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?
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.
(Read more on why alien life forms are likely to be water-based.)
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.