One of Mars’ strangest clouds may rely on unusual physics to emerge.
The Red Planet’s ribbonlike cloud — which stretches as long as four Grand Canyons laid end-to-end — may form without a speck of starting material to seed it, researchers report October 7 in Nature Geoscience. This manner of forming clouds, called homogenous nucleation, was thought to be possible on paper but not to occur naturally.
Tiny particles such as dust or salt floating in the air seed typical clouds on Earth. But that cloud physics doesn’t seem to apply to the Arsia Mons Elongated Cloud. Beyond its impressive length, this wisp of water ice is remarkably consistent, forming daily in the same spot roughly 45 kilometers high on the slope of a Martian volcano for several months each year.
The team tried and tried but couldn’t reproduce the cloud’s formation, including its lengthy tail and consistent cycle, with computer models employing standard cloud physics, says François Forget, an astrophysicist at the Laboratoire de Météorologie Dynamique, or LMD, in Paris.
There’s no shortage of dust on Mars. The trouble is that high up in a thin Martian atmosphere, few cloud-forming dust particles stay suspended. Instead, they probably clump together and eventually fall, says planetary physicist Jorge Hernández-Bernal, who did the work at LMD. This left cloud formation without seeds as the most convincing explanation, he says.
The team’s model suggests that the eye-catching plume originates when air flowing across the volcano triggers a rapid lifting of a moist parcel of air. The decrease in pressure at that altitude lets the air parcel expand, rapidly cooling it.
The newly cooled air now has 100,000 times more moisture than it can hold, the team’s model suggests. This excess moisture undergoes homogenous nucleation, rapidly forming more stable ice crystals. Winds sweep the forming cloud outward, creating the cloud’s distinctive long shape.
While the model doesn’t perfectly match satellite observations, it does capture how the phenomenon that leads to the cold air pocket reoccurs day after day, just like the real cloud does.
The study was surprising because “homogeneous nucleation is generally treated in our field as this theoretical concept,” says Samuel Atwood, an atmospheric and planetary scientist at the University of Maryland, Baltimore County and NASA Goddard Space Flight Center in Greenbelt, Md., who was not associated with the study.
“It’s a pretty compelling story that they’re telling with the model, but it’s only one view or one perspective on it,” Atwood says, adding that future measurements supporting the team’s finding will provide more certainty. Hernández-Bernal says that he plans to validate the study by testing alternative explanations.
Even beyond Mars, the study provides food for thought. Previous studies have suggested that on Earth, meteor smoke provides particles for cloud formation in an upper layer of the atmosphere called the mesosphere, though a few studies have suggested homogenous nucleation could be responsible. Now that scientists are proposing homogenous nucleation on Mars, Hernández-Bernal says, “maybe there will be an increased interest in investigating this on Earth.”
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