Scientists have now analysed the images and say that the formation, stretching for more than 1,000km, is larger than any seen before.

Writing in the journal Nature, the researchers believe the plume could be a large cloud or an exceptionally bright aurora.

However, they are unsure how these could have formed in the thin upper reaches of the Martian atmosphere.

"It raises more questions than answers," said Antonio Garcia Munoz, a planetary scientist at the European Space Agency.

Around the world, a network of amateur astronomers keep their telescopes trained on the Red Planet.

They first spotted the strange plume in March 2012 above Mars' southern hemisphere.

Damian Peach was one of the first stargazers to capture images of the phenomenon.

He told BBC News: "I noticed this projection sticking out of the side of the planet. To begin with, I thought there was a problem with the telescope or camera.

"But as I checked more of the images, I realised it was a real feature - and it was quite a surprise."