Water has flowed on Mars within the last few years, researchers announced Dec. 6. Images of Martian gullies show what appear to be the deposits left by little spurts of water as they trickled downhill. If an astronaut could bottle up some of this extraterrestrial water, could he drink it?
Probably not. At this point, it's impossible to know exactly which minerals are dissolved in the water, and in what concentrations. But the evidence from salt deposits elsewhere on the surface suggests that a Martian aquifer isn't exactly Poland Spring. There's a reasonable chance that any liquid you might find on the surface of the planet would be an acidic sludge of minerals.
The fact that there's any liquid water at all on the surface of Mars should be a warning sign. Under most conditions, the planet's thin and cold atmosphere would make water freeze or evaporate soon after it emerged from underground. The melting point of the water could be lowered—and this process could be delayed—if the fluid were particularly salty. (That's why we sprinkle salt on ice and snow in the winter.) Given that the Martian water stayed on the surface long enough to trickle into a gully, we might presume that it would be too salty to drink.
That's not the only reason to think the space water would be unpalatable. Mineral specimens collected by the Mars rovers (on other parts of the planet) have turned up significant quantities of jarosite, a salt deposit that tends to form only in very acidic solutions. Researchers guess that the jarosite precipitated from a liquid with a pH of between 0 and 4. For comparison, a human can handle an acidic beverage like Coca-Cola, which has a pH of about 2.4, but more concentrated acids will cause mucosal injuries to the esophagus, stomach, and intestines. (Anything with a pH of less than 5.5 can affect the surface of your teeth.)
Even if the Mars water weren't too salty or acidic for our canteens, it might still be laden with junk from underground. It's impossible to know whether the liquid emerged as a sludgy mess of minerals, or as a more dilute (and palatable) mixture. The difference could depend on the total volume of water that reached the surface—with more water being more dilute. But the deposit trails we see in the recent photos could have resulted from just the tiniest trickle of dirty liquid, enough to moisten up some sediment and carry it slowly down the hill.
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Explainer thanks Joel Hurowitz of the Jet Propulsion Laboratory and Nicholas Tosca of the State University of New York at Stony Brook.