A massive dust storm on Mars that threatened a NASA lander is finally fading away.
In late September, NASA's Mars Reconnaissance Orbiter sounded an alarm for missions studying the Red Planet: a dust storm was brewing. Such storms have a grim reputation among surface missions since a storm circling the whole planet ended the solar-powered Opportunity rover's mission in 2018. And NASA now has another solar-powered robot on the surface to worry about: Its InSight lander. For a few weeks, the storm left InSight in peace, but by early October, dust had darkened the skies above it and spacecraft personnel were worried their mission would come to an abrupt end.
Now, it's clear that although the InSight mission's end still looms, the lander is through the worst of the storm and dust is beginning to drop out of the skies, Claire Newman, an atmospheric scientist at Aeolis Research who works on weather observations from Mars surface spacecraft, told Space.com.
Scientists are still working to understand the nuances of the Red Planet's weather, and of its dust storms in particular, Newman said. Small local dust storms can occur year-round, but larger storms become more common as summer ends in the southern hemisphere, so a storm like the one that threatened InSight isn't beyond the norm.
"It's a type of event that we often see at this time of year," Newman said. "We were hoping that it would just turn out to be a regional one. It seems like, of its type, it's a fairly big one."
Storms follow a seasonal rhythm because they are triggered by heat imbalances that lift dust off the surface and into the thin Martian atmosphere. There, it triggers a vicious cycle. "You raise the dust, the dust gets heated, that tends to reduce these temperature gradients, so locally you tend to get some stronger winds, and then they tend to lift more dust," Newman said.
Orbiters can detect brewing storms in images and temperature data, but missions on the surface can also identify storms, even distant ones, because the dust causes the typical daily Martian cycle of atmospheric pressure to become more dramatic. The Perseverance rover, for example, detected these pressure changes in the early days of the storm, even with no dust in the skies above Jezero Crater.
"It's a global response to something that might only be happening in a third of the planet, or less," Newman said.
That's different from a global storm, which fills the atmosphere around the entire planet, east to west, with dust. These storms can form every few years because on Mars, it's difficult to stop the feedback cycle that dust in the atmosphere triggers.
"You don't get a global scale dust storm on Earth, and that's partly because the thick atmosphere is sort of preventing these really strong feedbacks," Newman said. "But it's also because you've got the oceans and you've got water and rainfall, and that's pulling the dust out of the atmosphere, whereas on Mars, you don't have any of that to slow down the dust."
Although the recent storm was a large regional event, it didn't manage to blanket the entire planet. The reprieve may stem from the fact that in general, at this time of year, surface winds on Mars blow from the north, and this storm began in the south. So while the upper atmosphere in the northern hemisphere (where Perseverance is posted) became somewhat dusty, the storm itself might have struggled to gain a foothold at the surface and grow northward.
"It could be that the time of year when this is happening and the background circulation was what made it more difficult for it to expand and become global," Newman said. "It's going to find it hard to expand toward the north and into the northern hemisphere at low levels, because the background flow is coming in the wrong direction."
It's unlikely, although not impossible, that Mars will see another large dust storm this year, Newman said. "We are probably coming to the end of the big dust storm season," she said. "You can never say never with Mars."
The recent storm is the second large one to occur this Earth year, following a storm that occurred near the Perseverance rover's outpost in Jezero Crater. That storm came unusually early in the season and was particularly intriguing, Newman noted.
"It was the first time we got to take detailed observations in a location where there was active dust lifting during the storm," she said. "We were actually in a storm source location."
Previously, other surface missions in locations where dust was lifting off the surface have had to hunker down, unable to observe the phenomenon. That said, Perseverance's January observations didn't quite go according to plan: the storm's winds were strong enough and carried enough debris that they damaged one of the rover's wind sensors, hampering the measurements the rover can take.
Fortunately, there was still plenty to see. "When that January storm was coming over us, we saw a big increase in dust lifting and dust devils even," Newman said. "We also saw a lot of motion on the surface; we saw basically little ripples that were moving."
InSight is particularly vulnerable to dust storms because of its reliance on solar panels, but the nuclear-powered Curiosity and Perseverance rovers still benefit from knowing a storm is approaching.
Scientists can advance activities that would be easier before a storm arrives, for example, or delay activities that might be more dangerous in heavy winds. And, of course, they can schedule additional observations of the storm itself. "Even if you don't have to worry about the dust, you still want to get your ducks in a row for doing weather monitoring," Newman said.
And weather observations, especially at the surface, are crucial for scientists working to develop models of dust storms that better match reality.
"There's no model that can really simulate the lifecycle of a dust storm really well," Newman said, unless scientists load them with lots of particular specifications. "All the models kind of diverge from reality at some point."
Those models are vital for the work she and other atmospheric scientists are doing using surface observations at individual locations in addition to orbital data to piece together the dynamics of Mars at a much bigger scale than any single spacecraft can observe.
"We're trying to understand where we are," she said of surface weather stations. "We're trying to understand the whole planet by understanding where we are. And we're also trying to understand the past by understanding the present."