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 from UV and cosmic radiation. Soil samples will be taken into the main body of the Rover and tested.
To moons beyond
Looking beyond Mars, are there any candidates for habitability amongst the moons of Jupiter?
Io – It is volcanically active, and is “violently turning itself inside-out”, Lewis said. “It is not a place to go on holidays.” We don’t expect to find water or organic chemistry there.
Calisto – Scarred from impact craters when the solar system was first made, it is a cold, dead world.
Europa – A warm, wet world, with a shell of hard water-ice but a global ocean beneath it with more water than the whole of the Earth. “It is Europa that is the water world in our solar system and not the Earth,” he said. “It ticks all the boxes for life. Its tidal heating that churns up the innards, warming the subsurface.”
Saturn’s mysterious moon Titan was “the largest area of unexplored real estate in the solar system” until we went there in 2005 with the Huygens Titan lander, which was carried by Cassini. This gave us our first glimpse of a hidden landscape – an earth-like place, in many respects. “This is the English countryside!” Lewis said. Titan has rolling hills and uplands, smooth flat dried-up lakes, and networks of river valleys. We have found seas and lakes full of fluid. “This is the only place that we know about in the entire universe beyond the earth where you can go on a sailing holiday,” he said. “It’s not very windy, but you could in principle go on a boating holiday!”
But this isn’t liquid water. It’s liquid methane. We don’t know enough about what kind of chemistry you can get in liquid methane. Can we talk about methane-based life, with a fundamentally different cell structure and molecules that look nothing like DNA? If so, this will make searching for life more difficult than searching for water-based Martian life.
Concentric rings of habitable zones
This would also turn on its head the idea of ‘habitability’. It could be that we have not a single habitable zone but concentric rings of habitable zones. You could have a water-habitable zone on the inside, a methane-habitable zone further out where Saturn and Titan lie, perhaps a liquid ammonia habitable zone in the dim distance. “This is the sort of thinking we’re considering when we start looking at planets orbiting other stars,” he said.
We’ve now discovered thousands of new worlds – exoplanets. Many of the earliest found exoplanets are much bigger than Jupiter and orbit their star very closely. These planets have metal vapour. But now we’re finding smaller planets orbiting further from their star. “We might be right on the brink of discovering a true twin to Earth – an Earth-sized rocky planet orbiting a sun-like star with an orbit of exactly one year,” he said.
Looking at the H-R diagram, we know that the more massive a star the hotter it is, therefore it’s bluer, therefore shorter-lived. “You do not want to find yourself [on an exoplanet] orbiting something like this because you’ve got about 10 million years to evolve and then get away again before it goes super-nova,” Lewis said. However, M-type red dwarfs last trillions of years. “You’ve essentially got infinite time to develop life if you are a planet orbiting a star like this.” Moreover, red dwarfs outnumber G-type ‘Sol’ type stars by about 10 to 1. So there are many potential homes for life.
Lewis described the discovery of the exoplanet Proxima b as “the discovery of the generation”. It’s not earth-like, but relatively Earth-massed, orbiting a red-class dwarf and it just happens to be the closest star to Earth.
“If you were playing God, when you were setting up the Universe, to tantalise humanity you would put what looks like an Earth-like planet right on their doorstep – and let them think about it for 200 years before we get the capability to actually travel there!” he said.
“It’s simultaneously one of the most exciting discoveries of our generation but also one of the most frustrating”. Spectroscopy will tell us a lot about it but we just can’t get there.
But one of the problems with planets that orbit red dwarf stars is tidal locking – like the Moon always faces Earth. The habitable zone is so close to the star. With one side in constant sunshine, the other in constant darkness, the atmosphere on the dark side may freeze and crash to the surface.
So how can you maintain habitability on a red dwarf? Deep ocean currents to redistribute the warmth would help. If there is a thick enough atmosphere there will be “brutal” winds. But that might be the only thing keeping the planet alive “and not spit-roasting”, Lewis said. There is some evidence that Proxima b might still have a magnetosphere – “So when you get there you can still use a compass,” he joked.
In the Q&A session, Lewis was asked why he hadn’t mentioned Saturn’s Enceladus as a potentially habitable moon. Apart from because of the time limit on his talk, Lewis said that Enceladus and Europa are similar: icy moons, tidally-heated. But he said that, whereas on Europa you’d have to drill down through perhaps kilometres of ice, Enceladus is actually “spitting its own ocean out into space through the cracks in the southern hemisphere”. So we could fly through the plumes of Enceladus, collect some of the ice crystals then loop back around Saturn and do it as a sample return mission. “That would be an exciting mission!”
Afterwards, Lewis signed copies of his books, The Knowledge: How To Rebuild Our World After An Apocalypse and Life in the Universe: A Beginner’s Guide to Astrobiology.
His blog is available here. One of the many articles is entitled How to become an astrobiologist.