitude. But stay a while on this red world, and you will soon see a towering
wall of dust and sand brewing on the horizon. You might think to yourself that this dust storm
is similar to those we see on Earth, and have no idea that this storm is about to grow so large,
its thick dust will swallow up not just a region, not just a continent, but the entire planet.
This all-enveloping superstorm arises on Mars usually every 3 Martian years, or about 5 ½
Earth years. Its choking dust will blot out light for weeks, or even months, and represents a huge
challenge to the continued functionality of any technology that we put up there. If humans ever
want to settle down on the Red Planet, it will be a hurdle we have to overcome.
Which is a problem, as there is much about the origins of these storms that we still don’t know.
I’m Alex McColgan, and you’re watching Astrum. Join with me today as we explore what we do
know about the characteristics, mechanisms and impacts of Martian super dust storms,
and what any humans would need to be aware of to make it through one.
Mars didn’t use to have to deal with planet-sized dust storms. Although Mars is now a barren, arid
planet, it once had a thick atmosphere that was warm enough to support the existence of running
water. However, over the course of the billions of years of Mars’ existence, Mars dried out, and
its atmosphere bled away until atmospheric volume was at less than 1% of what we have here on Earth.
Those dust storms didn’t just start happening because Mars became dry, though. Although we
don’t understand everything about such storms’ origins, we assume that a key component is Mars’
temperature. With less atmosphere, in spite of Mars’ high CO2 levels, Mars became far worse at
retaining heat. When the surface starts to cool, there is no air to catch the escaping warmth. It
is at the point where, if you were to stand on the planet’s equator during its warmest time of the
day, your feet might feel 23°C while at your head it would be 0°C. This means between day and night,
Mars has some intense temperature swings. Temperatures there now range from highs of
around 27°C, down to a freezing -133°C at night. Temperature difference can cause winds to form,
which can bring different weather systems across a planet. However, Mars’s arid weather is no longer
driven by rain and water cycles, but cycles of dust. Dust plays a surprisingly critical role
on Mars, and without it, those planet-spanning storms would likely never form. The atmosphere
might be too thin to capture and transport heat, but the Martian dust – now that’s another story.
It all begins with that Martian dust getting into the air.
There are a few mechanisms that make this happen. One is dust devils,
of which Mars experiences thousands every year, usually during the Martian Spring and
Summer. Warm rays from the Sun heat the ground, causing the air directly next to it to rise,
and cool air from the atmosphere to be drawn down to fill the vacuum. These contrasting
winds create rising spirals of air that can end up hundreds of metres wide and 8.5 km tall,
although many are much smaller. Regardless of their size, as they meander their way across
Mars’ all-encompassing deserts, they suck up dust and hurl it up into the atmosphere,
creating a haze of slightly darkened skies in their wake. This process is thought to
contribute to a miasma of background dust that constantly lingers in Mars’ atmosphere.
While this is the flashiest way by which dust gets into the atmosphere, it’s not the most prevalent.
Far more common is the simple influence of wind moving across Mars’ dusty surface, and a process
known as saltation. Mars’ dust is surprisingly difficult to get up into the air – small particles
have a lot of cohesion due to being slightly electrostatic, kind of like packing peanuts,
so they like to stick together – which means they need a certain amount of momentum to get
them going. Oddly enough, larger particles are actually easier for the wind to get moving as they
experience less cohesion. So, it’s these larger sand grains that are lifted by gusts of wind, and
are moved for short distances. But because they’re ultimately too heavy for the wind to suspend,
they crash down again, and the force of these tiny impacts imparts enough momentum to overcome
cohesion and get the lighter dust airborne. However, once it gets up there, Mars’ lower
gravity means that it’s easy for dust to remain in Mars’ atmosphere for a really long
time – weeks or months. And this is enough to start driving the formation of storms. Because,
unlike the thin air around them, particles of dust are really good at collecting heat from the Sun.
As the Sun warms dust in the atmosphere, they act like little radiators, slowly releasing
the gathered heat into the air around them where previously the warmth would have passed
through to the ground below. This makes the atmosphere nearby much warmer. Warm air rises,
but now it begins doing so on a much larger scale than when forming the
dust devils earlier. Wind has to be drawn in from the sides of the rising air mass,
but this only adds more fuel to the fire. More wind means more saltation,
more dust being thrown into the atmosphere, and more opportunity for the atmosphere to warm.
Eventually, this out-of-control process ramps up into a regional dust storm.
The good news is that on a local scale, a dust storm on Mars is not very dangerous in and of
itself. Wind speeds top out at 97km/h, only half the speed of hurricane winds on Earth. And even
this is misleading; due to the thinness of the atmosphere, even when it is travelling
quickly you wouldn’t feel much, in the same way a single person has less pushing power
than a whole crowd. Despite what you might see in some science fiction stories based on Mars,
dust storms are not powerful enough to push over spacecraft or break satellite dishes.
However, such storms carry their own incidental dangers. It’s very
bad news for any Martian rover that relies on solar power to function if a dust storm
filled with slightly sticky electrostatic dust particles suddenly passes overhead. Gradually,
dust deposited from such storms is enough to block sunlight from reaching a rover’s solar
panels, which has spelled the end of more than one mission from Earth.
But the problems magnify when you start scaling such storms to larger and larger sizes.
Mars does not have a perfectly circular orbit. Every year, around the Martian Southern
hemisphere’s Spring and Summer, the planet is at its closest to the Sun and as a whole is
warmest. For reasons we don’t fully understand, during this time of year regional dust storms
start merging into a superstorm the size of a continent. It’s of course possible that several
storms just happened to be forming anyway, and they started to merge just by proximity. Although
their regularity makes it seem like coincidence is insufficient an explanation – after all,
these happen yearly, like clockwork, once the temperature gets warm enough.
Those are just the continent-sized ones. Scientists as of yet do not have an explanation
for the stage above that – the planet-sized storms. These occur once every 3 Martian years,
which seems like a short enough timescale that you can rule out Milankovitch cycles as their origin;
those normally work on timescales of thousands to hundreds of thousands of
years. Perhaps there is some aspect to the dust cycle that needs to restock,
and it takes 3 Martian years, or around 5 ½ Earth ones, to do so? We don’t really know.
While most dust storms on Mars last only a few days, these apex of storms can last for weeks.
The impact of these storms can be profound for any human technology around the planet.
Scientists can measure the availability of sunlight on Mars using Aerosol Optical Depth,
or AOD, to check how much sunlight is absorbed or reflected by pollutants in the air. The AOD
on Mars is usually around 0.5. For context, an AOD of less than 2 is needed before rovers like
Opportunity or landers like Insight can charge their onboard batteries. The thick smoke caused
by a wildfire on Earth that starts to turn day into night is an AOD of 7. These planet-sized
dust storms hit AOD of 9 to 11. They represent a near total blockage of light. In 2018, a storm of
this variety killed the Opportunity rover, by swamping its solar panels and forcing it into
hibernation mode. Without power running through its onboard heaters, the wildly oscillating
day-night temperatures caused something critical to break in the rover, and it never woke up.
These storms are a problem for other rovers too – even ones with onboard nuclear batteries like
Curiosity. The clouds are thick enough to block communication to and from the surface,
meaning scientists have little choice but to wait them out. But that’s not all. Even satellites
can be affected by these storms. The colossal amounts of warming dust in the air causes the
entire atmosphere to expand, bringing some of it into the orbital paths of satellites. They have to
burn precious fuel making course corrections, to ensure that atmospheric drag doesn’t bring
them crashing down out of the Martian sky. If you were a human hoping to settle on Mars,
you’d need to find a way to overcome these issues. Solar-powered settlements would
not survive such storms without other sources of power – and humans don’t do well at such freezing
temperatures. Losing the ability to contact our satellites would also leave us isolated
and vulnerable for those few weeks. People would need to have a certain degree of self-sufficiency,
as if anything were to go wrong while a storm was blowing, you wouldn’t be able to call for help.
Still, in time these storms prove to be their own destruction. Blocking so much
light eventually means that the surface stops getting warmed enough to create the upswelling
winds that lift the dust into the air in the first place. Without more rising dust,
these storms starve themselves, and peter out. Quiet returns to the red planet. Things are still
once more. Any rovers that survived can emerge from hibernation mode,
ready to continue with their science now that communications with Earth have resumed.
As of 2023, there are 3 rovers active on Mars; NASA’s Curiosity and Perseverance,
and China’s Zhurong. Of the 3, Zhurong is the only one utilising solar panels as a
power source. It will have to be careful. The last super storm was in 2018. Given
that these occur on average every 5 ½ Earth years, the next one could be coming soon.
A little wind will start blowing, some dust will start to stir, and the cycle will begin anew.