These animals can survive until the end of the Earth, astrophysicists say – Washington Post
Tardigrades have a reputation as the toughest animals on the planet. Some of these microscopic invertebrates shrug off temperatures of minus 272 Celsius, one degree warmer than absolute zero. Other species can endure powerful radiation and the vacuum of space. In 2007, the European Space Agency sent 3,000 animals into low Earth orbit, where the tardigrades survived for 12 days on the outside of the capsule.
To a group of theoretical physicists, tardigrades were the perfect specimens to test life’s tenacity. “Life is pretty fragile if all your estimates are based on humans or dinosaurs,” said David Sloan, a theoretical cosmologist at Oxford University in Britain.
The tardigrade lineage is ancient. “Tardigrade microfossils are reported from the Early Cambrian to the Early Cretaceous, 520 million to 100 million years ago,” said Ralph O. Schill, an expert on tardigrades at the University of Stuttgart in Germany who was not involved with this research. “They have seen the dinosaurs come and go.”
Sloan, with his Oxford colleague Rafael Alves Batista and Harvard University astrophysicist Abraham Loeb, decided to try to rid the planet of tardigrades. In theory, anyway, in a report published Friday in the journal Scientific Reports. Through the powers of mathematical modeling they tossed three of the most devastating cosmic events at Earth: killer asteroids, supernovae and gamma-ray bursts.
“These are the biggest ways you can transfer energy to the planet,” Sloan said. The tardigrades kept on theoretically trucking, outlasting 10 billion years’ worth of cataclysms. Until the point that the sun failed or engulfed the planet.
In picking their apocalyptic poison, the scientists first tried to sterilize the planet with radiation. In the lab, some tardigrade species can survive radiation doses of 5,000 to 6,000 grays. (“You would be very, very lucky to walk away” from a dose of 5 grays, Sloan said.) But long before the scientists blasted Earth with enough radiation to kill all the tardigrades, they calculated that the radiation’s energy would boil the oceans away. The sticking point for tardigrades, then, was the evaporation of the planet’s water.
For an asteroid to deposit that much energy into the ocean, it would need a mass of at least 1.7 quintillion kilograms. Of all the asteroids in the solar system, only 19 fit the bill. (By way of comparison, the asteroid that finished the dinosaurs was six miles across; an asteroid called Vesta that is one of the potential ocean killers has a diameter of 326 miles.) The chances of such a massive collision are so small, the scientists said, that the sun would die first.
Likewise, the closest stars that could explode into supernovae are too far away to boil the oceans. Gamma-ray bursts were a bit more complicated — “we don’t really understand where they come from,” Sloan said — but not impossible to calculate. And though the bursts would strip off parts of the atmosphere, killing animals like humans, tiny and durable creatures under the ocean, huddled around hydrothermal vents, would be “sufficiently well-shielded,” Sloan said.
But lumping all tardigrade species into one unkillable chimera was a fatal flaw in this argument, according to tardigrade expert William R. Miller. “I can’t say anything about the physics,” he said, “but they can’t say anything about the animals.”
Not all tardigrades dwell in water; some species live in moss and lichens on trees. (Their variety of habitats is reflected in nicknames like “water bear” and “moss piglet.”)