The possibility of water on the moon has excited scientists and science fiction fans for decades. If we ever decide to maintain a human presence on the moon, clear evidence of water will be an important factor in the decision.
In recent years, that evidence has begun to mount. The data comes from several sources. First there was the pioneering Clementine mission in 1994, America’s first return to the moon in twenty years.
Clementine looked for water by bouncing radio waves off the surface–the returns giving a strong indication that water ice must lie beneath the surface.
Then there was the Lunar Prospector which found a signature for water by measuring the amount of neutrons emitted from the surface and which water ought to absorb).
Then there was Galileo’s flyby of the moon on its way to Jupiter, which also found evidence and more recently, the Indian spacecraft Chandrayaan-I in 2009 which used an infrared camera to spot evidence of water in lunar rocks.
All this has dramatically overturned the previous view that the moon was dry as a bone.
But an interesting question is how this view came to be. After all, there is no shortage of moon rocks on Earth–the Apollo missions brought back some 300 kilograms of the stuff, so much that NASA has lost track of much of it.
Today, Arlin Crotts at Columbia University in New York city throws some fascinating light on this question in a series of three articles about water on the moon and how it got there.
He points out that scientists believed that the Apollo samples were contaminated after their return to Earth.Apparently, the containers used to carry them could not be tightly closed because lunar dust clogged their seals. So any water found in these rocks was thought to have originated here.
What’s more, the Apollo missions confirmed beyond doubt that the river-like channels that earlier spacecraft had seen on the lunar surface were made by flowing lava rather than water. So the prevailing view was that there was that the moon was dry.
However, the Soviets had other ideas. Crotts has unearthed evidence that the Soviets found good evidence of water in moon rocks in the 1970s.
One of the least known missions is the Soviet Luna-24 sample-return mission which landed on the lunar surface in August 1976. This drilled some 2 metres into the lunar surface, extracted 300 grammes of rock and then returned to Earth. An impressive feat by any standards but one that has been largely forgotten in the west.
A Soviet team analysed the sample and found unambiguous signs of water in the rock–they reported that water made up 0.1 per cent of the sample’s mass. In 1978, they published the result in the Russian journal Geokhimiia. This journal also has an in English language version but it was not widely read in the West.
Crott says that today the work has been almost entirely forgotten. “No other author has ever cited the Luna 24 work,” he says.
Curiously, various scientists including the Nobel prize winning chemist Harold Urey, had predicted since the 1950s that water ice and other volatiles ought to be found in craters at the lunar poles, which are permanently in shadow.
Crott goes on to detail a number of other fascinating efforts to find water on the moon, including the famous impact experiment in which NASA slammed an empty rocket stage into one of these shadowy craters to see what the ejecta plume would look like. Sure enough, it contained plenty of water but lots of other stuff too including almost as much carbon monoxide as water.
Today, the idea of a dry moon has been completely overturned. “As recently as 2006 the settled value for the lunar bulk water content was below 1 part per billion. Most values now discussed well exceed 1 part per million,” says Crotts
That’s a remarkable turnaround but one that might have come a little sooner had the Soviet result been taken a little more seriously.