If there’s alien life on Mars, we’re probably not going to find it on the surface — not even tiny organisms. The planet already doesn’t seem too habitable, but new research indicates that the surface environment is even more inhospitable than it immediately appears, at least to life forms that are found on Earth.

Chemical compounds called perchlorates, which are types of salts that are also used in rocket fuel, are common on the Martian surface and when they interact with ultraviolet light, like the kind of radiation that beats down through the thin Mars atmosphere, they are deadly for a certain kind of bacteria, according to the University of Edinburgh in Scotland.

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Researchers from that institution imitated the Martian conditions in an Earth laboratory setting, finding that the combination of perchlorates and UV light killed Bacillus subtilis , which is common in soils, on rocks and on spacecraft, the university explained. The UV radiation activated compounds like magnesium perchlorate making it “capable of killing bacteria much more effectively than UV light alone,” Edinburgh said. “At concentrations of perchlorate similar to those found on the Martian surface, cells of B. subtilis quickly died.”

Usually it is heat that activates perchlorates, but the research shows that in the absence of heat on the normally chilly Mars, radiation can fill in to get the job done.

When iron oxides and hydrogen peroxide were further added to the mix — two more chemicals that are common on Mars, with iron oxides responsible for the abundant red color on the planet — the perchlorates became even more deadly, killing 10 times as many cells.

The results “could have implications for potential contamination from robotic and human exploration of Mars,” the statement said, in that those bacteria brought over to Mars by unmanned rovers or, in the future, humans would die quickly and would not pose a huge contamination risk.

However, the findings also demonstrate how uninhabitable the surface of Mars could be. While there may be other microorganisms and life forms that can survive the activated perchlorates, the death of the B. subtilis could be a negative indication of how further research could pan out.

“Although the toxic effects of oxidants on the Martian surface have been suspected for some time, our observations show that the surface of present-day Mars is highly deleterious to cells, caused by a toxic cocktail of oxidants, iron oxides, perchlorates and UV irradiation,” a study in Scientific Reports explains. “We show the bacteriocidal effects of UV-irradiated perchlorates provide yet further evidence that the surface of Mars is lethal to vegetative cells and renders much of the surface and near-surface regions uninhabitable.”

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This does not rule out, however, the presence of life forms beneath the surface. Scientists have been preparing to look underground for aliens during a Mars mission, using dry and dusty places on Earth like deserts as practice for finding signs of life that could be hiding below, perhaps even miles down.

It also does not rule out the idea that alien life may have existed on the Martian surface in the past, before it became the dry, cold landscape we know it as today. Research has suggested that Mars was once warm and wet, before it lost the majority of its atmosphere to space, completely changing its environment.