Have you ever wondered where all the Earth's water came from?
The inner part of the solar system is dry, but there’s lots of ice in the outer solar system, so it could have come when objects from the outer solar system hit Earth. In 2019 three German scientists reported that they put this idea to the test by studying the distribution of molybdenum in the Earth’s crust.
Molybdenum is a chemical element, and one of the heavy ones. Because it’s heavy, any molybdenum that came to Earth early in its formation, when it was still molten, would have sunk to the core. Molybdenum that came to Earth later, after the planet had partially solidified, would have stayed in the mantle. The researchers think that water came to the Earth in these same late impacts.
So, by studying molybdenum in rocks from the Earth’s mantle, researchers might learn where the water-bearing impacts came from.
Material from the outer solar system has a different ‘fingerprint’ of molybdenum isotopes than inner solar system material. The scientists found that the object that carried molybdenum and water to Earth probably came from the outer solar system, and that the impact was probably the one that formed Earth’s moon.
Planetary scientists think that the moon formed after a Mars-sized object called Theia hit Earth, hurtling material into space, which coalesced to form the moon. So, the German researchers think that Theia came from the outer solar system, and delivered most of Earth’s water, while also creating the moon.