for National Geographic News
For many, 2009 will be remembered as the year water on the moon was confirmed beyond any reasonable doubt.
"You're seeing the culmination of a whole bunch of missions that were instrumented specifically to address this question," said Paul Spudis of the NASA-funded Lunar and Planetary Institute (LPI) in Houston, Texas.
Earlier this year, NASA's Lunar Reconnaissance Orbiter and India's Chandrayaan-1 spacecraft detected possible chemical traces of lunar water. And just last week NASA announced that their LCROSS moon crashes had kicked up "significant" amounts of water from a crater.
But where did the moon's water come from?
"Was it deposited in a single big event that was recent? Or is this stuff that has been around for billions of years?" said Peter Schultz, an LCROSS scientist at Brown University in Rhode Island. "We don't know."
Right now there are three major scientific theories of how the moon got its water—and a "wildly speculative" fourth idea that can't be ruled out just yet.
Ancient Volcanoes Pushed Moon's Water to Surface
The moon's water was there from the start, one theory goes—water was an ingredient in the moon's creation, as it was for Earth's.
According to this idea, the water is concentrated in the moon's interior. In the distant past, when the now "dead" moon had a hot core, volcanic eruptions or gaseous "belches" slowly pushed water to the surface, where it's been frozen ever since, said LPI's Spudis, explaining the theory.
Video: Moon 101
Water is "Home Brewed" on the Surface
Lunar water could be home-brewed, with some help from the sun, some scientists speculate.
The sun constantly emits a stream of particles called solar wind. Positively charged hydrogen ions, or protons, in the solar wind may strike the moon and interact with oxygen-rich minerals in lunar soil to form H2O, aka water, according to this theory.
Forming water via solar wind would be a slow process, Brown University's Schultz said. But "even if you're only accumulating a molecule [of water] a day this way, over billions of years you can do a lot of things."
Comets and Asteroids Delivered Water to the Moon
Some say the moon's water may be a gift from water-bearing comets and wet asteroids that struck the moon in the distant past. (Related: "Comet Swarm Delivered Earth's Oceans?")
Most of the water from such an impact would have been ejected into space, but some sluggish molecules could have been captured by the moon's gravity.
"The idea is that comets or water-bearing asteroids hit the moon and create a cloud of water vapor that hangs around the vicinity of the moon's surface," LPI's Spudis said.
"Some of the water eventually migrates to the polar areas, where it might find its way into a cold trap"—a permanently frigid area, such as a polar crater where sunlight never reaches.
A cold trap is too chilly to allow ice to sublimate—turn directly into gas—and the airless moon is inhospitable to liquid water. As a result, the water would theoretically remain frozen for eons.
The Moon's Water Came From Earth
There are two ways Earth water could have ended up on the moon, and both would have been possible only when Earth and the moon were much closer, billions of years ago, Brown's Schultz said.
For starters, during prehistoric periods when Earth's magnetic field was absent or weak, solar wind could have stripped water vapor from our planet's atmosphere and deposited it on the moon.
Or perhaps catastrophic asteroid or comet impacts on Earth ejected seawater into space, and the orbiting moon passed through the vapor cloud, emerging somewhat soggier.
Both of these two scenarios are theoretically possible, though Schultz admits, "we're in speculation land." But then, that's exactly where lunar water lived until a few short days ago.
SOURCES AND RELATED WEB SITES