The phenomenon was observed on March 12, 2012 over the "terminator", the boundary between day and night on Mars. A second was spotted nearby on April 6, 2012, and lasted about 10 days
One of the plumes developed in around 10 hours and lasted for about 11 days, shifting shape from "double blob protrusions" to pillars which merged into a "finger", the study authors wrote.
Their trails were vast, extending between 500 and 1,000 kilometers (300 to 600 miles).
According to AFP, the "clouds" -- if that is indeed what they were -- were seen at high altitude, at about 200-250 kilometres, roughly above Terra Cimmeria, which is part of Mars' rugged southern highlands, according a paper published in the journal Nature Geoscience by a team led by Agustin Sanchez-Lavega of the University of the Basque Country in Bilbao, Spain.