Photo: Aaron J Hill

What animal cant be defeated?

Why tardigrades are so damn tough. It's important to note: Tardigrades are basically indestructible only when they enter a special state called cryptobiosis. In harsh environments, the animals tuck in their legs and expel all moisture from their bodies. In this state, they're called tuns.

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In the end, there will be tardigrades.

If a cataclysm wipes out most of life on the planet — including humans — it’s likely that tardigrades will survive. These oddly cute microscopic animals are among the toughest creatures on Earth. Some can withstand the shock of boiling water and temperatures that near absolute zero. Others have survived in the crushing pressures of the deep ocean or go without water for decades. Tardigrades have survived in the vacuum of space. In 2014, Japanese researchers thawed a group of tardigrades that had been frozen for 30 years. What did they do after they woke up? They reproduced.

But what would it take to ... kill them all? (Laughs maniacally.)

What would it take to end all life on Earth?

That’s roughly the question a group of physicists asked in a new paper in Nature’s Scientific Reports journal. The point of the paper isn’t to pick on tardigrades. Rather, it’s a thought experiment. “Much attention has been given in the literature to the effects of astrophysical events on human and land-based life,” the authors, physicists from Oxford and Harvard, write. “However, little has been discussed on the resilience of life itself.” This question is important because if a mass extinction event were to occur, it would be nice to know if life could again grow, evolve, and prosper. The asteroid impact that killed the dinosaurs 66 million years ago also took out 80 percent of the other species on the planet. But some small mammals survived. And that’s why we’re here today. Tardigrades are good benchmark on this question. If the extremely tough tardigrades go, then, the Earth, in the word of the researchers, would be “sterilized” of all life.

Why tardigrades are so damn tough

It’s important to note: Tardigrades are basically indestructible only when they enter a special state called cryptobiosis. In harsh environments, the animals tuck in their legs and expel all moisture from their bodies. In this state, they're called tuns. As tuns, the tardigrades produce glycerol (antifreeze), and secrete trehalose, a simple sugar with remarkable preservation properties. "Trehalose is viewed as a cocoon that traps the biomolecule inside a glassy matrix, like amber-encasing insects," explains a 2009 paper in Protein Science. When the trehalose crystallizes, the tardigrade becomes mummified in a glass suit of armor. As a tun, the tardigrade reduces its metabolism by 99.99 percent as it waits for a more suitable environment. Different species of tardigrades have different adaptations for a wide variety of environmental threats. In hot conditions, they release heat-shock proteins, which prevent other proteins from warping. Some tardigrades can form bubbly cysts around their bodies. Like puffer jackets, the cysts allow them to survive in harsh climates.

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(When tardigrades are not in tun form, "it's very easy to kill them," a biologist once assured me.)

Three scenarios that could wipe out tardigrades

So what would it take? The researchers made some assumptions about tardigrades’ upper limit for heat and radiation exposure, and examined three scenarios. A nearby star goes supernova and sends deadly radiation pummeling toward Earth A gamma ray burst from an unknown origin fries Earth Asteroid impact

Let’s start from the top.

In the case of 1 and 2, hell would be unleashed on Earth. The radiation would destroy the ozone layer, fry all life on land, and destroying our oxygen-rich atmosphere. However, in these scenarios, the tardigrades could still survive. “Even the complete loss of the atmosphere would not have an effect on species living at the ocean’s floor,” the study authors note. To really kill all the tardigrades — and all life on the planet — you’d need to vaporize the oceans. Which would take an incredible amount of energy.

How much?

560,000,000,000,000,000,000,000,000 joules of energy, the study’s authors compute. A joule is a measure of energy equal to the effort it takes to create one watt of power for one second. 560 septillion of them is “around a million years of total human energy production at current rates,” the authors note in the Conversation.

Luckily, there’s no star within striking distance that could produce such energy.

Gamma ray bursts are a lot more mysterious.

We’ve only seen them in other galaxies, and they’re some of the most energetic events ever observed in the universe. Scientists are not sure how or why they form. But they could absolutely destroy all life on the Earth. Luckily, this scenario is unlikely. The source of a gamma ray burst would have to be “within about 42 light-years,” the authors write. The chances of that are astronomically small. Again, we’ve never seen a gamma ray burst in our own galaxy. So it’s unlikely one will pop up in our neighborhood. Actually, few planets in the galaxy “would ever be sterilized by a gamma-ray burst.”

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Phew!

That leaves asteroids. Could a big enough asteroid produce the energy needed to end all life on the planet?

Yes!

The physicists estimate it would take an asteroid with a mass of 1.7 quintillion kilograms or greater to incinerate the oceans. There are only 19 of these in the solar system. None are on a collision course with Earth. The authors also suggest that if the dwarf planet Pluto slammed into the Earth, it would do the trick. And then finally we’d be rid of the little stubby-legged buggers (and absolutely everything else.)

A few caveats

As the Washington Post points out, there are some important caveats to this paper. The big one is researchers are lumping all tardigrades together in their analysis. There are thousands of species of tardigrade — and they all have slightly different adaptations that help them survive. But no one species contains all the defenses of the others. The ones that live in the deep, deep ocean may not be able to withstand as much heat, radiation, or other traumas as their counterparts on land. So it’s a bit of a rough sketch. Tardigrades might be a bit easier to kill than what’s implied here. But for all the talk of death and destruction, the study authors have an optimistic conclusion: "We do not fully understand the mechanisms by which life started, but once it exists on an Earth-like planet, the complete removal of all life (other than through evolution of the host star) is a very unlikely event.”

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