The tardigrade, or water bear, an animal that can only be seen under a microscope, is the likeliest candidate to survive a cataclysmic event that would wipe out all other life. A team of scientists studied the statistics of large asteroid impacts, supernovae (explosions of stars) and gamma-ray bursts, and found that while human life is fragile to nearby such events, the tardigrade can be resilient unless the event causes all oceans to boil. The study has been published in Scientific Reports.
The tardigrade can endure a wide range of pressure and temperature levels — between -272°C and 150°C for minutes, and at 20°C for decades — and high radiation levels. Half a millimetre long, it is essentially a water-dweller but also inhabits land and, a 2008 study found, can survive in the cold vacuum of outer space.
So, to find out what kind of astrophysical event can end life on an Earth-like planet, the new study chose the tardigrade as yardstick. “For complete sterilisation we must establish the necessary event to kill all such creatures.”
For example, while a large asteroid impact could lead to events that would be catastrophic for life dependent on sunlight, life around volcanic vents in the deep ocean would be unaffected — unless the oceans themselves boil away.
And although the solar system has 19 asteroids massive enough for an impact to cause such boiling, their orbits don’t cross Earth’s, said Rafael Alves Batista, an Oxford University astrophysicist and one of the study authors. Besides, the impact rate of objects with such masses is low; the likelihood of an impact strong enough to end life is less than 1 in 100,000 over a planet’s lifetime.
“Therefore, they [asteroid hits] don’t pose a real threat,” Batista told The Indian Express. For a supernova to kill all tardigrades, the team calculated that a star needs to be a fraction of a light-year from Earth. And no star is that close. “The closest star after the sun is about 4 light-years away, but it cannot go supernova. The nearest potential supernova known is about 130 light-years,” Batista said.
A gamma-ray burst could be disastrous for life on Earth’s surface but life could continue underground and for many marine species. A gamma-ray burst leading to boiling of the oceans — and annihilation of tardigrades — would be a very rare event, with the study calculating the probability at about 1 in 3 billion every billion years. “We can conclude that such event is unlikely,” the study says.