New data from an Indian probe strongly suggests there are massive chunks of water ice hidden in some of the moon's perennially shadowed polar craters.
The source of the six to 10-foot (one to three-meter) thick ice blocks may be comets that smashed into the moon eons ago, as well as an otherworldly lunar water cycle.
If the measurements are correct, the ice found just in the north pole area amounts to perhaps 600 million cubic meters. That's about two-thirds of a cubic kilometer of water -- an ample supply for spacecraft to use as fuel, should humans ever return to the moon and set up shop, according to Paul Spudis of the Lunar and Planetary Institute, Houston.
"The amounts are incredible," said Spudis, lead author on a paper in the latest issue of Geophysical Research Letters on the discovery.
One thing that appears to make the water ice possible is the surprisingly low temperatures in these forever sunless craters. Some are only 25 to 30 degrees Kelvin (-415 to -406 Fahrenheit), which makes it impossible for any water trapped in these places to escape.
"They are extremely cold," said Spudis. "Colder than the surface of Pluto."
The new measurements were made by the miniature synthetic aperture radar instrument on board the Indian Chandrayaan 1 spacecraft, which mapped most of the area near the north pole of the moon between February and April of last year.
As for where all that thick ice came from, the patchy way it's distributed suggests that at least some of it is from comets that smacked into the moon, said Spudis. That contrasts with the other way water is thought to possibly accumulate more evenly there: by way of a steady flow of hydrogen atoms from the solar wind.
"It can be shown on a time scale of 100,000 years the (lunar) surface becomes saturated with hydrogen," said Martin Wieser of the Swedish Institute of Space Physics in Kiruna. That hydrogen has been detected in lunar soils by other instruments as either water or oxygen-hydrogen molecules.
Over eons it's conceivable that those molecules waft around the moon until they get trapped in the frigid craters. In other words, the moon has a water cycle, of sorts.
"No one expected that the moon had a water exosphere," said Spudis, referring to the extremely tenuous gases that envelope the moon. Put that water cycle together with the ice blocks and the larger story of lunar water begins to take shape.
"It suggests that on top of this background (solar wind fed hydrogen) signal there are episodic (comet impact) signals" to make the patchy blocks, said Spudis.
The discovery also points to some very specific places to send a robotic probe to find out more, said Spudis.
"In a way it's easier because we know exactly where to go," Spudis said.
A larger message, however, is that this discovery underscores just how little we know about our nearest neighbor in space, despite having already sent humans there.
"It's a very interesting place," agreed Wieser. "There's still a lot to learn."