Life (as we know it) is adapted for Earth’s habitats, but some organisms may be able to survive on other planets or moons – worlds that are often exposed to dangers from outer space, including cosmic radiation.
So, which of our native species could tolerate such harsh environments, and why do we need to know?
What makes Earth special?
Chemistry and physics imply that biology requires three things: building blocks to form living cells, fuel to power the biochemical reactions that sustain life, and a liquid solvent – most probably water – where life’s reactions can occur.
Earth has those requirements and also protects its inhabitants from radiation – waves or particles that carry energy and radiate from a source, such as light from the sun. Radiation can transfer energy to substances in cells to produce energetic particles that then react with biological molecules, such as DNA, causing mutations.
Our planet has two shields to minimise damage: a magnetic field (generated by electric currents in Earth’s core) that deflects energetic particles, and an ozone layer (made up of oxygen produced by plants and algae) that absorbs harmful radiation, such as UV rays in sunlight.
Which animals can survive in space?
Microscopic plankton called bdelloid rotifers, nematode worms and larvae of the sleeping chironomid (a non-biting midge) can cope with the vacuum of outer space, where temperatures reach near absolute zero (-273ºC below water’s freezing point) and there’s no water or protection from radiation.
The toughest animals are tardigrades or ‘water bears’ – microscopic aquatic creatures that inhabit most ecosystems and can be found in wet moss, for example. In stressful conditions, a tardigrade can extend its lifespan (from months to years) by entering a state of suspended animation or ‘cryptobiosis’, withdrawing eight pairs of legs toward its body to become a ‘tun’ with a water content of just two to three per cent and an undetectable metabolism.
Amazingly, tardigrades can tolerate 10 days in space but, like all animals, they are only able to survive – not thrive – in such an extreme environment.
So which organisms might thrive on other planets?
Extremophiles. These microorganisms flourish in places with extreme temperature (hot or cold), pH (acidic or alkaline) or salt level. One species is so tough it was nicknamed Conan the Bacterium. Deinococcus radiodurans was discovered in cans of corned beef artificially exposed to 4,000 gray of radiation, which is 250 times higher than what is typically used to kill E. coli and 80 times the dose lethal to humans.
Though such high doses don’t occur naturally, stress from loss of water also breaks DNA. So the ability to resist extreme radiation may be a by-product of tolerance to desiccation. D. radiodurans achieves this by efficiently repairing genetic material, while tardigrades have ‘damage suppressor’ proteins that shield DNA.
Could any plants survive on other planets?
One especially tough species is the desert moss Syntrichia caninervis, whose cells can lose up to 98 per cent of their water content (called ‘drying without dying’) and yet recover to perform photosynthesis within seconds of rehydration.
The plant tolerates multiple stress factors: experiments simulating conditions on the planet Mars – including air that consisted of 95 per cent carbon dioxide (CO2) – showed it tolerates freezing at -196ºC for a month, -80ºC for five years, and resists extreme radiation (5,000 gray).
Okay, so some species could live on other worlds. Why does it matter?
Those species could provide vital resources – such as a breathable atmosphere and food – if future humans try to shape other worlds to become habitable and more like Earth (terraforming). Pioneer species can establish ecosystems in new areas – tough, photosynthetic organisms, such as moss or lichen (an association between fungi and bacteria or algae), are able to make oxygen and turn CO2 into carbon that can be used by other living things.
Extremophiles also hint at where to find alien life. Though many worlds in our solar system don’t have protective shielding, microbes could be living below the Martian surface, in oceans on Ceres (a dwarf planet) or on the moons Titan or Europa
