Astrobiology: What makes a planet habitable?
Professor Lewis Dartnell
Astrobiology, said Professor Lewis Dartnell, is “the science of hunting for aliens”! That may conjure up images of Martian death rays, wielded by tripods come to invade the earth. But if our next door neighbour planet ever did have an environment that was suitable for the origin of microbial life on the surface, the environment of Mars today is very harsh. There is no longer a thick atmosphere, no magnetosphere, so the Martian death rays are the constant cosmic radiation that beat down on the surface of Mars – and penetrating through several metres of Martian rock.
So what sort of life is going to be able to survive there? The ExoMars Rover will launch in 2020 and explore the top two metres of Martian land which has been irradiated. So is life long since dead?
One of the things that Lewis does is study bacteria in Antarctica – “It’s one of the most Mars-like places on Earth where we can test our instruments and experiments and test how life can survive in a very cold, dry, Mars-like environment,” he said.
It’s part of the study of ‘extremophiles’ – organisms that live in extreme environments. On Earth, at Yellowstone Park, there’s a big pool of magma just beneath the crust. “When that super-volcano goes up, it will collapse human civilisation,” he said cheerily. “There’s no two ways about it. But for now, it’s a great field site for astrobiology.”
There is a lake there which is heated by volcanic activity – steaming hot and very acidic. If you were to fall off the path beside it, you’d die. “The colours of the lake – green, yellow, orange, red – are the colours of life. The colours of thermophiles and acidophiles which have adapted to this hell-hole of a place,” he said.
By way of contrast, look at the side of an iceberg in the Arctic sea. There are thousands pockets and veins of very salty water which doesn’t freeze – and which contain bacteria, alive and metabolising down to -10°C, -20°C. Colder than your freezer.
There is, in other words, a huge range of conditions that can accommodate life on Earth, so there is a reasonable chance of extra-terrestrial life.
Mars, the most Earth-like planet
In many respects, Mars is the most Earth-like place we know. It once had seas and lakes and rivers of liquid water gushing across the surface. It once had a much thicker atmosphere. It would have had organic molecules – the building blocks of life – raining down on the surface aboard meteorites and comets.
So how habitable has it been? Was it for a very short period? Is it still habitable today? Lewis discussed the differences between Earth and Mars:
Water vapour – water is in all three phases on Earth, but especially liquid;
The thickness of the atmosphere;
There is a minimal greenhouse effect on Mars which keeps Earth alive: if it weren’t for the CO2 and other greenhouse gases, the Earth would be 20°C cooler. Mars has lost that;
A large moon may help stabilise a planet’s tilt axis, holding us upright so our axis doesn’t swing around and throw climate into disarray when the poles dip down to the equator;
This may be important for keeping planets habitable as volcanoes belch out CO2. Mars cooled down more quickly so its volcanoes have died and now it’s not topping up its atmosphere with CO2. The core can no longer generate a magnetic field that prevents the solar wind from stripping the atmosphere into space.
But all the signs of ancient Mars – Mars of 3.5bn to 4bn years ago – are that it has been habitable. The Exomars Rover is being built by ESA in cooperation with Roscosmos, and is due to launch in 2020. It has 6-wheel dive which is better than 4-wheel drive on the Martian dust, a triple-camera set-up and can see in 3D at about human head height, giving a very “human perspective”. A drill will enable it to bring up samples of soil that has been protected underground