Mapping the interiors of meteorites to learn how the Earth got its water

Mapping the interiors of meteorites to learn how the Earth got its water

Earth is a strange world. A hot, rocky planet covered in oceans of liquid water. This weirdness is central to life on Earth, but it’s a long-standing puzzle for astronomers. Why is our planet wet while other terrestrial worlds are dry? Where does all the water on Earth come from?

By itself, water is an extremely common molecule. It can be found throughout the universe, and it is abundant on most small worlds, usually in the form of ice. Even liquid water is common. The oceans of the Jovian moons Europa and Ganymede are larger than those of Earth. What makes Earth unusual is that it has so much water while being so close to the Sun.

Because water is so common, planets such as Earth, Venus, and Mars formed from dust and water-rich rocks. As the young planets settled, the water was rich with their early atmospheres. But much of that water evaporated into space before the planet could fully cool. Whether an inner planet could cool fast enough to accumulate water on its surface is a matter of debate. A study of Nature argues that it’s a matter of distance from its star, Venus being too close to the Sun to cool in time, while Earth and Mars cooled quite early. The idea here is that Mars then lost much of its water due to its lack of a strong magnetic field.

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Another idea is that planets like Earth boiled much of their original water, but were then seeded by water from comets and asteroids that later bombarded it. Proponents of this model point to a variance of molecular isotopes in Earth’s water. Common water is made up of one oxygen atom and two hydrogen atoms, but there is a rare variant of water that contains one deuterium atom instead of hydrogen. Only about 26 out of a million atoms are deuterium instead of hydrogen, but about 150 out of a million water molecules on Earth are the deuterium variant.

This is key because deuterium water is much more likely to have formed in deep space. The idea is that the first waters of the solar system were linked to asteroids and comets, then enriched the isotope levels of the Earth’s seas. But proving it is difficult. A relatively small number of meteorites have been broken up and analyzed at their basic molecular level. It would be difficult to do this for every asteroid we find to see if they have the right composition to explain Earth’s water. But a new study has found a way to make it easier.

A scan of X-rays and neutron beams showing compounds containing hydrogen. Credit: A. Treiman/Lunar and Planetary Institute/USRA

The NIST team discovered that instead of cutting slices from a meteorite to chemically analyze it, they could scan it using a combination of X-rays and neutron imaging. To prove its accuracy, the team scanned two meteorites that had already been chemically analyzed. They compared their scans to the known composition, and they matched up pretty well.

A big advantage of this new method is that it is non-destructive. Scans do not damage meteorites in any way. The scans also give a 3D picture of the location of water and other compounds in the rock, unlike chemical methods which only confirm the presence of these compounds.

In the future, the team hopes to use this method on many more meteorites, which could determine the chemistry of early asteroids and help solve the mystery of Earth’s water. It could even determine other compounds brought to Earth, such as the building blocks of life itself.

Reference: Hamano, Keiko, Yutaka Abe and Hidenori Genda. “Emergence of two types of telluric planet during the solidification of the magmatic ocean.” Nature 497.7451 (2013): 607-610.

Reference: Treiman, Allan H., et al. “Coordinated neutron and X-ray tomography of meteorites: detection and distribution of hydrogen-bearing materials.” Meteoritics and planetary sciences 57.10 (2022): 1820-1835.

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