NASA's Curiosity rover photographed an object on Mars last week that bore an uncanny resemblance to a femur or thigh bone.

But it's just a rock, the U.S. space agency says.

"If life ever existed on Mars, scientists expect that it would be small simple life forms called microbes," NASA said in the caption for the image. "Mars likely never had enough oxygen in its atmosphere and elsewhere to support more complex organisms. Thus, large fossils are not likely."

Mission scientists believe the rock was sculpted into its unusual shape by wind or water erosion.