Washington, Jun 28 (IANS): Rovers exploring Mars, such as NASA's Curiosity and Perseverance and others, may have to dig about 6.6 feet (two metres) or more under the surface of the Red Planet to find signs of ancient life, according to a new laboratory experiment by the US space agency.

It is because ionising radiation from space degrades small molecules such as amino acids relatively quickly, NASA said.

Amino acids can be created by life and by non-biological chemistry. However, finding certain amino acids on Mars would be considered a potential sign of ancient Martian life because they are widely used by terrestrial life as a component to build proteins that are essential to life.

"Our results suggest that amino acids are destroyed by cosmic rays in the Martian surface rocks and regolith at much faster rates than previously thought," said Alexander Pavlov of NASA's Goddard Space Flight Center in Greenbelt, Maryland, in a statement.

"Current Mars rover missions drill down to about two inches (around five centimetres). At those depths, it would take only 20 million years to destroy amino acids completely. The addition of perchlorates and water increases the rate of amino acid destruction even further," he added.

Twenty million years could be a relatively brief amount of time because scientists are looking for evidence of ancient life on the surface which would have been present billions of years ago when Mars was more like Earth.

This result suggests a new search strategy for missions that are limited to sampling at shallow depths.

Scientists have been exploring if life emerged on Mars by examining Mars rocks for organic molecules such as amino acids.

In the study, published in the journal Astrobiology, the team mixed several types of amino acids in silica, hydrated silica, or silica and perchlorate to simulate conditions in Martian soil and sealed the samples in test tubes under vacuum conditions to simulate the thin Martian air.

The samples were blasted with various levels of gamma radiation -- a type of highly energetic light -- to simulate cosmic-ray doses up to that received from about 80 million years of exposure in the Martian surface rocks.

The experiment is the first to mix amino acids with simulated Martian soil. "It turns out that the addition of silicates and particularly silicates with perchlorates greatly increases the destruction rates of amino acids," said Pavlov.

While amino acids haven't been found on Mars yet, they have been discovered in meteorites, including one from Mars. But as Martian meteorites typically get ejected from depths of at least 3.3 feet (one metre) or more, it is possible that the amino acids in it were protected from cosmic radiation, the team said.