In 2012, dozens of amateur astronomers spotted large plumes of dust rising off the surface of Mars. More than two years later, scientists still don't have a suitable explanation for the phenomenon.
In a new study, published this week in the journal Nature, scientists recount the strange occurrence and survey the various attempts to illuminate the plumes' origins or cause.
The phenomenon was observed twice, over the course of two ten-week long periods, in March and April of 2012. What began as a wide blob of dust slowly extended into a towering plume -- a finger of debris that stretched some 621 miles across, and the same distance lengthwise out into space.
"The features changed rapidly, their shapes going from double blob protrusions to pillars or finger-plume-like morphologies," researchers wrote in the new paper.
"I was really quite amazed that it was sticking out the side of the planet quite prominently," amateur astronomer Damian Peach, a resident of Selsey, England, and one of the first backyard sky-gazers to spot it, told New Scientist.
Both dust clouds kicked up by storms, as well as naturally occurring clouds of carbon dioxide and water crystals, have been observed on Mars; but neither in the shape or size of those seen in 2012.
There are two main explanations. One is that the plume was a traditional gas and water vapor cloud. The other is that it was an aurora, like the ones that turn Earth's polar skies into a scene from Fantasia. But both explanations have problems.
"Our explanation of this plume as an aurora would require an immense energetic flux, which our calculations show are highly unrealistic," physicist Agustin Sanchez-Lavega, a researcher at Spain's University of the Basque Country, explained to Discovery News via email.
"We favor the cloud scenario -- the condensation of water or carbon dioxide -- since the plume formed in the cold mornings," Sanchez-Lavega added.
The plumes were both seen at Mars's terminator, where night turns into day -- and where stark temperature differences might occur. Still, this explanation would require extreme cold, temperatures that the current Mars atmospheric models don't entertain.
"Therefore, very particular conditions occurred at that time in the area that currently we do not understand," he said.
Many find the cloud explanation just as unlikely at the aurora theory.
"Frankly, I'm puzzled by the observations," Bruce Jakosky, an astronomer at the University of Colorado, Boulder and head of NASA's Mars-atmosphere-observing MAVEN mission, told New Scientists. "I don't understand how material can get that high and stay there for so long."
Jokosky and others say the only way to explain the mystery is to wait for it to happen again, and to be ready to make more detailed observations.