(BBC) — A mysterious haze high above Mars has left scientists scratching their heads.
The vast plume was initially spotted by amateur astronomers in 2012, and appeared twice before vanishing.
Scientists have now analysed the images and say that say the formation, stretching for more than 1,000km, is larger than any seen before.
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Writing in the journal Nature, the researchers believe the plume could be a large cloud or an exceptionally bright aurora.