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Sedimentary structures have been found on Mars that mimic those formed by microbes on Earth.

It is the latest piece of evidence that suggests life was once on Mars, although it is not direct proof of its existence.

The structures were revealed in images of a lakebed that is thought to have undergone seasonal flooding billions of years ago.

A geobiologist from Old Dominion University in Virginia has revealed pictures that show structures on Mars similar to ones on Earth formed by microbes in images from Curiosity in Gale Crater. Shown is an overlay of a sketch on a photograph from above to assist in the identification of the structures on the rock bed surface

Dr Nora Noffke, a geobiologist at Old Dominion University in Virginia, has spent the past 20 years studying these microbial structures.

On Earth, carpet-like colonies of microbes trap and rearrange sediments in shallow bodies of water such as lakes and coastal areas, forming distinctive features that fossilise over time.

These structures, known as microbially-induced sedimentary structures, are found in shallow water settings all over the world and in ancient rocks spanning Earth's history.

In a paper published in the journal Astrobiology, she detailed the striking similarities between Martian sedimentary structures in the 3.7 billion-year-old Gillespie Lake outcrop and microbial structures on Earth.

The distinctive shapes include erosional remnants, pockets, domes, roll-ups, pits, chips and cracks, which on Earth can extend from a few centimeters to many kilometers.

She adds that it is not definitive proof that these structures on Mars were shaped by biology. Such confirmation would require a rock sample to be returned to earth for microscopic analysis.

'All I can say is, here's my hypothesis and here's all the evidence that I have,' Dr Noffke said, 'although I do think that this evidence is a lot.'

The findings follow the recent detection of methane on Mars that could have originated from organics on the planet.

'Along with the recent reports of methane and organics on Mars, her findings add an intriguing piece to the puzzle of a possible history for life on our neighboring planet,' said Dr Penelope Boston, a geomicrobiologist at the New Mexico Institute of Mining and Technology. 

On the left is a comparison of cracks in Gillespie Lake outcrop on Mars and in a modern microbial mat in Bahar Alouane, Tunisia. On the right, knob-shaped structures on Mars are compared to similar structures caused by erosion of microbial mats at Carbla Point, Western Australia. On Earth, both are formed by microbes

Gillespie Lake is an ancient body of water that was once believed to once be at the bottom of Gale Crater on Mars, shown here in this illustration. The patterns found in the Gillespie Lake outcrop are consistent with the microbial structures found in similar environments on Earth

Dr Noffke spent several weeks investigating images centimetre by centimetre, drawing sketches and comparing them to structures on Earth.

The photos showed striking morphological similarities between the terrestrial and Martian sedimentary structures.

The distribution patterns of the microbial structures on Earth vary depending on where they are found.

Different types of structures are found together in different types of environments.

For instance, microbial mats that grow in rivers will create a different set of associations than those that grow in seasonally flooded environments.

The patterns found in the Gillespie Lake outcrop are consistent with the microbial structures found in similar environments on Earth.

In her paper, she also describes alternative processes through which these could have formed. For instance, the chips, pits and cracks could be the product of erosion by salt, water, or wind.

'But if the Martian structures aren't of biological origin, then the similarities in morphology, but also in distribution patterns with regards to microbially-induced sedimentary structures on Earth would be an extraordinary coincidence,' she said.

Last month, scientists in California revealed evidence that suggests there is life on Mars based on readings taken by the Curiosity rover (shown). They say methane spikes on the planet could be produced by bacteria. This sedimentary finding is the latest feather in the cap for the possibility of life once existing on Mars