The photo, taken by the Curiosity rover on August 14, had conspiracy theorists in a fit online over the past week because they believed it showed a bone-like object lying on the surface of the red planet.

"Simply put, this shows that there were some living things on the Mars [sic]," an anonymous writer at Northern Voices Online claimed, while a popular UFO blog compared the alleged bone to previous images of a "fossilised, reptilian spine" and a "human finger".

In an effort to stop the speculation on numerous sites, some of which said it proved there had been life on Mars, NASA finally decided to give its opinion on Friday, a week after it was taken.

"Seen by Mars rover Curiosity using its MastCam, this Mars rock may look like a femur thigh bone," NASA's scientists said. "[But] mission science team members think its shape is likely sculpted by erosion, either wind or water. If life ever existed on Mars, scientists expect that it would be small simple life forms called microbes. Mars likely never had enough oxygen in its atmosphere and elsewhere to support more complex organisms. Thus, large fossils are not likely."

Astronomical Society of Victoria's Perry Vlahos, who is an astronomer and also a columnist for Fairfax Media, publisher of this article, said the human brain often played tricks on us.

"One of the fantastic things about the human brain is that we're wired for pattern recognition," Mr Vlahos said. "And so [it means] we look for familiar things in whatever it is that we're glancing at.

"It's been a huge part of evolution in being able to understand patterns and visualise things that help us avoid predators ... and it's something that's grown up with humans over a long period of time.

"So I'm not surprised that people see a 'man in the moon' and a 'thigh bone' on Mars."

He added that he would be "extremely surprised" if the object seen in the photo was anything other than what scientists already expected to see on Mars. "If there was something genuine to this image as being something that could be anything other than just a natural occurrence and just light and shadow playing with our minds and our eyes I would be very surprised," Mr Vlahos said.

So far there had been no evidence that life had ever existed on Mars.

"If there was evidence of [life] on Mars it would have been front-page news way before any of this conspiratorial stuff came up," Mr Vlahos said. "There have been a number of press conferences that have been held by scientists where they reported such discoveries.

"[For example] there was a meteorite that was found in Antarctica some 10 years ago and we thought that we had found evidence of bacteria in it coming from Mars and later on it was decided that that wasn't the case. So no, there's been [no evidence] and if there ever was it would be at the bacterial level. It won't be anything like a large animal that would give us such bones."

Although there have been findings that would suggest there was water on Mars, which is now likely to be in ice form, Mr Vlahos said this would not be enough alone to support life on the planet.

"Water on Earth works in conjunction with a lot of other things," he said. "It's not just water on its own. There's an atmosphere, there's vegetation, there's all sorts of other systems that work together, precipitation etc [to support life]. So there's a combination of factors."

The Curiosity rover has been on Mars for just over two years now, producing many images that have intrigued scientists and conspiracy theorists.

Earlier this year its counterpart, the Opportunity rover, spotted a mysterious rock shaped like a jelly doughnut that appeared to come out of nowhere when comparing two separate images of the same spot. It was later explained as a piece of a larger rock that had been broken and moved by the Mars rover Opportunity's wheel in early January.