A martian meteorite frozen in antarctic ice suggests we may need to dig deeper into Mars to search for traces of past life, U.S. researchers say.

Scientists at Tufts University say the meteorite is full of chemicals related to those used in household bleach, increasing the likelihood any carbon-bearing compounds -- strong indicators of life -- on the martian surface may have been broken down by chemical reactions.

"We're speculating that you perhaps cannot find organics on the surface of Mars," chemist Sam Kounaves said. "You have to be below the surface or inside sedimentary rocks."

Kounaves and colleagues studied a martian meteorite collected in 1979 in Antarctica where it fell 12,000 years ago and found it to contain a form of nitrate and chloride minerals, which can create bleaching agents.

The chemical are not Earthly contaminants, they said.

"It's clear to us it's martian," Kounaves told New Scientist at a meeting of the American Association for the Advancement of Science in Boston.

In the presence of even minute traces of water, the bleaching agents will break down any organic compounds present, a process that can be even stronger in the presence of cosmic radiation, the researchers said.

NASA's Curiosity rover, exploring the surface of Mars for traces of past life, is equipped with a drill that may reach deep enough to find any organics preserved in rocks below, shielded from the oxychlorine-forming processes, they said.

"If organics were present under several inches of soil or in a sedimentary rock, they could be protected from radiation," Kounaves said.