The question of where Earth's large amount of water came from may find its answer by looking to the Moon. According to research by planetologists at the University of Münster, a collision with a Mars-sized planetoid that formed the Moon 4.4 billion years ago brought with it almost all the water that makes our planet habitable, as well as creating our relatively large satellite that stabilized the planet's axis.

Earth is a highly unusual planet, if not, to our current knowledge, unique. Aside from the fact that it's the only one we know of that supports not just life, but intelligent life, it also has many odd physical properties. Among these are that three quarters of the Earth's surface is covered with water that is liquid and that it has a moon that is so relatively large that it has major effects on the planet's habitability.

According to the most recent theories, the Moon was formed when the primordial Earth collided with a planetoid called Theia that was about the size of Mars. This impact not only supplied the material that became the Moon, but the Münster study indicates that it also brought large amounts of Earth's water.

What makes this conclusion interesting is that the Earth is in the inner solar system, which is very dry. Earth has lots of water, but Mercury, Venus, and Mars have almost none. The wetter part is the outer solar system with whole worlds that are mostly ice, and it's from this region that the carbonaceous meteorites come from.

These are meteorites that are made up of silicates instead of nickel-iron, and contain organic molecules, as well as being composed of up to 20 percent water. According to the Münster team, it's thought that these carbonaceous meteorites brought water to Earth. The question was, how? The answer lay in looking at the isotope ratios, which are a powerful tool for finding the origin of a material.

"We have used molybdenum isotopes to answer this question," says Gerrit Budde of the Institute of Planetology in Münster. "The molybdenum isotopes allow us to clearly distinguish carbonaceous and non-carbonaceous material, and as such represent a 'genetic fingerprint' of material from the outer and inner solar system."

The isotopic signature of material from the inner and outer solar system differ significantly. However, the Münster team found that the molybdenum isotopic composition of the Earth is halfway between the two groups, indicating that a lot of material in the Earth's crust and mantle came from beyond the orbit of Mars, while the core material seems to be from the inner solar system. In addition, this phenomenon showed up relatively late in the Earth's formation, which explains the different concentrations.

If this is the case, the team concluded, then the mantle molybdenum arrived during the collision with Theia 4.4 billion years ago. This, in turn, means that Theia had to come from the outer solar system rather than the inner region, and that it brought almost all of Earth's water as well as creating the Moon.

"Our approach is unique because, for the first time, it allows us to associate the origin of water on Earth with the formation of the Moon," says Thorsten Kleine, Professor of Planetology at Münster. "To put it simply, without the Moon there probably would be no life on Earth."

The research was published in Nature Astronomy.