For decades, the moon was thought to be mostly dry, particularly inside of it, since the moon is believed to have formed when a body the size of Mars (or larger) slammed into Earth. That collision would have released so much energy that both the Earth and the Moon would have melted to depths of hundreds of kilometers, forming what we call magma oceans (picture an entire planet/moon covered completely by magma). After that impact, the bodies would have been so hot that it is hard to explain how either the Earth or Moon could have held onto water…but this was particularly troublesome for the Moon, since it has less gravity, making it easy to lose the water to space
In 2008, the research group from Brown discovered that, yes, despite the violence of the Moon’s formation; the rocks returned by the Apollo astronauts did contain water.
One explanation for the presence of this water could be that it came in late. Just like today, there were comets throughout the early solar system, and those comets could have delivered water after the Moon formed, after the magma ocean.
Well, it turns out there is a way to test that, and it uses a fairly famous rock. A rock from Apollo 15 is an “anorthosite”. It’s made almost entirely of the mineral anorthite, a type of plagioclase. This rock is special because it probably formed directly from the Lunar Magma Ocean, from plagioclase that floated to the surface of the moon. Basically, it’s the oldest rock we have from the moon, leading to the nickname “The Genesis Rock”. If the water came in late, that rock should still be dry, even if every other rock from the moon isn’t.
Just published in Nature Geoscience this week, a group of researchers at the University of Michigan investigated this rock, and found that yes, in fact, it contains water. Not only that, it has a surprisingly high amount of water…enough water that the Lunar mantle must have had a comparable amount of water to Earth’s mantle when this rock formed.
So, the water just keeps being there. We thought for a long time that the moon’s interior had to be dry since its formation was so violent…but every time we look at the rocks, we find a different story. How the water came to the Moon, and to the other solid bodies in the solar system, is a bigger mystery now than it was just a couple weeks ago.
Image copyright: NASA/Johnson Space Center