Tardigrades are legendarily tough. The microscopic animals, also colloquially referred to as “water bears,” can survive extremes of pressure, radiation, temperature, and dehydration. They’re capable of surviving in environments where these extremes are presented simultaneously, including outer space. This makes them useful for testing certain theories regarding the origin of life on Earth.
One of the most significant scientific questions our species has yet to answer is whether Earth is unique, which is to say: Has life arisen elsewhere in the solar system/galaxy/universe other than here? If life existed on Venus or Mars prior to those planets becoming uninhabitable, there’s a chance that an impact event could have carried life from one world to another. The relatively recent detection of ‘Oumuamua and the comet 2I/Borisov implies that interstellar rocks might swing through the solar system on a frequent enough basis to have deposited preserved biological material on the early Earth.
The idea that life might be able to hitchhike its way across solar systems or between stars is known as panspermia. The concept is not necessarily as far-fetched as it seems. While there is no proof of panspermia, the argument for its possibility looks something like this: Preserved impact craters across the solar system are evidence for a large number of interactions after planetary formation. Some of Earth’s water may have been delivered by these impacts; an analysis of meteorite fragments from the asteroid 4 Vesta show nearly identical hydrogen and nitrogen ratios to that of Earth, implying that Vesta is itself a good example of the kind of body that delivered Earth’s water.
4 Vesta, incidentally, is also proof that long-term gravitational interactions can deliver material over tens of millions of years. About a billion years ago, an impact excavated a large quantity of material from Vesta’s crust and mantle. Today, 5 percent of all known meteorites on Earth are believed to come from this event. Meteors from this ancient impact continue to fall on Earth; a recent report identified Vesta as the likely parent body of 2018 LA, a small meteor that fell over Botswana in 2018.
So, we’ve got enough plausible bits and pieces to cobble together a theory. Only problem is, can life actually survive slamming into another planet at the sorts of temperatures and speeds associated with meteorite impacts? This is where the tardigrades come in.
As scientists Alejandra Traspas and Mark J. Burchell write in Astrobiology: “The ability of tardigrades to survive impact shocks in the kilometer per second and gigapascal range was investigated.” Tardigrades, it turns out, can only withstand a shock pressure of ~1.14 gigapascals. Above 2,000 miles per hour, or above 900 meters per second, even the tardigrade’s legendary toughness fails it.
This has implications for the types of scenarios in which panspermia could occur. Spores and microbes are hardier, surviving impacts of up to 5000 meters per second and pressures of 40 gigapascals or more. According to the authors, tardigrades have been tested for static loading of up to 7.5 gigapascal (and survived it), but no one has tested to see if they can survive the impact. The scientists write: “Accordingly, we have fired tardigrades at high speed in a gun onto sand targets, subjecting them to impact shocks and evaluating their survival.”
As you can see, the tardigrades aren’t quite hardy….grades (I regret NOTHING) enough to survive the higher-pressure, higher-speed impacts. This has implications for the types of planets and landings where life could plausibly survive. This is already a tricky problem because the amount of radiation your typical asteroid is exposed to is more than enough to sterilize the rock during a long voyage from, say, Mars to Earth. According to the researchers, it might be possible to catch samples of life from the geysers of Enceladus, depending on the materials used for the attempt, but impact velocities above Europa would be above the tardigrade survival limit.
If this research is accurate, the only places where panspermia might plausibly occur with organisms as tough as tardigrades would be between the Moon and Earth and possibly Mars and Phobos. Other contemplated transfers either result in unsurvivable impacts or a too-high dose of radiation while in space. Panspermia might still have occurred with spores or microcellular life, but even tardigrades have limits.
Top image credit: Rebecca Smith/Flickr (Public Domain)