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Covered round-the-clock by rovers and orbiters, researchers know a lot about Mars and its vast desolate plains. Yet, some mysteries remain. Of course, researchers with major space agencies continue to look for evidence of life and of conditions hospitable to support possible manned missions, but even more so researchers are interested in the anomalies above the surface.

Originally discovered in 2012 by amateur astronomers, a large plume in Mars' southern hemisphere has left researchers wondering exactly what could it be. Stretching more than 1,000km and far up into the thin upper Martian atmosphere, it's larger than anything they've encountered before. And now researchers a taking a stab at figuring out exactly what it could be...though they're not certain they will find many answers, at least not until it returns again. Because as quickly as it arose, 10 days later the plume disappeared and was only seen once after that.

Publishing a new study this week in the journal Nature, researchers with the European Space Agency led by Dr. Antonio Garcia Munoz reveal that the plume could very well be a large cloud or even a particularly bright aurora, much like what we see here on Earth. Both would fit the bill, and would also explain the temporary nature of the plume, although they are unsure as to how they could have formed in Mars' thin atmosphere.

"It raises more questions than answers" Munoz says. 

Could clouds be the answer?

Well, while clouds of carbon dioxide or even water particles are possibilities as they can form on the red planet, researchers question if even high hanging clouds would have been able to reach such heights.

"We know there are clouds on Mars, but clouds up to this point have been observed up to an altitude of 100km" Munoz says. "And we are reporting a plume at 200km, so it is significantly different."

"At 200km, we shouldn't see any clouds, the atmosphere is too thin. So the fact we see it for 20 days in total is quite surprising."

Could it be Mars' version of the southern lights?

They're not like auroras we see here on Earth, and they're not quite as dazzling or frequent, but auroras have been reported on Mars. Researchers just aren't sure if the conditions present at the time of the plume in 2012 would have caused such an abnormal show. Waiting for another plume to return may better answer this question later on.

"We know in this region on Mars, there have been auroras reported before" Munoz says. "But the intensities we are reporting are much much higher than any auroras seen before on Mars or on Earth."

"It would be 1,000 times stronger than the strongest aurora, and it is difficult to come to terms that Mars has such an intense aurora."

While Munoz's team has posited some great theories as to what may have originally given rise to mysterious plume, he hopes that in publishing a paper with an incomplete conclusion may spark interest in other astronomers to help find the answer.