It turns out the moon is a lot wetter than we ever thought.
When Apollo astronauts returned from the moon 40 years ago, they brought back souvenirs in the form of moon rocks to be used for scientific analysis, and one of the chief questions was whether there was water to be found in the lunar rocks and soils.
The problem was they faced was complicated by the fact that most of the rock boxes containing the lunar samples had leaked. This led the scientists to assume that the trace amounts of water they found came from Earth air that had entered the containers. The assumption remained that, outside of possible ice at the moon's poles, there was no water on the moon.
Forty years later, a team of scientists including Larry Taylor of the University of Tennessee, Knoxville, has found evidence that the old assumption may be wrong. To do so, they used a high-tech instrument on a satellite in orbit around the moon.
"To some extent, we were fooled," said Taylor, a distinguished professor of earth and planetary sciences, who has studied the moon since the original Apollo missions. "Since the boxes leaked, we just assumed the water we found was from contamination with terrestrial air."
The team of researchers used a NASA instrument called the Moon Mineralogy Mapper – M3 for short – housed on the Indian Chandrayyan-1 satellite, India's first lunar expedition, which was launched into orbit around the moon late last year.
M3 analyzes the way that light from the sun reflects off the lunar surface to understand what materials comprise the lunar soil. Light is reflected in different wavelengths off of different minerals, and scientists can use those differences – mostly imperceptible to the human eye – to know what is present in the thin layer of upper soil – so-called reflectance spectrometry.
In this case, the instrument detected wavelengths of reflected light that would indicate a chemical bond between hydrogen and oxygen. Given water's well-known chemical symbol, H2O, which represents two hydrogen atoms bonded to one oxygen atom, this discovery was a source of great interest to the researchers.
The instrument can only see the very uppermost layers of the lunar soil – perhaps to a few centimeters below the surface, but what it saw, according to the scientists, was water, previously theorized but not proven to exist only in permanently shadowed craters at the lunar poles. What scientists did not understand, though, was where this newly observed water came from.
There are potentially two types of water on the moon: exogenic, meaning water from outside sources, such as comets striking the moon's surface, and endogenic, meaning water that originates on the moon. Taylor and his colleagues suspect that the water they're seeing in the moon's surface is endogenic.
Since the rocks and soils that compose the moon contain about 45 percent oxygen, mostly combined in silicate phases, the question before researchers is where the hydrogen component of the water they're seeing with M3 came from. In this case, they believe it may have come from an astronomical phenomenon called the solar wind.
As the sun undergoes nuclear fusion, it constantly emits a stream of particles, mostly protons, which are positively charged hydrogen atoms. On Earth, the atmosphere and magnetism prevent us from being bombarded by these protons, but the moon lacks that protection, meaning the oxygen-rich minerals and glasses on the surface of the moon are constantly pounded by hydrogen in the form of protons, moving at velocities of one-third the speed of light.
When those protons hit the lunar surface with enough force, suspects Taylor, they break apart oxygen bonds in soil materials, and where free oxygen and hydrogen are together, there's a high chance that trace amounts of water will be formed. These traces are thought to be about a quart of water per ton of soil.
"The isotopes of oxygen that exist on the moon are the same as those that exist on Earth, so it was difficult if not impossible to tell the difference between water from the moon and water from Earth," said Taylor. "Since the early soil samples only had trace amounts of water, it was easy to make the mistake of attributing it to contamination."
Taylor and other M3 team members believe their findings will be of particular significance as mankind continues to plan for a return to the moon. The maps created by M3 could provide mission planners with locations prime for extraction of needed water from the lunar soil.
The M3 team, made up of scientists from the U.S. and India, reported its findings in this week's edition of the online journal Science Express. The team, funded by NASA, is led by researchers at Brown University, which collaborates with Taylor and UT Knoxville's Planetary Geosciences Institute.
Contacts:
Larry Taylor, lataylor@utk.edu, (865) 974-6013
Charles Primm, charles.primm@tennessee.edu, (865) 974-5180