A couple of questions about water on other planets.

Previously published Slate articles made new.
Nov. 13 2009 3:13 PM

The Water From Another Planet

Should you drink it? Plus, why do we think that alien life would be water-based?

At least 24 gallons of water were discovered on the moon when NASA researchers slammed a satellite into a lunar crater last month. "We practically tasted it with the impact," said one member of the mission team on Friday. In a pair of "Explainer" columns first published in 2006, and excerpted below, Daniel Engber reviewed the significance of extraterrestrial water.

Liquid water is often seen as a precondition for extraterrestrial life. What makes us so sure?

Daniel Engber Daniel Engber

Daniel Engber is a columnist for Slate

Every living thing on Earth needs water to survive. That doesn't mean life on other planets would necessarily be based on liquid water, but it gives us one of our best clues as to what to look for. Since water works so well for us, we may as well focus our attention on planets or moons that have it, too.

AP video: Water on the moon

What makes water so useful? First of all, it serves as a substrate for all the chemical reactions you need to make a living thing. To get something as complicated as biology, you've got to have a system that allows a wide variety of molecules to interact in a wide variety of ways. Water, which is a polar molecule—i.e., it has both positively and negatively charged ends—acts as a "universal solvent." That means it can dissolve many chemicals—including the organic compounds that are the building blocks of life on Earth—and allow them to recombine or attach to one another in various arrangements.

It also helps that water remains liquid at a wide range of temperatures. That's important because solids are too rigid to allow for the necessary chemical reactions and gases aren't stable enough to maintain them. If you started to mix up ingredients for living things in a liquid that's not as stable as water, climate changes on your planet might send the whole experiment down the toilet.

A few years ago, signs of flowing water were discovered on the surface of Mars. 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.)

(Read more on why it might not be a good idea to drink the water on Mars.)

Got a question about today's news? Ask the Explainer.

TODAY IN SLATE

Medical Examiner

Here’s Where We Stand With Ebola

Even experienced international disaster responders are shocked at how bad it’s gotten.

It Is Very, Very Stupid to Compare Hope Solo to Ray Rice

The U.S. Is So, So Far Behind Europe on Clean Energy

Even if You Don’t Like Batman, You Might Like Gotham

Friends Was the Last Purely Pleasurable Sitcom

The Eye

This Whimsical Driverless Car Imagines Transportation in 2059

Politics

Meet the New Bosses

How the Republicans would run the Senate.

A Woman Who Escaped the Extreme Babymaking Christian Fundamentalism of Quiverfull

Nicolas Sarkozy, Thrice Married, Says Gay Marriage Humiliates the Family

  News & Politics
Politics
Sept. 22 2014 5:33 PM The Politics of Stigma Why are lighter-skinned Latinos and Asians more likely to vote Republican?
  Business
Moneybox
Sept. 22 2014 5:38 PM Apple Won't Shut Down Beats Music After All (But Will Probably Rename It)
  Life
Outward
Sept. 22 2014 4:45 PM Why Can’t the Census Count Gay Couples Accurately?
  Double X
Doublex
Sept. 22 2014 4:06 PM No, Women’s Soccer Does Not Have a Domestic Violence Problem Or, why it is very, very stupid to compare Hope Solo to Ray Rice.
  Slate Plus
Slate Plus
Sept. 22 2014 1:52 PM Tell Us What You Think About Slate Plus Help us improve our new membership program.
  Arts
Brow Beat
Sept. 22 2014 5:45 PM The University of California Corrects “Injustice” by Making Its Rich Chancellors Even Richer
  Technology
Future Tense
Sept. 22 2014 12:14 PM Family Court Rules That You Can Serve Someone With Legal Papers Over Facebook
  Health & Science
Medical Examiner
Sept. 22 2014 4:34 PM Here’s Where We Stand With Ebola Even experienced international disaster responders are shocked at how bad it’s gotten.
  Sports
Sports Nut
Sept. 18 2014 11:42 AM Grandmaster Clash One of the most amazing feats in chess history just happened, and no one noticed.