Mars might have its own defense mechanism against life from Earth, in a kind of reverse of “The War of the Worlds” scenario that could help protect the Red Planet from contamination by terrestrial bugs.
Scientists led by Corien Bakermans, a professor of microbiology at Penn State University, were experimenting with exposing tardigrades, which are microscopic animals nicknamed “water bears” that grow to at most half a millimeter in length, to simulants of Martian regolith.
The intention was to determine how well tardigrades did in the regolith, with an eye on one day converting Martian regolith — which is dead, inorganic dirt — into organic soil in which plants can grow. For that to happen, life needs to be able to flourish in the regolith in order to help fertilize it. Think of microbes and earthworms in Earth‘s soil, continually processing the soil to keep it healthy.
Simulants are representations of real Martian regolith. Because we have no samples of Martian regolith on Earth, scientists are forced to replicate it in simulants based on readings of its chemical composition by Mars rovers. In particular, Bakermans’ team used two simulants, both informed by measurements from NASA’s Curiosity rover, which is exploring Gale Crater on Mars.
One of the simulants, known as MGS-1, is designed to mimic the general properties of regolith across Mars. The other simulant, OUCM-1, is designed to more specifically represent the Rocknest area in Gale Crater, where Curiosity took the sample that the simulant is based on.
“We know a lot about bacteria and fungi in simulated regolith, but very little about how they impact animals — even microscopic animals, like tardigrades,” said Bakermans in a statement. “We investigated the specific, isolated impact of the regolith on tardigrades.”
What Bakermans’ team found shocked them. Tardigrades have two states of living: active and dormant, the latter usually resulting from dehydration. When exposed to MGS-1, the tardigrades entered dormancy within two days.
“For the MGS-1 simulant, we saw significant inhibition — reduced activity — within two days,” said Bakermans. “It was very damaging compared to OUCM-1, which was still inhibitory but much less so. We were a little surprised by how damaging MGS-1 was, [so] we theorized that there might be something specific in the simulant that could be washed away.”
So the team did just that, washing a sample of MGS-1 and then applying a new batch of tardigrades to it. This time the little water bears did much better, showing only minor ill effects.
However, the identity of whatever it was that affected the tardigrades remains uncertain.
“It seems that there’s something very damaging in MGS-1 that can dissolve in water — maybe salts or some other compound,” said Bakermans. “That was unexpected, but it’s good in a sense, because it means that the regolith’s defense mechanism could stop contaminants. At the same time, it can be washed to help support plant growth or prevent damage to humans who come in contact with it.”
NASA has a planetary protection office, which is currently led by the microbiologist J. Nick Benardini. “Planetary protection” brings to mind defending Earth from slavering aliens, but it’s actually mostly focused on the reverse scenario: protecting other worlds from contamination by microbes from Earth.
There are two main reasons why we might want to do this. One is that, if life does exist on another planet — and it is still an open question for Mars, given the ambiguities in the measurements made by NASA’s Viking landers and the anomalous methane plumes that could be biological or geological in origin — then the introduction of microbes from Earth could potentially destroy that alien biosphere. Any microbial life native to Mars could have evolved to resist the toxic element(s) in the planet’s regolith, as extremophiles do here on Earth.
The other reason is that, even if terrestrial contamination doesn’t destroy the native biosphere, it will confuse our measurements. If we detect life on Mars, how could we be sure that it is truly native and not a microbe imported from Earth that has contaminated the Red Planet?
It is for this reason that all missions that land on Mars, such as the Curiosity and Perseverance rovers, undergo the strictest regime of sterilization before launch. Guidelines from COSPAR, the Committee on Space Research, state that any mission to land on a planetary surface that may host a biosphere must have only a 1-in-10,000 chance of carrying an Earthly microbe to that world. (Complete sterilization is impossible as you could never be sure you’ve got all the microbes.)
However, if Mars has its own defenses, then things might be a little less worrisome from a planetary protection perspective, and constraints on future missions could be loosened.
“With this research, we’re looking at a potential resource for being able to grow planets as part of establishing a healthy community — but we’re also looking at whether there are any inherent damaging conditions in the regolith that could help protect against contamination from Earth, which is a goal of planetary protection,” said Bakermans.
Her team’s findings do seem to be the best of both worlds. Mars’ surface no longer has running water — it is far too cold and its atmosphere too thin — but there is ice at the poles and permafrost at mid-latitudes to extract water from. So as it stands, Mars’ regolith may be able to defend against Earthly contamination, especially if it is accidental.
These findings are consistent with previous experiments that have found Martian regolith to be detrimental to active cells. On the other hand, simply washing the regolith makes it much more hospitable, raising the promise that one day it really could be converted into a soil for growing plants in, producing oxygen and feeding astronauts living on Mars.
The new findings were published in December 2025 in the International Journal of Astrobiology.
