Astronomers are scratching their heads over mysterious plumes that have been sighted in the atmosphere of Mars. First seen by amateur astronomers using Earthbound telescopes, the plumes are at an altitude much higher than that of any clouds yet seen on the Red Planet, and may not even be clouds.

Clouds have been seen on Mars since the 1870s and observations sent back by probes like Curiosity and the Mars Reconnaissance Orbiter have greatly increased our understanding of the Martian atmosphere, but that doesn't mean that our neighbor can't still surprise us.

The mysterious plumes were first reported by amateur astronomers in 2012 as they rose to altitudes of 250 km (155 mi) over the the same region of Terra Cimmeria (195 degrees west, 45 degrees south) on two occasions. They were seen on the limb or observed edge of the planet at the day/night terminator during the local Martian sunrise, where the lighting conditions made them stand out much like the illuminated clouds at dawn.

According to the European Space Agency (ESA), similar plumes have been seen before, but never at altitudes above 100 km (62 mi), and this extremely high altitude is significant because it marks the boundary between the Martian atmosphere and space with the plumes extending into the ionosphere and exosphere. The plumes covered an estimated area of 1,000 x 500 km (620 x 310 mi) and took under 10 hours to develop. Though the structure of the plumes changed over time, they didn't last more than 10 days and weren't seen by Mars-based spacecraft because lighting conditions weren't right.

Plume seen by the Hubble Space telescope (Image: JPL/NASA/STScI)

The nature of the plumes has yet to be determined and the extreme altitude raises some interesting possibilities. Clouds on Mars are usually made of water vapor, dry ice, or dust, but to reach the high altitudes observed, the clouds would have to use an atmospheric circulation system different from any understood on Mars. According to ESA scientists, water and carbon dioxide clouds remains below 100 km and dust below 60 km (37 mi).

However, the possibility also exists that the plumes may not be conventional clouds, but a phenomenon similar to Earth's northern lights, though a thousand times brighter. This idea is supported by the fact that the region is home to a large magnetic anomaly, which would support the creation of such auroras.

ESA says Hubble Space Telescope data, along with amateur observations, are being used by astronomers to try to find out the exact cause of the plumes. Already an examination of the backlog of Hubble images has uncovered similar plumes from May 17, 1997. One promising angle will be information sent back by the space agency's ExoMars Trace Gas Orbiter (TGO), which is scheduled to launch next year.

A research report on the plumes was published in Nature.

Source: ESA

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