War of the Worlds Martian cylinder-cannon!
Scientists are at a loss to explain strange high altitude clouds found hovering over the surface of the red planet.
The massive mists, first discovered by amateur astronomers on 12 March 2012, defy science's current understanding of atmospheric physics on Mars.
At over 1000 kilometres wide, the clouds are far larger than any previously observed on Mars, according to a report today in the journal Nature.
The phenomenon was spotted along the observed edge of the planet at the night day boundary, between 200 and 250 kilometres above the Terra Cemmeria region of the Martian south-eastern hemisphere.
"I first noticed it when processing image data from a session and saw something that looked like an artefact," says amateur Florida astronomer Wayne Jaeschke, who is one of the study's authors.
"After processing the data it seemed to show up on all the images, so I animated some frames and found that we had a real feature, but one I couldn't explain!"
The massive clouds which lasted for at least 11 days, rapidly changed shape going from double blob protrusions to pillars or finger-plumes.
Jaeschke sent the animation to a wider group of amateur and professional astronomers.
On 6 April 2012, a second similar cloud was observed in the same area and at similar altitudes, lasting about 10 days.
The discovery prompted a review of more than 3500 images of Mars from both the Hubble Space Telescope and amateur image databases.
The authors found other images showing the occasional presence of clouds, however none were as massive as the 2012 features, with the exception of a similar abnormal plume in Hubble images from 17 May 1997.
Martian auroraThe research team looked at a range of possible explanations for the unusual plumes, including Martian aurora activity.
Highly concentrated and localised patches of auroral emissions generated by magnetic field anomalies in the Martian crust have been previously observed at altitudes of 130 kilometres, in the same area.
"I think there's still some discussion about the magnetic field as a contributing factor because of the high solar winds at the time," says Jaeschke.
"We saw this feature as several large coronal mass ejections from the Sun were passing by Mars."
However, the authors calculated that the intensity of any auroral activity would have been far less than the degree of brightness seen in the features.
The findings mean aurora activity are not fully consistent with the feature Jaeschke and colleagues found.
Dust and iceClouds of both carbon dioxide and water ice crystals are frequently observed in the Martian atmosphere, but always below 100 kilometres in altitude.
Scientists have also detected Martian dust at altitudes up to 60 kilometres during major dust storms.
The authors compared the properties of the mystery clouds with known properties of both carbon dioxide and water ice crystals, as well as Martian dust particles.
They quickly ruled out dust particles for the 1997 event, and found dust was also unlikely to have caused the 2012 clouds.
Getting dust to the clouds high altitude would require unusually strong up-drafts more likely to occur when temperatures are higher around noon rather than when the clouds are seen at dawn.
"Morning was the only time we were able to see the plume," says Jaeschke.
The researchers found either water ice or carbon dioxide ice particles provided the best fit for both the 1997 and 2012 events.
"If it was a condensate [ice] cloud, then it's possible that it dissipated during the day," says Jaeschke. "Or, because of the change in the Sun angle, the cloud may not have been bright enough for us to see against the bright backdrop of the planet's surface."
However, condensation of ice would require unusually cold temperature drops of over 50 degrees for water clouds and double that level for carbon dioxide clouds, say the researchers.
"The most probably reason was a drop in the temperature in the layer from 100-200 km above surface, for unknown dynamical reasons," says the study's lead author Dr Agustin Sánchez-Lavega of the Universidad del País Vasco in Spain.
He says extended monitoring will be needed before this latest Martian mystery can be solved.