As Mars approaches its southern summer solstice, scientists and spacecraft are preparing for a new season of dust storms, mighty wind-driven clouds of dust that can cover thousands of square miles or even the entire planet.
To learn more about what drives dust storms on Mars, Space.com spoke with Claire Newman, an atmospheric scientist at Aeolis Research and the lead author of a new study exploring how dust devils and winds can fill the Martian atmosphere with dust.
Space.com: How much do we know about how dust storms start?
Newman: I could talk for hours about dust storms! One of the big questions that we have about Mars is how the big dust storms start, and will we ever be able to predict them. To answer them, one of the really useful things we need to know is how the dust gets lifted off the surface in the first place. Another important thing to know is how much dust there is on the surface to be lifted. We can understand the processes really well, but if we don't actually know how much dust there is at a particular location, then it's really hard to be able to estimate it accurately. Then, the third thing you need is to have a weather model that describes the environmental conditions. There might be certain types of atmospheric waves or pressure patterns that might make Mars predisposed to having a certain type of dust storm.
Space.com: What do the models tell us?
Newman: I published a paper with my colleague Mark Richardson in 2015, where we modeled the dust storms but with a restricted amount of dust. We found that the places that had the strongest winds never retained enough dust to really produce a lot of dust storms. But the places that produced an average amount of wind, there was dust coming and going from them, because it had enough time to get redeposited and then lifted up.
Then, there were interesting things such as one region might run out of dust completely during one storm, and then it wouldn't be able to produce the same storm the next year because there wouldn't be dust there — and it might take three years for the dust to build back up and that wind pattern to repeat. Then, you have to factor in where the winds peak due to seasonal effects and the shape of the land — where the mountains are, where the slopes are, where the ice caps are. There's also the possibility for variability in the wind, whether it repeats exactly from year to year. There's this notion that the system is a little bit chaotic, because one little difference could trigger a big feedback.
Space.com: But isn't the dust-storm season predictable to some degree?
Newman: Yes, there's usually a very repeatable storm track, and there are certain smaller storms that appear in certain places at certain times of year. But the big storms typically don't seem to occur at the same exact times. So, in a way, I think we have a sense of when storms might occur and what they might look like, but there can be differences. For example, one year a storm might travel south, and the next year it doesn't because it's blocked by a combination of pressure patterns. And the year after that, there might be a different combination that allows the storm to propagate. They really are very hard to fully understand, and you can attack it from lots of different ways. But there's lots of information we don't yet have.
Space.com: Are there any similarities between dust storms in Earth's desert regions and the dust storms on Mars?
Newman: Mars has some big differences compared to Earth: It has a much thinner atmosphere, it's colder and it has a lower gravity. So Mars is a great way for us to try and test our theories about dust lifting and sand motion, because we have many ideas based on theory and decades of wind-tunnel research and fieldwork being done out in Earth's deserts.
And it's a good question: Are they the same processes? For example, do electrostatic effects have a bigger impact on Mars because Mars is drier? On Earth, water is an extremely important factor in how much dust gets lifted; it limits the size of dust storms and prevents global storms from developing, since clearly, there's no dust to lift over oceans and lakes or over places that are more vegetated. On Mars, is water still a factor, or is there so little water that it doesn't have an effect?
Space.com: How does dust in Mars' atmosphere affect the planet's climate?
Newman: When dust is in the atmosphere, it absorbs heat from the sun. Because Mars has a very thin atmosphere, a hundred times less dense than Earth, usually most of the solar radiation that comes through to the surface is then reflected back into space without being absorbed. But when you put 10 times more dust into the atmosphere, it absorbs a lot more solar radiation. This ends up heating the middle atmosphere, about 25 kilometers [16 miles] high, where you might get a 40-degree-Celsius [72-degree-Fahrenheit] increase, while at the surface, the temperature will go down because the dust is absorbing the sunlight. Then, at night, the surface cools off faster than the atmosphere, and because it is warmer, the atmosphere begins to warm the surface below it. And there are huge temperature gradients that drive winds and thermal tides that alter the atmospheric circulation, particularly during a global dust storm.
Space.com: Is there any connection between dust storms and Mars losing its water to space?
Newman: There's some interesting questions about dust storms and the rate of water loss. During the 2018 global [Mars] storm, there was a big increase in water being raised to higher altitudes. A dust storm causes an increase in vertical wind velocity and, consequently, the height reached by the dust, and that can allow more water vapor to be transported higher because it's warmer and there are also greater updrafts. So dust storms have a big impact on water-loss rates.
Space.com: Can the dust warming the atmosphere pose a danger to spacecraft?
Newman: As the dust heats the atmosphere, the atmosphere inflates upward, which must be taken into consideration if you're trying to aerobrake a spacecraft ahead of entry, descent and landing. You have to worry about the air density and wind strength, and all those things. By understanding these things better, we can give early warnings that there may be dust storms. Also, from measuring the air pressure at different landing sites, we can get a sense of when thermal tides are growing. And that is due to a lot of dust being in the atmosphere, and so we can give some early warning there. If there are astronauts on the surface, it would be great if they could have this information so they can take cover.