Mars is a hostile planet. Its distance from the sun makes it cold and barren – average daily temperatures on Mars are around -60C and this falls to - 126C in winter near the poles.

What’s more the thin Martian atmosphere means the planet is bombarded with intense life-destroying radiation.

Scientists believe a handful of single celled creatures could have what it takes to survive on the Red Planet

And there’s little oxygen there. In fact 95% of the atmosphere is made up of carbon dioxide.

Yet scientists believe that a handful of simple single celled creatures found in some of the most extreme places on Earth – sulphurous lakes and permafrost for instance – could have what it takes to survive on the Red Planet.

To vet the prospective applicants, astrobiologists are replicating Martian conditions in the laboratory, zapping the microbes with gamma and UV radiation, and freezing them to see if they survive. Some microbes have even been taken up to the International Space Station for the ultimate test of how they cope off our planet.

As a result the researchers now have a list of potential candidates that could survive the Red Planet’s sub zero temperatures, vacuum conditions and intense solar radiation. So what are the candidates like?

The most obvious potential Martian is nicknamed “Conan the Bacterium” for its toughness

The most obvious potential Martian is Deinococcus radiodurans, a bacterium that is the most radiation resistant lifeform that has ever been found. The species is nicknamed “Conan the Bacterium” for its toughness. The almost indestructible microbe can survive doses of ionising radiation thousands of times stronger than those that would kill a human.

The bacterium isn’t phased by extreme temperature either. Scientists chilled D. radiodurans to -79C, the average temperature at Mars’s mid-latitudes. Then they bombarded the cells with gamma rays to simulate the dose they would receive living under 30cm of Martian soil over long periods of time. The beings were so hardy that researchers estimated it would take 1.2 million years under these conditions to shrink a population of the bacteria to a millionth of its original size.

Another contender well suited to life on Mars is the Halobacteriaceae family. These microbes are examples of ancient bacteria-like microbes called archaea, which are probably the oldest forms of life on the planet. They might have first evolved on the primordial Earth as long ago as 3.5 to 3.8 billion years ago.

Halobacteriaceae could theoretically survive in salty brines on Mars

Halobacteriaceae live in salty places on Earth, such as the Dead Sea. They could theoretically survive on Mars too following the discovery of salty brines, or reservoirs of liquid salt on Mars. Two members of the family – Halococcus dombrowskii and Halobacterium sp. NRC-1 – have already proved that they can survive a simulated Martian atmosphere. An experiment showed that they could happily cope with pressures six times greater than Earth’s standard atmospheric pressure, an atmosphere of 98% carbon dioxide, and an average temperature of -60C for up to 6 hours.

Now Stefan Leuko, an astrobiologist at the German Aerospace Centre has put three other salt-tolerant "halophilic" archaea through their paces to see whether they could survive when exposed to UV solar radiation as powerful as that found in outer space. He found that two of the organisms, Halobacterium salinarum NRC-1 and Halococcus morrhuae were much more resistant to radiation than the third species, Halococcus hamelinensis, despite the fact that the three species are all from the same family.

There’s another reason to believe that salt-tolerant microbes could live on Mars. There’s controversial evidence that here on Earth, some have survived for million of years inside salt – or “halite” – crystals.

“We have organisms that can stay viable for millions of years enclosed in halite and are highly radiation resistant,” says Leuko. Given that we now know saline brines exist on Mars, halophilic microbes seem like a very good model for the kind of cells that could survive on the Red Planet. “When it comes to radiation resistance, the clear winner is still Deinococcus radiodurans, but, this strain cannot survive in high saline environments,” says Leuko. “Therefore I think that halophilic archaea are good candidates when it comes to look for life (extinct or extant) on another planet.”

Methanogens are perfect candidates for life on Mars, as they don't need light, oxygen, or organic nutrients to survive

Another type of archaea may also have what it takes to survive on Mars: the methanogens. Instead of breathing oxygen, they use hydrogen and carbon dioxide as their energy source, and generate methane as a byproduct, hence the name.

Methanogens are widespread in nature and often make their homes in extreme environments. They have been found in hot springs, salty ponds, acidic and alkaline lakes and in the sub-freezing soils of Siberian permafrost. They have also been found living in the guts of cattle, termites and in dead and decaying matter.

It’s the permafrost dwelling microbes that are of most interest to astrobiologists, as the permanently frozen soils of Arctic tundra closely mimic conditions that exist just below the Martian surface. In fact a study recently showed that a giant piece of ice as big as California and Texas combined lurks beneath the surface of Mars between its equator and north pole.

Methanogens are perfect candidates for life on Mars, as the simple organisms don't need light, oxygen, or organic nutrients to survive – none of which are plentiful on our neighbouring planet. In an experiment in 2007, species of methanogenic archaea were exposed to simulated conditions of Mars – and they survived.

Why have any of these microbes evolved to be so tough?

Now a team led by Dirk Wagner from the GFZ German Research Centre for Geosciences in Potsdam has found another indestructible methanogen, called Methanosarcina soligelidi, living in the permafrost soils of Samoylov Island in Siberia. He calls the microbe “our superhero” because of the conditions it can withstand. The average daily temperature on Samoylov is -14.7C, although it can drop as low as -48C. The island is also very dry, with just 190mm rain falling a year, and the soils remain permanently frozen.

Wagner has already discovered microbes and other methanogens living in the frozen soils that can survive intense cold and dehydration, but his superhero is almost indestructible.

He bombarded the microbe with solar ultraviolet and ionising gamma radiation to test its survival limit. It could withstand up to 13.8 times more UV radiation, and 46.6 times more ionising radiation than another species of methanogen, Methanosarcina barkeri. This means the microbe can absorb a level of radiation similar to that which would have been prevalent on early Earth and on present day Mars.

A question still remains though. Why are these microbes so tough? Why evolve to survive levels of radiation that are common in space and on Mars but that are not generally found on Earth?

Archaea evolved when the Earth lacked an ozone layer and was exposed to the full UV spectrum from the sun

The dose of background ionising radiation in permafrost, for example, is about 2 milligrays per year, about the same as the radiation from a single brain CT scan and very far below the radiation threshold displayed by some microbes living in the environment.

One reason for the discrepancy lies in the age of the microbes. Many of the most radiation-resistant species are archaea, one of the earliest and most primitive group of organisms. Archaea evolved when the Earth lacked an ozone layer and was exposed to the full UV spectrum from the sun. Solar radiation would have been much greater than it is today, and so early colonisers of Earth would have needed coping mechanisms, which they perhaps never lost even once our planet gained an ozone layer. However most researchers now think that life began deep in the oceans, where radiation would have been less of a problem even before there was an ozone layer.

Another theory is that the microorganisms have developed their resistance to radiation purely by accident, as a consequence of adapting to their extreme environments on Earth.

Another theory is that the microorganisms have developed their resistance to radiation purely by accident

“In general organisms that are resistant to one stress are also resistant to others,” explains Wagner. “The bacteria Deinococcus radiodurans is highly radiation resistant, but it’s also resistant to drying out. The two are most likely based on the same mechanisms.”

In other words all the Martian candidates – Conan the bacterium, the Halobacteriaceae and the methanogens – have developed unique ways of surviving in their environments. Radiation resistance is just a by-product.

How exactly do the microbes protect themselves from radiation?

Some of Leuko’s salt tolerant microorganisms do so by simply hiding away from the sun’s UV radiation. The Halococcus morrhuae cells cluster together forming layers upon layers of microbes. The cells deep within the cluster are protected from solar radiation, which is absorbed by the cells closer to the surface. As the microbes naturally live in low oxygen, salty habitats, they don’t suffocate.

However, as Leuko explains, this strategy only works for UV radiation and not ionising gamma radiation, which has more energy radiation and will penetrate through the cluster to reach cells deep in the centre. This means that the microbes could escape UV radiation on Mars by hiding within soils or ice, but they would still be prone to ionising radiation.

Other microbes use a different approach. Radiation triggers the release of reactive oxygen species (ROS), which damage cell constituents such as proteins and DNA. To counteract the problem, salt-tolerant archaea have a purple pigment called bacteriorhodopsin that can mop up ROSs and protect the cell from damage. They may have evolved the pigment because ROSs are also generated when the cell dries out – which is a common problem for cells living in such salty environments.

Even if radiation and drying does damage DNA, many of the microbes seem to be able to repair this damage. A study found that Conan the bacterium was able to stitch broken DNA back together with repair proteins. As long as the repair systems are intact, the bacteria can survive.

If radiation and drying damages DNA, many of the microbes seem to be able to repair it

The Conan microbe has another trick too. It carries multiple copies of its genes on different chromosomes. If one or two copies are damaged by radiation, the cell can use another copy of the gene to stay alive while it repairs the DNA damage. Leuko found that one of his archaea, Halobacterium salinarum NRC-1, was also able to actively repair its cells during the exposure to radiation, and Wagner believes that DNA repair may also be at the heart of the success of his superhero Methanosarcina soligelidi.

Some microbes accumulate salt and sugar inside their cells to protect themselves from drying out. This also seems to offer protection from radiation by somehow preventing the DNA double helix from breaking apart. How salts and sugars help is unknown, but the evidence is mounting that one sugar in particular, trehalose, does indeed offer protection and stops proteins and cell membranes from unravelling when they get hot and dry out.

Promising though all of this sounds, neither Leuko nor Wagner truly believe that any of these microbes could really survive on the surface of Mars today. Conditions there are just too extreme for even the toughest Earth-based life.

However the conditions on Mars’ surface early in its history were similar to those on early Earth. The planet may be barren and dry now, but lots of evidence suggests that rivers, lakes and seas once flowed on Mars. Perhaps life could have evolved on Mars back then and subsequently adapted as conditions worsened.

Mars may be barren and dry now, but lots of evidence suggests that rivers, lakes and seas once flowed there

“If we have a look at the environmental conditions on early Mars and early Earth they are comparable,” says Wagner. “Both had moderate temperature and pressure conditions, there was no oxygen on both planets and whilst Earth was dominated by oceans there are strong indications that there was also liquid water on Mars’ surface.”

We know that life developed in those conditions on Earth, and it probably could have done so on early Mars too. What happened to those lifeforms when Mars became a harsher place to live is unknown. “It could have become extinct, but could also be buried deep within the Martian crust,” says Wagner.