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Mysterious haze on Mars puzzles scientists
A curious plume-like feature was observed on Mars on 17 May 1997 by the Hubble Space Telescope. It is similar to the features detected by amateur astronomers in 2012, although appeared in a different location. - photo by Natalie Crofts
The discovery of an unusually large, hazy plume on Mars by amateur astronomers is now puzzling the experts.

The plumes appeared on two separate occasions during March and April of 2012 and reached altitudes of 250 km, compared to previous known highs of 100 km, according to the European Space Agency. Scientists published a report that validated the amateurs' findings but only offered theories of what could have created the plumes in the journal Nature on Monday.

"It raises more questions than answers," one of the studys authors, ESA planetary scientist Antonio Garcia Munoz, told BBC News.

The plumes are very unexpected because the division between the planets atmosphere and outer space is very thin at 250 km, according to lead researcher Agustin Sanchez-Lavega. Both times it appeared, the high-altitude plumes changed shape over 10 days before vanishing.

One of the two theories presented in the report is that the plume is caused by a reflective cloud of water-ice, carbon dioxide or dust particles. The second theory is that the plumes were created by an auroral emission similar to Earths aurora borealis.

However, Munoz told BBC News that if either of the theories presented in the report are correct, that means scientists current understanding of Mars atmosphere is wrong.

"I've heard of about four or five different possible explanations," NASA planetary scientist Bruce Jakosky, who did not participate in the research, told Popular Mechanics. "And honestly? I don't like any of them."

The plumes have not yet returned, but the ESA reported that scientists found a set of Hubble images from May 1997 that show an abnormally high plume, similar to that spotted by the amateur astronomers in 2012. The ESA is planning to launch its ExoMars Trace Gas Orbiter in 2016.