Evidence of life on Mars could come not from a mission to the Red Planet but from one sent to the martian moon Phobos, U.S. researchers said.

"A sample from the moon Phobos, which is much easier to reach than the Red Planet itself, would almost surely contain martian material blasted off [Mars] from large asteroid impacts," Purdue University Professor Jay Melosh said.

"If life on Mars exists or existed within the last 10 million years, a mission to Phobos could yield our first evidence of life beyond Earth," he said in a university release Friday.

"We are talking little green microbes, not little green men," Melosh said.

Melosh led a team chosen by NASA's Planetary Protection Office to evaluate if a sample from Phobos could contain enough recent material from Mars to include viable martian organisms.

The team concluded a 200-gram sample scooped from the surface of Phobos could contain, on average, about one-tenth of a milligram of Mars surface material launched in the past 10 million years.

"Approximately one ton of martian material lands on Earth every year," Melosh said. "There is a lot more swapping back and forth of material within our solar system than people realize."

There is ongoing international interest in a Phobos mission, he said, and such a possibility will likely be a recurring topic as NASA reformulates its Mars Exploration Program.

A NASA report issued June 26 said the martian moons are "important destinations that may provide much of the value of human surface exploration at reduced cost and risk."