We're always looking for traces of water on other planets, but is there water on our own lunar satellite?

The obvious answer would be no. The Moon has virtually no atmosphere and the temperature on lunar surfaces can get up to 123 degrees Celsius (253 Fahrenheit) during a lunar day - high enough to boil away any water. And seeing as one lunar day equals roughly two Earth weeks, it makes sense that, for decades, scientists assumed the Moon was completely dry.

But in 2009, a very different picture emerged. As Derek Muller explains in the latest episode of Veritasium, we now know that the Moon does contain water. 

Samples collected during the Apollo missions did actually contain traces of water, but seals on the containers were damaged by lunar dust, and it was always assumed to be terrestrial contamination rather than actual water from the Moon. 

But in 2009, researchers intentionally crashed part of a rocket onto the surface of the Moon, making an impact crater 25 metres wide and 4 metres deep, and blasting 10,000 tonnes of material into space.

Half of that ejected material made it high enough to be lit by the Sun, which allowed cameras on an orbiting spacecraft to look for water emission lines.

Surprisingly, they showed that 5.6 percent of the ejected mass was actually water.

So how is this possible?

To answer that, you just need to look at where the rocket crashed - in one of the permanently shaded craters near the Moon's poles.

As Derek explains above, these regions are some of the coldest places in the Solar System, colder even than the surface of Pluto. They can reach –249 degrees Celsius (–416 degrees Fahrenheit), which means that they're cold traps where water will remain frozen in ice.

But where does the water come from in the first place? There are actually three sources of water on the Moon.

Firstly, water-containing comets and asteroids that have struck the Moon over time. Then there's water that came from the Moon's core back when it was still volcanic, and became trapped in tiny glass beads.

And finally, water is actually being created on the Moon's surface from the reaction between oxygen and hydrogen.

Oxygen makes up 45 percent of the lunar surface, mostly bound in oxide minerals, and the free hydrogen is supplied by the solar winds that are constantly bombarding the Moon's surface.

So scientists now know that our lunar satellite is far from dry. But what does that mean for the future of space exploration? We'll let Derek explain that in the video above. Let's just say, Moon water is going to be very valuable one day.

If you want more science videos, Derek's also launched a new channel you should check out, called Sciencium.