There are certain areas on Mars where we don't dare tread. NASA forbids spacecraft from visiting spots that possibly host liquid water, and so where life might be able to thrive, for fear of contaminating (感染) Mars with Earth microbes. But an analysis of the salty liquids on Mars suggests we needn't worry, because life as we know it should be unable to exist anywhere on the planet's surface.
Edgard RiveraValentin at the Lunar and Planetary Institutein Texas and his colleagues used readings of the temperature and relative humidity across Mars to map the presence of salty water. Any water on the surface is likely to be salty, simply because the surface is. This boosts the chances of water being liquid because salt lowers its freezing point.
It is like when you throw salt on an icy sidewalk, says Danielle Nuding at NASA's Jet Propulsion Laboratory in California. "It's the same chemistry happening. "
Unfortunately, the saltier the water, the less likely anything can survive in it. The team found that even though there could be briny (盐分多的) 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.
Life as we know it is not going to find these brines and survive because it's either going to be way too cold or way too salty," says RiveraValentin, who presented the results at the Lunar and Planetary Science Conference in Texas.
That doesn't mean we can't 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't dig down, it might be highly unlikely or even impossible for rovers (飞行器) such as Curiosity to contaminate Mars.
"The level of sterilization (杀菌) that we've done with Curiosity should be good enough to ignore the ban on visiting what we've been calling special regions until now, says Jennifer Hanley at Lowell Observatory in Arizona. I think that we're OK to go. 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 Curiosity, which is near an area where the flows often form, could solve it once and for all.
Even if areas with water are inhospitable to Earth life, they could still be home to native Martian life forms.
"If 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," says RiveraValentin.
