Scientists are scratching their heads after large gray circles were spotted on Mars by the Reconnaissance Orbiter.

NASA reported the unusual sand dunes on the Red Planet, which they say are "almost perfectly circular," and just another of the strange phenomena that NASA has observed over a decade of thoroughly surveying Mars.

The space agency's Mars Reconnaissance Orbiter spotted the shapes as it studied how frost thaws at the end of Martian winter.

"Sand dunes of many shapes and sizes are common on Mars. In this example, the dunes are almost perfectly circular, which is unusual," the agency explains. "They are still slightly asymmetrical, with steep slip faces on the south ends. This indicates that sand generally moves to the south, but the winds may be variable."

"This is part of a series of images to monitor how frost disappears in the late winter; this observation appears to be free of frost."

This is the second set of publicized images from the Reconassaince Orbiter of Martian winter sand. In December, the orbiter found a remarkable group of frosty talons thawing out after a long winter.

Also earlier this year, the orbiter reported what looked like the face of a bear on the barren surface.

Taken in December from a height of 251 km, with NASA asking "a Bear on Mars? This feature looks a bit like a bear's face. What is it really?"

The photo shows a hill with a V-shaped collapse structure (the nose), two craters (the eyes), and a circular fracture pattern (the head). The circular fracture is probably a layer of dust covering the rim of another, larger impact crater.

The Mars Reconassaince Orbiter has been in action since 2006 studying the geology of mars. These are just three of its many and important contributions to science.

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