Throughout the Apollo missions, NASA believed there was no water on the Moon.

In the nearly one thousand pounds of rock and soil samples that these missions brought back, they did find traces of water. But they assumed these were contamination from gathering and transport.

Then in 2008, the Indian space agency launched a Moon orbiter carrying NASA’s Moon Mineralogy Mapper—or M3—which was designed to search for minerals, like water.

The tilt of the Moon’s axis prevents sunlight from reaching into some lunar depressions.

M3 used infrared light absorption to discover ice hidden in the dark corners of these craters— especially near the frigid lunar poles.

But the Moon’s atmosphere is too thin to keep water from escaping into space. As it dissipates, how is it being replaced?

Scientists realized that solar wind bathes the Moon in hydrogen ions for three weeks of its four-week orbit around Earth. The hydrogen reacts with oxygen compounds in Moon rocks to form water.

During the one week of full moon, when the Moon is hidden behind Earth, it’s also shielded from the solar wind.

At that time, Earth wind—the tail of our magnetosphere, which is deformed by solar wind into a teardrop that stretches toward the Moon— also bathes it in hydrogen and oxygen.

Without these constant supplies of the building blocks of water, from both Sun and Earth, the Moon would, in fact, be totally dry.