At least 18 sightings were reported during two 10-day periods in March and April 2012, when plumes appeared over an area known as Terra Cimmeria, located in Mars' southern hemisphere.
The plumes extended over 500- to 1,000 kilometers (311- to 621 miles) in both north-south and east-west directions and changed in appearance daily. They were detected as the sun breached Mars' horizon in the morning, but not when it set in the evening.
"Remarkably... the features changed rapidly, their shapes going from double blob protrusions to pillars or finger-plume-like morphologies," scientists investigating the sightings wrote in a paper published in this week's Nature.
Spacecraft and ground-based telescopes previously had seen clouds made of carbon dioxide and water ice crystals on Mars, but the clouds were typically layered and never rose above 100 kilometers, or 62 miles, from the planet's surface, the researchers noted.
Clouds of dust can be kicked up into the atmosphere during storms, but these max out at about 37 miles in altitude, they added.
The plumes could be some sort of very unusual aurora, 1,000 times brighter than anything seen on Earth, triggered by a very strong magnetic pocket in the planet's crust that drove solar wind particles out into the atmosphere.
Current understanding of Mars' atmosphere, however, leaves all explanations wanting.
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