Where did Earth’s water come from? That’s one of the most compelling questions in the ongoing effort to understand life’s emergence. Earth’s inner solar system location was too hot for water to condense onto the primordial Earth. The prevailing view is that asteroids and comets brought water to Earth from regions of the Solar System beyond the frost line.

But a new study published in the journal Nature Astronomy proposes a further explanation for Earth’s water. As the prevailing view says, some of it could’ve come from asteroids and comets.

But most of the hydrogen was already here, waiting for Earth to form.

To understand the origin of Earth’s water, scientists study its isotopic compositions. Not only Earth’s water but also the evidence of water in meteorites, asteroids, and anywhere else in the Solar System they find it. In this new study, scientists developed a novel technique to determine the isotopic composition of water in the most ancient meteorites ever found.

The study is “Determination of the initial hydrogen isotopic composition of the solar system.” The first author is Jérôme Aléon, a researcher at the Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (CNRS / MNHN / Sorbonne University).

Scientists have known about isotopes since the early 20th century. There are three naturally occurring isotopes of hydrogen: 1H, 2H, and 3H. 1H and 2H are stable, while 3H is unstable. The three hydrogen isotopes are also called protium, deuterium, and tritium. Water contains different amounts of the three, especially protium and deuterium. Scientists express these relative amounts as a ratio. The D/H ratio is the ratio of deuterium to hydrogen. (Researchers often use the terms 1H, protium, and hydrogen interchangeably.)

Deuterium can take the place of hydrogen in the water molecule. When deuterium takes hydrogen’s place in a quantity of water, it’s called heavy water. Earth water only occasionally contains H2O molecules with deuterium rather than hydrogen.

We know the D/H ratio of water on Earth to an exact degree. It’s 1.5576 ± 0.0005) × 10-4. The ratio is a fingerprint scientists use to compare Earth’s water with water reservoirs elsewhere in the Solar System. For example, hydrogen contained in minerals inside ancient meteors, though it’s not water, can have different hydrogen isotope ratios that reflect the D/H present when the minerals form.

That fact is at the heart of the new study. We can turn the clock back to the Solar System’s formation to understand this research. Back to when the Sun was born in a molecular cloud.

After the Sun took shape in that cloud, it was surrounded by a smaller collection of gas called a solar nebula. The planets formed from the material in the solar nebula. This new study focuses on the 200,000 years after the Sun formed, but before any planetary embryos formed. To understand that time period, the researchers examined calcium-aluminum-rich inclusions (CAI) in a type of meteorite called carbonaceous chondrites.

CAIs are the oldest rocks we have. The minerals inside them are some of the first solids to condense out of the protoplanetary disk. They’re 4567.30 ± 0.16 million years old. Scientists use CAIs to define the Solar System’s age because they date back to the primordial Solar System before planets formed.

In this study, the researchers measured the isotopic composition of hydrogen in the CAI’s captured minerals. Those measurements determined the isotopic composition of hydrogen at the birth of the Solar System. The minerals captured inside the CAI are called xenoliths. Xenoliths were captured inside the CAIs while cooling from a magma state.

This image from the paper shows some of the detailed imaging in the study. It shows CAI E101.1, one of four CAIs that have been accurately dated. The images show the location of a sinuous xenolith in the CAI. The panel on the right shows the mineralogy of the xenolith. CAI E101.1 has experienced only modest metamorphic modification and little fluid circulation, and according to the authors that helped preserve its primordial D/H fingerprint. Image Credit: Aléon et al. 2022.
This image from the paper shows
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