The Moon is even drier than astronomers believed, based on new findings. Earlier studies of a rare mineral may have caused scientists to overestimate the amount of water present on our celestial companion.
Apatite, a group of minerals rich in hydrogen, often forms in the presence of water on Earth.
Jeremy Boyce of the University of California (UCLA) led an investigation of how apatite is created. Computer models were developed to model the creation of crystals of apatite in cooling magma, during the early days of the Moon.
Simulations suggested apatite may have gathered hydrogen from other sources. Earlier theories to explain the presence of the material suggested the crystals may have formed in an environment containing molecules of water, embedded in rock or ice.
Apatite is believed to be an indicator of water on alien landscapes. This new study shows the mineral may not be good evidence of a water-rich past.
"The mineral apatite is the most widely used method for estimating the amount of water in lunar rocks, but it cannot be trusted. Our new results show that there is not as much water in lunar magma as apatite would have us believe," Boyce said.
Astronomers once believed the Moon was bone-dry, nearly barren of any sources of water. In 2010, deposits of apatite were found in the lunar surface, which astronomers declared to be evidence of water deposits in the ancient past. Computer simulations suggest the material could have been formed without the need for water.
As crystals of apatite grow, they want to bond with chlorine and fluorine. If those elements are not present, the mineral will take hydrogen from the environment.
Earlier theories for the origin of lunar apatite suggested water was the source of these hydrogen atoms. The 2010 findings reported the lunar surface contained water at a concentration of up to one part in ten million.
"Early-forming apatite is so fluorine-rich that it vacuums all the fluorine out of the magma, followed by chlorine. Apatite that forms later doesn't see any fluorine or chlorine and becomes hydrogen-rich because it has no choice," Boyce said.
The first findings of apatite on the lunar surface were found in samples returned from the Apollo missions to the Moon. Boyce's research may show the apatite in the lunar surface was newer, hydrogen-rich apatite which formed after the depletion of chlorine and fluorine.
Research into the development of apatite on the Moon was published in the journal Science.
