On March 12, 2012, Mars watchers spotted a mysterious blob rising from the southern hemisphere of the planet. The plume reached 155 miles from the surface. As astronomers watched, the plume continued to grow until it was more than 600 miles across.

Unfortunately poor weather hampered visibility and obscured the phenomenon for a week and by the time visibility returned on April 2 it had disappeared. Then four days later another plume appeared and lasted for 10 days. Nothing like it has been seen since and no plausible explanation has been offered.

Agustin Sanchez-Navega from the University of the Basque Country, Spain and colleagues sought out images from around the world. They collected pictures from 18 observers as well as a collection of old images from the Hubble Space Telescope which had spotted a similar phenomenon in 1997.

In a letter published in the journal Nature the astronomers lay out their findings. However, those findings primarily explain why none of the proposed explanations works very well.

On Earth, when a sufficient number of charged particles from the sun interact with Earth’s magnetic field it generates auroras, such as the famed Aurora Borealis that occurs high in the Northern Hemisphere.

There are a few serious problems with that idea however. First, the sun wasn’t very active in March of 2012 and despite the lack of solar activity, the borealis would have needed to be a thousand times stronger than any ever observed on Earth.

“Our explanation of this plume as an aurora would require an immense energetic flux, which our calculations show are highly unrealistic,” Sanchez-Lavega wrote in an email to Discovery News.

The possibility of volcanic activity was also considered. The problem with that scenario is that volcanic eruptions are usually visible around the clock and not just in the mornings. There are also no known active volcanos on Mars.

“You would think that something large enough to dump that much vapour in the atmosphere would be picked up,” Nicholas Heavens of Hampton University in Virginia told New Scientist.

A massive dust storm was considered as a possibility, however dust storms don’t reach higher than 37 miles into the atmosphere and the plumes didn’t carry the signature red color of the martian dust.

The researchers even considered the possibility of a biological explanation. The problem with that idea is perhaps the most obvious.

“No life past or present [has been] detected so far on Mars, so it cannot be. If there is no positive evidence, you should probably exclude something biological,” said Heavens.

It is also not clear what sort of biological activity would have caused a plume 600 miles wide reaching more than 150 miles into the atmosphere.

The best guess, according to the researchers, is that it was caused by a cloud of frozen carbon dioxide and water in the upper atmosphere. Even that most likely scenario, however, has some serious problems. No cloud on Earth or Mars has ever been seen at an altitude above 62 miles (100 km).

“If the phenomenon is a cloud, then the most similar phenomena on Earth will be the mesospheric clouds that form at 80 kilometres altitude on polar regions,” says Sánchez-Lavega.

So, after three years some of the world’s leading Mars watchers are no closer than they were on March 12, 2012 to knowing what caused the strange blobs on Mars.

At this point the researchers simply have to hope that it happens again, now that there are several active spacecraft monitoring Mars including the Mars Atmosphere and Volatile Evolution (MAVEN) satellite which arrived last year.

“MAVEN should see something like this very easily if it occurred again, if we were at the right place at the right time. I’ve given a heads-up to our science team, so they’ll be keeping an eye out for it,” said Bruce Jakosky of the University of Colorado, Boulder, who leads NASA’s Mars-atmosphere-observing MAVEN mission.