But the search for life on the red planet isn’t over yet.

Mars is not a very welcoming place. It’s cold, there’s hardly any atmosphere, and its bombarded with deep space radiation. Even the soil wants to kill you; as the Phoenix lander confirmed in 2009, the Martian regolith is laced with perchlorates—chlorine-based compounds that, when heated up, can rip apart organic materials, like cells and their building blocks.

To try to find out exactly what those perchlorates might do to Martian microorganisms, researchers at the University of Edinburgh pitted bacteria against this compound in a battery of tests. What they found does not bode well for the search for life on Mars: Under Mars-like conditions, the perchlorates were indeed toxic to the microbes.

“We knew before that any life would have an incredibly hard time to survive on the surface, and this study experimentally confirms that,” says Dirk Schulze-Makuch, an astrobiologist at Washington State University, who wasn’t involved in the new study.

Bacillus subtilis, a species of bacteria that often contaminates spacecraft, served as the unfortunate test dummy. Vials of these little guys were exposed to a variety of uncomfortable conditions, including UV radiation, environments without oxygen, and low temperatures, all in the presence of perchlorates. The results are published today in Scientific Reports.

Mars has very little atmosphere to protect it from radiation, so the researchers were particularly interested to see how the combination of perchlorates and UV radiation—like what microbes would experience on the surface of Mars—would affect the bacteria in the lab. UV light by itself killed off a ton of bacteria, but when perchlorate was included, too, the test tubes were completely sterilized within 30 seconds. Meanwhile, cells that were only exposed to perchlorates (no radiation) were still alive and kicking when the researchers left them for up to an hour.

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Low temperatures and rock simulant improved the bacteria’s survival rate a little, but the perchlorate+UV combo was still devastating.

The researchers also tested whether iron oxides and hydrogen peroxide—two other compounds found on the Martian surface—would “act in synergy with irradiated perchlorates to make the surface of Mars inimical to life,” the authors explain in the paper. This combo, too, resulted in another huge drop in B. subtilis population.

The authors conclude 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.”

What does this mean for life on Mars?

If you’re still hoping to find life on Mars, don’t despair yet. The study supports a hunch that many scientists have had for a while: if there’s life on Mars, we’re going to have to dig for it.

“Although the surface of Mars may be uninhabitable, there’s a whole potential subsurface habitat to be explored,” says lead study author Jennifer Wadsworth, an astrobiology PhD student. She and her co-author, Charles Cockell, are still working out exactly how the UV and perchlorate mixture delivers the one-two knockout punch, so they aren’t sure how wide-ranging its effects are, but it might extend the uninhabitable zone.

“If that’s the case we may have to dig at least a few meters into the ground to ensure the levels of radiation would be relatively low,” she says. “At those depths, it’s possible Martian life may survive.”

Schulze-Makuch agrees that our robots might need to dig a little deeper than previously thought to look for life—or search in habitats where there is natural protection from radiation, such as lava tubes, below ice crusts, or areas with hydrothermal activity.

Do we still have to disinfect our spacecraft?

Before NASA sends a spacecraft to Mars, it spends millions of dollars trying to scrub the robot clean of microbes, so that we won’t contaminate another planet. If the Martian topsoil is so deadly on its own, does this mean we can skip that step?

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No, says Wadsworth. “Life can survive very extreme environments. The bacterial model we tested wasn’t an extremophile so it’s not out of the question that hardier life forms would find a way to survive.”

Experiments on the International Space Station have shown that bacteria can survive for hundreds of days while exposed to extreme temperature swings, intense radiation, and the vacuum of space. Plus, Schulze-Makuch adds, Mars is a big planet, and the disinfecting perchlorates aren’t likely to be present everywhere.

“It’s important we still take all the precautions we can to not contaminate Mars,” says Wadsworth.

How will this affect my future Martian colony?

Setting up a base on Mars is still just as dangerous as before, since we already knew the perchlorates were there.

“In a greenhouse kind of setting, much of the UV radiation would be reduced,” says Schulze-Makuch, “and also the perchlorate would vanish with watering. Only the initial preparation of the soil used for growing plants in a greenhouse is likely a bit more challenging.”

“Perchlorate can be dangerous to humans so we’d just have to make sure we keep it out of the living quarters,” says Wadsworth. “I think the more immediate threat to Martian colonies would be the amount of radiation reaching the surface.”