The weather report from Mars may soon come with a forecast for dust storms, following the discovery that, in 78% of cases, a Red Planet dust storm is preceded by an unusually high amount of solar heating.
Dust storms are one of Mars' natural wonders, and also one of its greatest hazards. They begin on small scales, but some can blow up into global storms that enshroud the Red Planet in obscuring dust, hiding even the largest surface features from our prying eyes.
The thin Martian atmosphere limits the damage that the dust-strewn winds can do — they certainly can't build up enough force to threaten to blow a spacecraft over, as was the perceived danger in Andy Weir's novel "The Martian," but they can deposit large amounts of dust onto solar panels. In this way, NASa's Mars rover Opportunity was deprived of solar energy in the middle of a global dust storm and could not be reactivated later.
"Dust storms have a significant effect on rovers and landers on Mars, not to mention what will happen during future crewed missions to Mars," said Heshani Pieris of the University of Colorado, Boulder. "This dust is very light and sticks to everything."
Related: Mars dust storm mysteries remain as scientists study the Red Planet
Pieris and her Colorado colleague, Paul Hayne, scrutinized eight Martian years' (equivalent to 15 Earth years; one Martian year is 687 Earth-days long) worth of data from the Mars Climate Sounder instrument on board NASA's Mars Reconnaissance Orbiter.
The Climate Sounder observes the surface of Mars and the atmosphere above the limb of the Red Planet on the horizon in visible and near-infrared light, monitoring the way temperature changes on the surface can affect the atmosphere and vice versa.
Pieris and Hayne used measurements performed by the Climate Sounder to investigate two types of dust storm on Mars, denoted as "A" and "C" storms. These storms start in the Red Planet's northern hemisphere and extend along two well-worn dust-storm paths through Acidalia Planitia and Utopia Planitia. (Another category, "B" storms, begin around the south pole, often using dust from deep within Hellas basin as fuel.)
The researchers found that 78% of dust storms during that period were preceded by an extended period of what passes for "warm" and sunny days on Mars. (The average surface temperature on Mars is minus 60 degrees Celsius, or minus 80 degrees Fahrenheit, but can on occasion reach up to 20 degrees C, or 70 degrees F, at the equator.)
Although it's not incontrovertible proof that warmer days lead to dust storms, the correlation between the two is strong, argue Pieris and Hayne. And, perhaps, it's not too surprising, given that something similar happens on Earth. How often are hot spells brought to an end by a thunderstorm, for example?
"When you heat up the surface, the layer of atmosphere right above it becomes buoyant, and it can rise, taking dust with it," said Pieris.
The researchers suggest that this is a big leap toward being able to forecast the appearance of dust storms. Already, they have developed an algorithm that can predict the appearance of an "A" or "C" storm, based on surface heating, with a confidence of 64%.
"This study is not the be all, end all of predicting storms on Mars," said Pieris. "But we hope it's a step in the right direction."
Even if their model is correct, there remain many unanswered questions about the physics of Mars dust storms. For example, why are some dust storms localized events, while others grow into global phenomena?
"We need to understand what causes some of the smaller or regional storms to grow into global-scale storms," said Hayne. "We don't fully understand the basic physics of how dust storms start at the surface."
Supporting Pieris and Hayne's "sunny days" model are findings from NASA's now-deactivated Mars Global Surveyor, which orbited the Red Planet between 1997 and 2006, and was on hand to witness the global dust storm of 2001. The spacecraft's Thermal Emission Spectrometer measured a large imbalance between the amount of solar energy being absorbed by the surface of Mars, and the amount radiated away. Warmer spells, particularly during southern summer when dust storm activity peaks, would exacerbate this imbalance, providing the energy for the dust storms.
On Earth, we long for warm, sunny days. It could well be that future crewed missions to the Red Planet might not be so eager for the onset of summer, for the threat of dust storms that it brings.
Pieris and Hayne presented their research Dec. 10 at the annual fall meeting of the American Geophysical Union in Washington, D.C.
