Hardy water bears survive bullet impacts—up to a point

2021-05-18

They can survive temperatures close to absolute zero. They can withstand heat beyond the boiling point of water. They can shrug off the vacuum of space and doses of radiation that would be lethal to humans. Now, researchers have subjected tardigrades, microscopic creatures affectionately known as water bears, to impacts as fast as a flying bullet. And the animals survive them, too—but only up to a point. The test places new limits on their ability to survive impacts in space—and potentially seed life on other planets.

The research was inspired by a 2019 Israeli mission called Beresheet, which attempted to land on the Moon. The probe infamously included tardigrades on board that mission managers had not disclosed to the public, and the lander crashed with its passengers in tow, raising concerns about contamination. “I was very curious,” says Alejandra Traspas, a Ph.D. student at Queen Mary University of London who led the study. “I wanted to know if they were alive.”

Traspas and her supervisor, Mark Burchell, a planetary scientist at the University of Kent, wanted to find out whether tardigrades could survive such an impact—and they wanted to conduct their experiment ethically. So after feeding about 20 tardigrades moss and mineral water, they put them into hibernation, a so-called “tun” state in which their metabolism decreases to 0.1% of their normal activity, by freezing them for 48 hours.

They then placed two to four at a time in a hollow nylon bullet and fired them at increasing speeds using a two-stage light gas gun, a tool in physics experiments that can achieve muzzle velocities far higher than any conventional gun. When shooting the bullets into a sand target several meters away, the researchers found the creatures could survive impacts up to about 900 meters per second (or about 3000 kilometers per hour), and momentary shock pressures up to a limit of 1.14 gigapascals (GPa), they report this month in Astrobiology. “Above [those speeds], they just mush,” Traspas says.

The results suggest the tardigrades on Beresheet were unlikely to survive. Although the lander is thought to have crashed at a few hundred meters per second, the shock pressure its metal frame generated hitting the surface would have been “well above” 1.14 GPa, Traspas says. “We can confirm they didn’t survive.”

The research also places new limits on a theory known as panspermia, which suggests some forms of life could move between worlds, as stowaways on meteorites kicked up after an asteroid strikes a planet or moon. Eventually, the meteorite could impact another planet—along with its living cargo.

Charles Cockell, an astrobiologist at the University of Edinburgh who was not involved in the study, says the research shows how unlikely panspermia is. “What this paper is showing is that complex multicellular animals cannot be easily transferred,” he says. “In other words, Earth is a biogeographical island with respect to animals. They’re trapped, like a flightless bird on an island.”

Traspas, however, says it shows panspermia “is hard,” but not impossible. Meteorite impacts on Earth typically arrive at speeds of more than 11 kilometers per second. On Mars, they collide at least at 8 kilometers per second. These speeds are well above the threshold for tardigrades to survive. However, some parts of a meteorite impacting Earth or Mars would experience lower shock pressures that a tardigrade could live through, Traspas says.

Objects strike the Moon at still lower speeds. When impacts on Earth send bits of rock and debris hurtling toward the Moon, about 40% of that material could travel at speeds low enough for any tardigrades to survive, Traspas and Burchell say, theoretically allowing them to jump from our planet to the Moon. A similar passage, they add, could take place from Mars to its moon, Phobos. And other life might have an even better chance of surviving; compared with water bears, some microbes can survive even faster impacts of up to 5000 meters per second, according to previous research.

The new experiment also has implications for our ability to detect life on icy moons in the outer Solar System. Saturn’s moon Enceladus, for example, ejects plumes of water into space from a subsurface ocean that could support life, as might Jupiter’s moon Europa. If the findings of the new study apply to potential life trapped in the plumes, a spacecraft orbiting Enceladus—at relatively low speeds of hundreds of meters per second—might sample and detect existing life without killing it.

No such orbiting mission is currently planned for Enceladus or Europa—upcoming NASA and European flyby missions will swoosh by the latter at high speeds of several kilometers per second. But perhaps one day far in the future an orbiter might be in the cards, with an ability to detect life at gentler speeds. “If you collect it and it died on impact, how do you know whether it’s been dead for millions of years?” asks Anna Butterworth, a planetary scientist at the University of California, Berkeley, who has studied plume impacts on spacecraft. “If you collect microscopic life and it’s moving around, you can say it’s alive.”

sciencemag.org, 18 May 2021
; https://www.sciencemag.org