Scientists Finally Found Something Tardigrades Can’t Survive

Tardigrades are tiny, endearing, and practically invincible. As it turns out, however, not even tardigrades can survive the harsh conditions of the Martian surface. Or at least, simulations of the Martian surface.

For a recent study published in the International Journal of Astrobiology, researchers plopped tardigrades into lab-made reconstructions of the Martian regolith, or the loose mineral deposits covering the planet’s bedrock. The experiment had several twists and turns. Initially, tardigrades struggled to survive on Martian soil, but a simple pre-wash of the regolith appeared to greatly boost tardigrade survivability.

“We know a lot about bacteria and fungi in simulated regolith, but very little about how they impact animals—even microscopic animals, like tardigrades,” Corien Bakermans, the study’s lead author and a microbiologist at Penn State University, said in a statement.

Microbial astronauts for Earth

The study’s primary goal was to assess the Martian regolith’s impact on planetary protection, or the practice of keeping extraterrestrial bodies safe from Earthly contaminants—and vice versa. Specifically, the researchers investigated whether Martian soil was at all compatible with plant growth, in addition to the presence of “inherent damaging conditions” to “protect against contamination from Earth,” Bakermans explained.

As microscopic animals with experience surviving outer space—not to mention extreme temperatures, radiation, dehydration, and starvation—tardigrades appeared to be ideal candidates for this mission, according to the study.

Lab-made Martian soil

For the study, the researchers created two types of Martian regolith. The replica soil was based on samples collected by NASA’s Curiosity rover during its journey through the Rocknest Deposit at the Gale Crater.

One sample, named MGS-1, served as a “global” regolith representing the planet’s overall surface, whereas another sample, OUCM-1, was subsequently developed with “specific attention to chemical composition [and] mineral makeup,” the researchers explained.

The team studied both the dormant and active states of tardigrade activity. The former occurs when tardigrades are severely dehydrated; this was also the state that allowed them to survive the vacuum of space or the depths of the ocean. On the other hand, active tardigrades will be “vigorously crawling or swimming,” according to the paper.

Tardigrades in desolate soil

The team mixed active tardigrades into each regolith sample, checking their activity levels over several days. They found that MGS-1 significantly reduced tardigrade activity, pushing some to become completely inactive by day two. By contrast, OUCM-1 tardigrades were “reasonably energetic at all time points,” the paper noted. In both samples, the tardigrades—dead or alive—had mineral particles near their mouths.

“We were a little surprised by how damaging MGS-1 was,” Bakermans said. “We theorized that there might be something specific in the simulant that could be washed away.”

Indeed, when the researchers gave MGS-1 a simple rinsing with water, the tardigrades added to the modified mixture showed far more vigor for several days and managed to survive, according to the paper. Bakermans noted that this was unexpected, but it suggests the regolith contains substances that are highly damaging to contaminants.

Far from Mars

That said, the experiment comes with some important caveats. First, the researchers say it’s relatively reassuring that something as simple as rinsing regolith with water might remove harmful substances. But if humanity moves to Mars, water will be a huge issue, whether or not there’s a need to wash the soil.

Also, this was a simulation that relied on pretend Martian regolith; for better or worse, no tardigrades have ever set foot on Mars (as far as we know). As the researchers noted, further investigations that account for pressure or temperature differences may play a significant role in tardigrade survivability. Finally, tardigrades are fascinating test subjects, but they don’t tell the whole story about contamination risks on Mars. How they perform in Mars-like conditions doesn’t necessarily show how other organisms—including hardy extremophile microbes—would survive or spread there.

Still, the new results offer some insight into how “any single piece could be a drawback or benefit the larger understanding of planetary protection,” as Bakermans describes it. Also, it’s another example of how tardigrades are perhaps the toughest creatures ever to exist. Seriously.