Tardigrades have a reputation for being some of the toughest creatures on the planet.
These miniscule animals can survive in space, inside a volcano, and nearly a mile deep in an Antarctic lake. After being frozen for three decades, they have even recovered to normal functioning. This has ramifications for the concept of panspermia, which proposes that some life forms may be able to travel across worlds by catching a ride on a comet or meteorite.
However, for hardy small critters like tardigrades to accomplish such a journey, they must not only survive space travel but also impact. The Israeli lunar lander Beresheet crashed on the moon in 2019 carrying thousands of dehydrated tardigrades, among other things.
Alejandra Traspas, a Ph.D. student at Queen Mary University of London, was intrigued by this and wondered if the little astronauts could have lived. Traspas collaborated with her supervisor, University of Kent planetary scientist Mark Burchell, to construct an experiment to imitate the impact of a planet or moon on another body in space.
They were able to get their hands on several tardigrades, which were frozen and loaded into a two-stage light gas gun, which NASA uses to mimic impacts in space.
To see how the extremophiles fared, the water bear bullets were shot into sand targets at various velocities. They were shown to be able to withstand hits at speeds of nearly a kilometer per second (2,000 miles per hour) and shock pressures of up to 1.14 gigapascals.
The tardigrades could be revived after being shot out at speeds of less than 900 meters per second (approximately 2,000 miles per hour) – quicker than an average bullet. They wouldn’t have made it if they had gone much quicker, according to Alejandra Traspas, a co-author of the study.
She goes on to say that, though Beresheet may have crashed at a slower speed, the shock pressure from the metal spacecraft colliding with the moon’s regolith would have been much stronger, bringing the tardigrades on board to their deaths.
“We can confirm they didn’t survive.”
All of the findings have been published in the journal Astrobiology. They are pessimistic about panspermia and interplanetary travel by way of an unplanned crash landing.
“Arrival of a tardigrade on Earth, for example by way of a meteorite impact, is not likely to be a viable means of a successful transfer even for such hardy organisms,” the study reads. “There are other places in the Solar System, however, where biological material, during transfer, would encounter low shock pressures.”
Tardigrades are also known as water bears or moss piglets, which are fitting titles given that under a microscope, these 0.05-inch-long critters resemble eight-legged potatoes with crumpled up noses and small claws.
The critters can tolerate temperatures ranging from minus 458 degrees Fahrenheit (minus 272 degrees Celsius) to 304 degrees Fahrenheit (151 degrees Celsius), as well as pressures up to six times that of Earth’s deepest oceans.
Water bears, like their namesake, may enter a condition of hibernation, which allows them to endure fatal radiation and temperatures. Tardigrades can go for long periods of time without water or oxygen by entering a state of suspended animation known as cryptobiosis, in which their bodies dry up and their metabolisms shut down. When a dehydrated, hibernating tardigrade is submerged in water, it quickly recovers its full function.
Transfer between a planet and its moons, such as Mars and Phobos, or even Earth and our own moon, might still be conceivable, the researchers note, because some rocks and debris in space might collide at lower speeds and pressures.
The findings have an impact on future missions to the outer solar system that may try to find life by sampling the watery plumes emitted by Enceladus and Europa.
The conclusion for anyone expecting to uncover extraterrestrial water bear cousins is to slow down as much as possible and collect samples with materials that allow for a soft landing to avoid killing any extraterrestrial water bear relatives on impact.