There are certain areas on Mars where we don鈥檛 dare tread. NASA forbids spacecraft from visiting spots that possibly host liquid water where life might be able to thrive, for fear of contaminating Mars with Earth microbes. But an analysis of the salty liquids on Mars suggests we needn鈥檛 worry, because life as we know it would be unable to聽exist anywhere on the Martian surface.
Edgard Rivera-Valent铆n at the Lunar and Planetary Institute in Texas and his colleagues used maps of the temperature and relative humidity across Mars to map the presence of salty water 鈥� any water on the Martian surface is likely to be salty, simply because the surface itself is. This聽improves the chances of water being liquid, because salt lowers the freezing point of water.
鈥淵ou can think about it as if you throw salt on ice on the sidewalk,鈥� says Danielle Nuding at NASA鈥檚 Jet Propulsion Laboratory in California. 鈥淚t鈥檚 the same chemistry happening.鈥�
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Unfortunately, the saltier the water, the less likely anything can survive it. The researchers found that even though there could be briny water on the surface of Mars up to 18 per cent of the year depending on the season, no microbe we have ever seen on Earth would be able to reproduce there.
鈥淟ife as we know it is not going to find these brines and survive, because it鈥檚 either going to be way too cold or way too salty,鈥� says Rivera-Valent铆n, who presented the results on 18 March at the Lunar and Planetary Science Conference in Texas.
That doesn鈥檛 mean we can鈥檛 contaminate Mars: brines with different types of salts mixed together might be friendlier to life, and temperatures just below the surface are much less extreme.
Nevertheless, as long as we don鈥檛 dig down, it might be impossible or at least highly unlikely for rovers to contaminate Mars. 鈥淭he level of sterilisation that we鈥檝e done with Curiosity should be good enough to ignore what we鈥檝e been calling 鈥榮pecial regions鈥� until now,鈥� says Jennifer Hanley at Lowell Observatory in Arizona. 鈥淚 think that we鈥檙e OK to go.鈥�
Visiting these regions would be particularly helpful because, while they are in theory the most vulnerable areas on Mars, they are also the most interesting. For example, arguments have been raging for over a decade about whether dark streaks on Martian slopes called recurring slope lineae are flowing water or just dust. A quick visit by the Curiosity rover, which is near an area where these flows often form, could solve it once and for all.
Even if these wet areas aren鈥檛 hospitable to Earth life, they could still be home to native Martian life forms. 鈥淚f you had life that originated on Mars when it was more habitable, it could be that as Mars changed life could have gradually adapted to the new, more extreme conditions,鈥� says Rivera-Valent铆n.
