— -- A mysterious large plume that has been observed intermittently around Mars is baffling astronomers.
The cloud was first observed by amateur astronomers in March 2012 quickly appearing and then vanishing. A second iteration of the haze followed the same pattern in April of the same year and stretched more than 600 miles across the thin Martian atmosphere, according to scientists.
What the features had in common was how they developed over the span of just a few hours and remained visible for around 10 days, with the structure consistently changing.
The haze was not photographed by spacecraft orbiting Mars at the time because of "their viewing geometries and illumination conditions at the time," the European Space Agency said.
An image taken by the Hubble Telescope on May 17, 1997, reveals a similar cloud lingering high in Mars' atmosphere. Scientists are now studying the image and comparing it to the ones taken by amateur astronomers in the spring of 2012.
Agustin Sanchez-Lavega, the lead author on a paper about the Mars mystery published in the journal "Nature," said that while there isn't an official explanation, he has several theories about what may have caused the haze.
"One idea we've discussed is that the features are caused by a reflective cloud of water-ice, carbon dioxide-ice or dust particles, but this would require exceptional deviations from standard atmospheric circulation models to explain cloud formations at such high altitudes," he said.
Another theory: The haze may be an auroral emission.