– The last refuge for possible life on Mars might be deep below the planet’s surface, beyond the reach of any currently planned missions, according to a new study.


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


The finding was 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, leaving the planet vulnerable to billions of years of radiation.


“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 the amount of solar and galactic radiation affecting Mars. 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 ice from a frozen sea recently discovered on Elysium Plantia, a major volcanic region on Mars, might be a place to find living cells. Scientists think the sea formed only within the last five million years.


“That’s very, very recent,” Dartnell said. “Five million years ago is yesterday in terms of geology.”


The researchers estimate that life buried about 7.5 meters beneath Elysium’s ice could survive for long periods of time. However, no currently planned missions will attempt to reach this depth. The only mission that will come close, Dartnell said, is ExoMars, a European rover slated for launch in 2013, which will be equipped with a drill that can dig about 2 meters for samples.


Life in a crater


Other potential digging sites are young craters that contain exposed rock, which is denser and provides better shielding against space radiation than the ice that covers much of the planet. In these craters life might be able to survive closer to the surface, and most of the vertical digging work for excavation 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 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.


“After 450,000 years, our model says that only one in a million cells will survive,” Dartnell said in a telephone interview Jan. 29.


The cells would not be killed so much by radiation in this case, but by an inability to repair themselves due to the frigid environment.


“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 said. “There’s actually less radiation on the surface of Mars than some natural locations on Earth.”


But the subzero temperatures on Mars’ surface makes it extremely difficult for cells to repair radiation damages that do occur, or to divide. While the cells would be held in a frozen stasis, the radiation damage would accumulate until they are killed off, Dartnell said.


Follow the water


Other good candidates to search for extant martian life are the new gullies recently discovered by Mars Global Surveyor, the researchers said. 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 new. Subterranean microbes have been discovered more than three kilometers beneath Earth’s surface. 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, 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,” Dartnell said.