Mars may have its own defense mechanism against life from Earth, in a sort of reverse of “The War of the Worlds” scenario that could help protect the Red Planet from contamination by Earth insects.
Researchers led by Corien Bakermans, professor of microbiology at Penn State University, experimented with revealing tardigradeswhich are microscopic animals nicknamed “water bears” that grow to a maximum of half a millimeter in length, into simulants of Martian regolith.
The aim was to find out how well tardigrades did in the regolith, with an eye towards one day converting the Martian regolith – which is dead, inorganic dirt – into organic soil where plants can grow. For that to happen, life must be able to flourish in the regolith to help fertilize it. Think about microbes and earthworms Earth‘s soil, continuous cultivation of 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, Bakerman’s team used two simulants, both informed by measurements from NASA Curiosity rover, exploring Gale Crater on March.
One of the simulants, known as MGS-1, is designed to mimic the general properties of the Martian regolith. The second simulant, OUCM-1, is designed to more specifically represent the Rocknest area of Gale Crater, where Curiosity took the sample on which the simulant is based.
“We know a lot about bacteria and fungi in simulated regolith, but very little about how they affect animals – even microscopic animals, like tardigrades,” Bakermans said in a statement. “We investigated the specific, isolated impact of the regolith on tardigrades.”
What Bakerman’s team found shocked them. Tardigrades have two life states: active and dormant, the latter usually due to dehydration. When exposed to MGS-1, the tardigrades went into hibernation within two days.
“For the MGS-1 simulant, we saw significant inhibition — reduced activity — within two days,” Bakermans said. “It was very damaging compared to OUCM-1, which was still inhibitory, but much less. 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 group of tardigrades to it. This time the little water bears did much better, showing only minor damage.
The identity of whatever it was that affected the tardigrades remains uncertain.
“It appears that there is something very harmful in MGS-1 that can dissolve in water — perhaps salts or some other compound,” Bakermans said. “It was unexpected, but it’s good in a way, because it means the regolith’s defense mechanism can stop pollutants. At the same time, it can be washed to support plant growth or prevent harm to people who come into contact with it.”
NASA has a Planetary Protection Office, which is currently headed by microbiologist J. Nick Benardini. “Planetary protection” brings to mind defending the earth from slavery aliensbut 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 exists on another planet – and that remains an open question for Mars, given the ambiguities of measurements made by NASA’s Viking landers and that abnormal methane plumes which may be biological or geological in origin – then the introduction of microbes from Earth could potentially destroy the alien biosphere. Any microbial life native to Mars could have evolved to resist the toxic elements in the planet’s regolith, as extremophiles do here on Earth.
The second reason is that even if terrestrial pollution does not destroy the original biosphere, it will confound our measurements. If we discover life on Marshow can we be sure that it is truly native and not a microbe imported from earth that has polluted the red planet?
It is for this reason that all missions that land on Mars, such as Curiosity and Endurance rovers, undergo the strictest sterilization regime before launch. Guidelines from COSPAR, the Committee on Outer Space Research, state that any mission to land on a planetary surface that could host a biosphere must have only a 1-in-10,000 chance of carrying an Earth microbe to that world. (Complete sterilization is impossible, as you can never be sure you have all the microbes.)
However, if Mars has its own defenses, things may be a little less worrisome from a planetary protection perspective, and constraints on future missions may be loosened.
“With this research, we’re looking at a potential resource for being able to grow planets as part of establishing a healthy society — but we’re also looking at whether there are any intrinsically harmful conditions in the regolith that could help protect against contamination from Earth, which is a goal of planetary protection,” Bakermans said.
The team’s findings appear to be the best of both worlds. The surface of Mars no longer has running water – it is far too cold and the atmosphere too thin – but there is ice at the poles and permafrost in mid-latitudes to draw water from. As it stands, Martian regolith may be able to defend itself against terrestrial contamination, especially if it is accidental.
These findings are consistent with previous experiments that have found the Martian regolith to be harmful to active cells. On the other hand, simply washing the regolith makes it much more hospitable, raising the promise that one day it could truly be transformed into a soil to grow plants in, produce oxygen, and feed astronauts living on Mars.
The new findings were published in December 2025 in International Journal of Astrobiology.
