3 Sources of Water on the Moon

Is there water on the Moon? The obvious answer seems to be no. Because during a day on the Moon, which lasts 2 Earth weeks, the temperature on the lunar surface gets up to 123 degrees Celsius, which would be enough to boil away any water if the Moon had an atmosphere like Earth, which it doesn't.

And the reason that it doesn't is because the Moon's gravity is weak, just a sixth of Earth's gravity, which means it can't hold onto light fast-moving molecules like water. Anything going faster than 2.4 kilometers per second—the Moon's escape velocity—is gone. But that might sound pretty quick. However, at any given temperature, there are always a small number of molecules going significantly faster than the average. On the Moon, these molecules flap into space, and then other molecules take their place until all the light fast-moving molecules are gone.

Plus, since the Moon has a very weak magnetic field, there's little to protect the lunar surface from the solar wind—that's the stream of charged particles coming from the Sun. These particles can effectively blast molecules from the surface of the Moon into space. In fact, that's how Mars lost most of its atmosphere.

So in short, if the Moon ever had any water, it should be long gone by now. And this appeared to be confirmed by the Moon landings. Rock samples brought back by Apollo astronauts contained virtually no hydrated minerals. Some water was detected, but it was assumed just to be terrestrial contamination rather than actual Moon water, because seals on some of the sample-return containers were damaged by the gritty moondust.

But in 2009, a very different picture emerged when scientists intentionally crashed part of a rocket into the surface of the Moon. It was going two point five kilometers per second on impact, making a crater 25 meters wide and 4 meters deep. That injected 10,000 tons of material, with half of it making high enough to be lit by the Sun. That allowed cameras on an orbiting spacecraft to look for the emission lines of water in the dust.

And what did they find? The water emission lines were bright. Scientists estimated that five point six percent of the mass of the objective debris was water. But how is this possible? Where did the water come from, and how hadn't it remained on the Moon?

Well, strangely enough, there are actually three different sources of lunar water. To understand the first source, you just have to look at where they crashed that rocket into the Moon. Now, it's a misconception that the Moon has a dark side. Although the same side always faces Earth, all sides of the Moon do receive sunlight. However, at the poles, there are some craters that, due to their location and depth, are always in shade. They're called permanently shadowed regions.

Now, since they never see the Sun, these craters are some of the coldest places in the whole solar system—colder than the surface of Pluto. They can reach negative 249 Celsius. And it is here that scientists decided to crash that piece of rocket. Since the permanently shadowed regions maintain such a consistently low temperature, they act as cold traps—places where volatiles like water remain frozen as ice.

But where did the water come from in the first place? Well, from water-containing comets and asteroids that have struck the Moon over time. Any water molecules that found their way to the permanently shadowed regions would be trapped there—forever as ice.

But this is not the only source of water on the Moon. In lunar samples from Apollo 15 and 17, scientists found tiny glass beads. These beads were formed back when the Moon was still volcanically active. Fire fountains launched droplets of lava into space, where they cooled rapidly and then fell back to the surface. In 2007, these beads were sliced in half and tested for water.

The results showed significant amounts of water, with concentrations highest in the middle of the beads, and this indicates that the water must have been trapped there during the initial eruption rather than leaching in due to contamination once those beads were back on Earth. Now, this water came from the interior of the Moon, so the Moon must have contained water to begin with, which kind of makes sense, considering the Moon is likely a piece of Earth that broke off billions of years ago in an impact with a Mars-sized body.

The third and final source of water on the Moon is water created on the Moon’s surface from the reaction of oxygen and hydrogen. Now, the source of oxygen is easy. Oxygen makes up 45 percent of the lunar surface, mostly bound in oxide minerals. But where do you get the free hydrogen? The answer is in the solar wind, which is constantly bombarding the Moon's surface with protons.

These can react with oxygen to form hydroxide, and ultimately water. This water can then drift over the surface of the Moon until it either flies off into space or lands in the cold traps and becomes ice. So there is water on the Moon.

And this is important because it means that future missions can take advantage of this valuable resource, not only for Moon colonists to drink and grow crops, but also for processing into rocket fuel. If you can make your own rocket fuel on the Moon, that dramatically reduces the challenges and costs of space travel.

For a long time, we used to think the Moon was completely dry. But now we know that hypothesis is all wet.

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