The hope for Martian life took another blow today. As Ian Sample at The Guardian reports, a new study suggests that in the presence of ultraviolet light, perchlorates, a class of chemical compounds widespread on Mars' surface, turn deadly for bacteria.

The presence of perchlorates isn't new. Viking 1 and 2 spacecraft detected perchlorates when they landed on the Martian surface in 1976, Jeffrey Kluger reports for Time. Since then, other spacecraft have confirmed the presence of the compounds. The 2009 Phoenix lander found that perchlorates make up between 0.4 and 0.6 percent of the soil sample it collected.

While perchlorates, which are composed of chlorine and oxygen, are toxic to humans, microbes typically love the stuff. And researchers have been optimistic that their presence could support bacterial life on Mars. As Kluger reports, some bacteria on Earth use naturally occurring perchlorate as an energy source. The compound also lowers the melting point of water, which could improve the chance of liquid water existing on the Red Planet.

But the latest study, published in the journal Scientific Reports, suggests that in the presence of ultraviolet light perchlorate is not so microbe-friendly. Mars has a thin atmosphere, which often leaves its surface bathed in UV rays. And when heated, chlorine-based molecules like perchlorates cause heavy damage to living cells, reports Sarah Fecht at Popular Science.

Researchers at the University of Edinburgh wanted to see just how much damage those perchlorates would cause to any Martian bacteria. So they exposed test tubes of a common bacteria, Bacillus subtilis, to conditions similar to ones they might encounter on Mars. They started with low temperatures and low oxygen in the presence of perchlorate. Bacteria under these conditions survived for up to an hour, Fecht reports. But when the researchers added UV light to the mix, the test tube was completely sterilized within 30 seconds. The researcher also found that two other common Martian soil components, iron oxide and hydrogen peroxide, reacted with irradiated perchlorate to make the soil hostile to bacteria.

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

That doesn’t completely rule out the possibility that bacteria may exist on Mars. “I can’t speak for life in the past,” co-author Jennifer Wadsworth tells Sample. “As far as present life, it doesn’t rule it out but probably means we should look for life underground where it’s shielded from the harsh radiation environment on the surface.” As Sample reports, the ExoMars rover, scheduled to launch in 2020, will test this idea, digging about 12 feet into the Martian soil to look for bacteria.

There still remains some hope for surface microbes. As Kluger reports, the researchers found that the colder temperatures offer some small protection for the bacteria. And the average temperature on Mars is -67 Fahrenheit. Also, the concentrations of perchlorate are not uniform, meaning there may be some pockets where life could exist.

It's also possible that hypothetical Martian bacteria could be much tougher than the common Bacillus subtilis. On Earth, researchers have found all types of extremophile organisms with the ability to survive under intense heat and pressure, in the presence of acid, without water and even inside rocks. “Life can survive very extreme environments,” Wadsworth tells Fecth. “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.”