The water locked underneath the mantle of the Moon came from the same place that likely supplied the Earth’s oceans: the asteroid belt near Jupiter.
New research from a team of researchers led by Brown University geochemist Alberto Saal and published online Thursday by the journal Science indicates that water has been around on the Moon since it was first formed.
Until 2008, most scientists had believed the Moon was bone-dry, thanks to the event that formed the satellite: a cataclysmic impact about 4.5 billions years ago between a still-forming Earth and a Mars-size planet.
“Clearly such [an] event was so intense that all the hydrogen was believed to be lost from the molten material that finally accreted to form the Moon,” Saal said in an email.
Hydrogen was thought to be especially likely to be lost during the Moon-formation event, since it’s the lightest element. And no hydrogen means no water.
But Saal and his team have been chipping away at the assumption of a dehydrated Moon. They scrutinized samples brought back from the moon by the Apollo missions. In 2008, they presented evidence in the journal Nature that there was hydrogen dissolved inside pieces of lunar volcanic glass that are trapped inside greenish mineral crystals called olivine.
Then, Saal said, in a 2011 Science paper, the team showed “irrefutable” evidence that the material that formed lunar lava early in the Moon’s history contained an amount of hydrogen similar to the basalts that formed on the ocean floors of Earth.
Now, the new paper draws a link between Earth’s hydrogen and the Moon’s hydrogen. If the residue of water on the Moon and the oceans of Earth came from the same source, there should be distinct chemical fingerprints linking the Moon samples to stuff on Earth. In this case, the "fingerprints" the researchers were looking for on the lunar samples were the levels of an isotope called deuterium.
Deuterium is almost identical to hydrogen, except for the fact that its atomic nucleus carries one extra neutron. The amount of deuterium in a water molecule tends to vary depending on where in the solar system it formed. Closer to the sun, you have less deuterium; farther out, you have more.
The Moon samples had a low deuterium-to-hydrogen ratio that is similar to the ratio observed in a type of meteorite hailing from near Jupiter called carbonaceous chondrite. This points to a source of lunar water that originates from these meteorites, not comets, as one alternative explanation had it.
Other research has shown that Earth’s water has a deuterium-to-hydrogen ratio similar to the same class of meteorites, so it seems likely -- though not yet certain -- that the water came from the same place.
"The simplest explanation for what we found is that there was water on the proto-Earth at the time of the giant impact," Saal said in a statement on Thursday. "Some of that water survived the impact, and that's what we see in the Moon."
SOURCE: Saal et al. “Hydrogen Isotopse in Lunar Volcanic Glasses and Melt Inclusions Reveal a Carbonaceous Chondrite Heritage.” Science published online 9 May 2013.