Small iron-oxide spheres discovered in Martian rocks may indicate that life existed on Mars millions of years ago, according to researchers.
Nasa 's Opportunity rover has found many of the hematite balls - nicknamed "blueberries" by researchers - since landing on the planet eight years ago.
They were originally thought to have provided the first evidence of liquid water on Mars but their existence may hold an even more important implication.
Researchers from the University of Western Australia and University of Nebraska have found that similar spheres, when they appear on Earth, are formed by microbes.
If those found on Mars are made up of a similar composition, it could show that life once existed on the planet.
On Earth, the spheres are commonly found on beaches and deserts and consist of a hard shell of iron oxide surrounding a softer sandy interior.
Similar examples to those discovered on Mars have been found in the Jurassic Navajo Sandstone near the Colorado River, Utah, where they range in size from small marbles to cannonballs.
They were once thought to have formed through simple chemical reactions but researchers have found that biological elements such as carbon and nitrogen are needed to create them.
Researchers said the spheres would make an excellent target for Nasa’s advanced Curiosity rover, which landed on the planet in August.
The research was published in the latest issue of Geology .