Hopes of finding life on Mars, at least on the surface, were dealt a blow Thursday by a study revealing that salt minerals present on the Red Planet kill bacteria. The most hospitable environment may lie two or three metres beneath the surface where the soil and any organisms are shielded from intense radiation.
One way to turn a perchlorate into an oxidant is to expose it to UV light, and since Mars has a much thinner atmosphere than Earth, there's plenty of this kind of radiation on its surface.
In lab tests on Earth, the compounds known as perchlorates killed cultures of the bacteria Bacillus subtilis, a basic life form, a research duo from the University of Edinburgh's School of Physics and Astronomy reported.
"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 who was not involved with the study, tells Popular Science. She found that the bugs were wiped out twice as fast when perchlorate was present.
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Further tests found that the UV rays broke down the perchlorate into other chemicals, namely hypochlorite and chlorite, and it is these that appear to be so destructive to the bacteria. Writing in Scientific Reports, the researchers say that the inhospitable conditions on Mars are caused by a "toxic cocktail of oxidants, iron oxides, perchlorates and UV irradiation".
However, the findings also demonstrate how uninhabitable the surface of Mars could be. "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". On the plus side, it means that any microbes that hitch a ride on landers sent to Mars will be swiftly destroyed on the surface, alleviating concerns about contaminating a potentially inhabited planet. From a Mars exploration point of view, he said the results were both good and bad news.
The European Space Agency's ExoMars rover is set to launch in 2020 and is equipped with a drill capable of digging several feet below the surface, where there are possibly signs of life.
Additionally, the fact that the surface of the planet was hostile to life doesn't eliminate the possibility that subterranean life may have thrived on Mars, including the possibility of marine life.