Cold nights on Mars



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Cold War nuclear radiation testing and the inhospitable living conditions on Mars was the subject of this month’s Packed Lunch at Wellcome Collection. Dr Lewis Dartnell, an astrobiologist at the Centre for Planetary Sciences at UCL, was in to discuss his research into whether some forms of life might be able to survive on Mars.

From what we know of it, Mars isn’t the friendliest of places. It’s bitterly cold; the air temperature is rarely above freezing. And it’s very dry. Although we can see that the planet has polar ice caps, there’s no sign of any liquid water – which is essential for life – anywhere. It all seems to be either frozen or evaporating into the atmosphere.

To make matters worse, Mars has no magnetic shield, no ozone layer, and only a very thin atmosphere, so its surface is exposed to intense levels of radiation, including UV and ionising cosmic rays, which damage and destroy DNA.

As Dartnell explained, we’re protected from cosmic radiation by our planet’s “lovely deep atmosphere,” which absorbs high-energy particles in cosmic rays, and a magnetic shield, which deflects them. The high-energy radiation that reaches the surface of Mars would kill us.

But what about other forms of life, such as microbes? Might they be able to survive the harsh cold and fierce cosmic radiation on Mars?

To find out, Dartnell has been looking at bugs he calls “ultra hardy survivors” which live in the Dry Valleys of Antarctica, the place on our planet that is most like Mars. With only 1mm of snowfall a year, and cosmic and solar radiation streaming through the hole in the ozone above Antarctica, no plants and animals survive in those desolate valleys.

But in the cracks of the rocks, protected from the fierce UV light and cold, dry winds, lives a tiny bug called Deinococcus radiodurans. Its name means ‘enduring radiation’, and indeed Deinococcus is the most radiation-resistant organism we’ve found on this planet. It even grew on the walls of post-explosion Chernobyl.

To find out precisely how much radiation these bacteria can survive (and therefore, whether similar microbes might be able to survive on Mars), Dartnell cultured them in his lab. Deinococci are rich in pigments that protect them from UV radiation, and the cultures sprawl in lurid shades of pink, across the Petri dishes, easily visible to the naked eye.

Dartnell then took his cultured specimens to an old Ministry of Defence research facility at the University of Cranfield where, during the Cold War, researchers tested whether tanks could protect against radiation from a nuclear bomb. There, he bombarded them high doses of gamma radiation from a cobalt-60 source.

Gamma rays are extremely energetic forms of light that that can break bonds in our DNA and shatter our genomes. A dose of a few Grays of ionising radiation would kill a person, yet Dartnell found his deinococcus bugs could all survive doses of 5000 grays “without blinking an eye”.

How do they do it? Dartnell says the bacteria have a number of quirks in their biochemistry that protect them. While they can’t directly protect themselves from the gamma rays, which are powerful enough to go through rock, they are very good at repairing the damage those rays do to their DNA. A suite of repair enzymes piece the damaged genome together again. Moreover, each cell or bacteria has up to 12 or more copies of its genome, whereas humans only have one copy in each of our cells.

The deinococcus’s ability to piece itself together again, in the right order, is of great interest to genetic engineers, who are looking to apply its molecular biology to other cells to develop new medicines or forms of bio-remediation. It might be possible to engineer organisms that can survive radiation and clean up oil slicks and toxic waste, for example.

As well as testing his bugs’ impressive survival rates, for his PhD Dr Dartnell worked with physicists, mathematicians and computer programmers at UCL to develop a computer simulation of the radiation on the surface of Mars. These kinds of multi-disciplinary collaborations are, he says, at the forefront of modern science: mathematicians condense biological complexity into equations, which a computer programme then codes for analysis.

That work also involved some hard-core, very high-energy particle physics. Dr Dartnell and his team used the same codes that researchers at the Large Hadron Collider at CERN (the European Organization for Nuclear Research near Geneva) use to simulate particle energies. The collisions that occur in a huge underground circular tunnel at CERN also happen right over our heads in the Earth’s atmosphere, and on and around a metre below the ground on Mars.

It’s there, in the crevices of Martian rocks, that Dartnell is hoping to find some hardy little deinococcus -like organisms hiding from the radiation underground. And he may soon find out whether that is the case when, in 2018, the European Space Agency and NASA launch their robotic ExoMars mission.

As we know from watching CSI, organic molecules (blood, sperm, and other proteins) glow in the dark under UV light. So the ExoMars robot will wait for night on Mars, and then scan the landscape and rocks with a UV laser, looking for protective nooks and cavities, where microbes might be hiding. If those crevices fluoresce in the dark, it could be a sign of life. The robot will then fish out an armful of soil and deliver it into the onboard instruments for analysis.

In the meantime, researchers are looking at the space material that’s already delivered to us. We know that the building blocks of life can begin in outer space. Meteorites, what Dartnell calls the “builder’s rubble left over from the making of the planets”, made of silica rocks, contain large amounts of carbon. And carbon is good at sticking together to make organic compounds. Add sources of energy from the sun or from radioactive decay in the asteroids, and sometimes water to the mix, and the chemistry results in organic molecules.

The Murchison meteorite, which fell near Murchison in Australia in 1969 apparently, reeked of petro-chemicals and organic molecules, when it was first picked up. And researchers found 70 different amino acids (the building blocks of protein) inside the meteorite – yet there are only 20 amino acids on Earth – definitive proof that the building blocks of extra-terrestrial life flicker into being far out in the depths of space.

Penny Bailey

Penny Bailey is a writer at the Wellcome Trust.

UPDATE: 25/6/2010

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