The discovery of compounds found in the Martian soil show that they are turned into toxic bactericides by the ultraviolet light that bathes the planet, effectively sterilizing the upper layers of the landscape has wide-ranging implications for the hunt for alien life and suggests that missions will have to dig deep underground to find past or present life if it exists there. The most hospitable environment may lie two or three meters beneath the surface where the soil and any organisms are shielded from intense radiation.
NASA scientists theorized that alien bacteria could potentially use the perchlorates as an energy source, implying that perceived damp streaks spotted on Mars’s surface were some of the best bets for places to look for microbial life.
Wadsworth and colleague Charles S. Cockell of the University of Edinburgh looked at how a soil bacterium fared when it was mixed with perchlorates and then subjected to ultraviolet rays similar to those on Mars. Two other components of the Martian surface were added, iron oxides and hydrogen peroxide, and the result was rapid cell death.
“These data show that the combined effects of at least three components of the Martian surface, activated by surface photochemistry, render the present-day surface more uninhabitable than previously thought, and demonstrate the low probability of survival of biological contaminants released from robotic and human exploration missions.”
The new study was “a big step forward” in understanding the ramifications of finding high levels of perchlorate on Mars," said Chris McKay, a planetary scientist at Nasa Ames Research Center in California. "From a Mars exploration point of view, he said the results were both good and bad news. 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. “This should greatly reduce planetary protection concerns as well as any concerns about infection of astronauts,” he said. “But the bad news is that this means we have to dig to quite some depth to reach a biological record of early life that is not completely destroyed by the reactive UV-activated perchlorates.”
Working with Charles Cockell, an astrobiologist at Edinburgh, Wadsworth looked at what happened to Bacillus subtilis, a common soil bacterium and regular Earthly contaminant found on space probes, when it was mixed with magnesium perchlorate and blasted with ultraviolet rays similar to those witnessed on Mars. She found that the bugs were wiped out twice as fast when perchlorate was present. Other perchlorates found on Mars had a similar bactericidal effect.
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.”
The end result of the findings mean that damp streaks on the Martian surface may not be prime spots to find alien microbes. These briny patches would be likely to harbor perchlorates, making the streaks even more toxic than the surrounding soil.
“I can’t speak for life in the past,” said Wadsworth. “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.”
“This, combined with the solar and galactic particle radiation environment at the Martian surface, makes it all the more important to sample underneath the surface in the search for biomarkers,” said Andrew Coates, a planetary scientist at UCL who leads the ExoMars panoramic camera team.
“With the ExoMars rover, we will drill to retrieve and analyse samples from up to 2m under the surface,” he added. “This is important as a millimetre or two will get us below the harmful ultraviolet, one metre will get us below the oxidants such as perchlorates, and 1.5m gets us below the ionising radiation from the sun and galaxy.”