Scientists studying a mineral often used to estimate how much water exists in the interior of the Moon may have been deceived into overestimating the amount, researchers say.
Scientists have long assumed the amount of hydrogen present in a mineral known as apatite can be taken as an indicator of the total water content of the moon, but a new study at UCLA suggests that's not the case.
The research suggests hydrogen-rich crystals of apatite seen in many rock samples from the moon could have formed in environments not as rich with water as generally considered.
"The mineral apatite is the most widely used method for estimating the amount of water in lunar rocks, but it cannot be trusted," says Professor Jeremy Boyce, lead researcher of UCLA Department of Earth, Planetary and Space Sciences. "Our new results show that there is not as much water in lunar magma as apatite would have us believe."
A 2010 finding of apatite with high hydrogen levels inside lunar rocks led scientist to reconsider the long-held belief that our moon has almost no water in its makeup and they began to search for more evidence of water in the moon's past.
Analysis of a small amount of apatite was considered useful in estimating the water content level of a large percentage of lunar magma, they thought, but Boyce, in his study, suggests apatite may be deceptive as an indicator.
Its water content may be the result of a peculiarity in the process of crystallization and not because of the water content of its surrounding environment, Boyce says.
Apatite forming during the cooling of lunar magma can incorporate hydrogen in its crystal structure, but only if chlorine and fluorine, apatite's preferred crystal building blocks, have been exhausted.
"Early-forming apatite is so fluorine-rich that it vacuums all the fluorine out of the magma, followed by chlorine," Boyce says. "Apatite that forms later doesn't see any fluorine or chlorine and becomes hydrogen-rich because it has no choice."
As a result, much of the apatite in lunar rocks does not accurately mirror the water content in the original magma.
While most lunar samples are actually dry and mostly devoid of water, apatite crystals rich in hydrogen present in many moon rock has been something of a scientific paradox.
"We had 40 years of believing in a dry moon, and now we have some evidence that the old dry model of the moon wasn't perfect," Boyce says. "However, we need to be cautious and look carefully at each piece of evidence before we decide that rocks on the moon are as wet as those on Earth."