The moon used to be considered a rocky, bone-dry world. Although it was speculated to contain water for decades, it was only confirmed last year. But the exciting discovery prompted a new question: where did the water come from?
The Science Explorer reported back in October 2015 that the moon likely received its considerable reservoirs of water from asteroids, rather than from icy comets. A new study published in the journal Nature Communications has gathered even more evidence to support this idea.
In order to investigate the source of the moon’s water, an international team of scientists compared the isotopic ratios of hydrogen to deuterium, or “heavy hydrogen” from samples brought back by the Apollo missions, with the ratios found in comets and asteroids. They then calculated the amount of water and heavier elements that could have been delivered by either of those objects.
“[The hydrogen isotopes are] like a fingerprint or a barcode for where water may have come from in the solar system - these different types of objects have different hydrogen isotope compositions,” Jessica Barnes, a planetary scientist at the Open University, who co-authored the research, told The Guardian. Comets, which form far out in the solar system carry water rich in deuterium, while water formed closer to the sun has more hydrogen.
According to their results, more than 80 percent of the water found within the moon’s interior came from asteroids that have a similar composition to carbonaceous chondritic meteorites (rare type of stony meteorite which contains large amounts of the magnesium-rich minerals).
But to get into the moon's interior, the asteroid impacts would have had to happen while the moon was still surrounded by a magma ocean — before a crust formed — nearly 4.5 to 4.3 billion years ago. After these impacts, the formation of a crust over the magma prevented the water from escaping.
There are still a lot of uncertainties surrounding the results. Only two percent of the samples brought back by the Apollo missions have been examined for their water content, and every single one comes from the near-side of the moon. “We are completely restricted geographically about what we can understand about the entire planetary body,” explained Barnes.
Also, not everyone, including Alberto Saal of Brown University who has also conducted studies on the presence of water inside the moon, is convinced that the puzzle has been solved. Saal explained to The Guardian, “They are doing the best they can with the information that we have, but I think that this is not the last that we will hear about this.”
But new lunar missions could help solve the mystery. In all honesty, who wouldn’t want to journey to the dark side of the moon?
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