This story was updated at 11:50 am EST.

The last refuge for Martian life, if it exists, might be deep below the planet's surface and beyond the reach of any currently planned missions, according to a new study.

After mapping cosmic radiation levels at various depths on Mars, researchers have concluded that any life within the first several yards of the planet's surface would be killed by lethal doses of cosmic radiation.

The finding will be detailed in the Jan. 30 issue of the journal Geophysical Research Letters.

Unlike Earth, Mars is no longer protected by a global magnetic field or thick atmosphere. As a result, the planet has been vulnerable to radiation from space for billions of years.

"Even the hardiest cells we know of could not possibly survive the cosmic radiation near the surface of Mars for that long," said study leader Lewis Dartnell of University College London.

Dartnell and his team developed a radiation dose model that calculates how much solar and galactic radiation Mars is subjected to. They tested three surface soil scenarios and calculated particle energies and radiation doses on the surface and at various depths underground. From this, they calculated the length of time that the hardiest known cells on Earth could survive.

The team believes a good place to look for living cells on the red planet is in ice from a frozen sea recently discovered on Elysium Plantia, a major volcanic region on Mars. Scientists think the sea formed only within the last five million years.

"That's very, very recent," Dartnell told SPACE.com. "Five million years ago is yesterday in terms of geology."

The researchers estimate that life could survive for long periods of time about 8 yards (7.5 meters) beneath Elysium's ice.  However, this is still beyond the range of any currently planned missions..  The only mission that will come close, Dartnell said, is ExoMars, a European rover slated for launch in 2013. ExoMars will be equipped with a drill that can dig about 6.5 feet (2 meters) for samples.

Life in a crater

Other potential sites to dig for life are young craters, the researchers say. The exposed rock in craters is denser than the ice that covers much of the planet, and it provides better shielding against space radiation than ice. As a result, life might be able to survive closer to the surface. Another bonus of excavating in craters is that most of the vertical digging work would already be done.

"In effect, the meteorites dug tens of meters deep for you," Dartnell said.

The researchers estimate that on a typical Martian exterior-one not covered in yards of ice or bombarded by meteorites-life might survive as close as 2 meters beneath the surface. While this is just within range of ExoMars' drill, the team's model also predicts that even Earth's toughest life forms would survive for a relatively brief amount of time-about 450,000 years-on Mars at those depths.

 "After 450,000 years, our model says that only 1 in a million cells will survive," Dartnell said in a telephone interview.

The cells wouldn't be killed so much by radiation as by an inability to repair themselves due to the frigid environment of Mars.

"If you had a beaker of water [filled with cells] and put it on the surface of Mars where radiation is highest, those cells would be perfectly happy," Dartnell explained. "There's actually less radiation on the surface of Mars than some natural locations on Earth."

The problem is that the subzero temperatures on Mars' surface makes it extremely difficult for cells to repair radiation damages that do occur or to divide. The cells would be frozen solid and held in stasis and the radiation damages would accumulate, until the point where they're killed off, Dartnell said.

Follow the water

Another good candidate for finding extant Martian life is new gullies recently discovered by Mars Global Surveyor, the researchers say. Evidence suggests the gullies might have flowed with water within the last five years, possibly ferrying underground cells to the surface.

The notion of life surviving deep underground is not unheard of. Subterranean microbes have been discovered two miles and deeper beneath Earth's surface. Furthermore, those bacteria used the planet's internal heat, in the form of radioactive elements like uranium and potassium to convert water molecules into useable energy.

"If you extend that to Mars," Dartnell said, "even if things are getting nuked off by radiation on the surface, if you dig deep enough, the internal heat of the planet might have melted down the permafrost ice into aquifers of liquid water where even today you could have an active ecosystem."