The images, taken by
Nasa's Mars Explorer Spirit, seem to show a figure striding purposefully across the surface of the Red Planet.
Sceptical people might suggest it is just a rock whose outline resembles that of a person.
The pictures were sent back from the surface of the Red Planet by the six-wheel robot four years ago.
But a space fan has since spotted the figure, which was just a pinprick in the original photograph. It was blown up and posted on a website.
One stargazer said: "I couldn't believe my eyes when I looked and saw what appears to be a naked alien running around on Mars."Experts have said for years that if the planet - the fourth one out from the Sun in our Solar System - once had surface water, there was the potential to support life, which has prompted hundreds of conspiracy theories about Martians.
Analysis of soil showed it contained 90 per cent silica - a concentration only likely to have formed when water is present.
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