Strange water — the source of water in our solar system

Featured Image: The star-forming nebula W51 is one of the largest “star factories” in the Milky Way galaxy, NASA/JPL, Public Domain (CC0)

Paper: Origin of hydrogen isotopic variations in chondritic water and organics

Authors: L. Piani, Y. Marrocchi L.G.Vacher H. Yurimoto M. Bizzarro

Vast blue oceans, swirly rain or fluffy white snow – water is ubiquitous on Earth. But where does the water of our solar system come from?

A group of researchers were able to investigate the isotopic composition of water in different components of meteorites. Their findings hint that some of the water on Earth may have originated from a source beyond the solar system.

Hydrogen forms two stable isotopes. The most abundant one has only a single proton in its core, but the rarer and heavier isotope, called deuterium, carries an additional neutron. Most of the hydrogen in meteorites is either contained in water absorbed on minerals or in organic matter. When scientists started to look at the isotopic composition of hydrogen in meteorites, they were puzzled by the unusual heterogeneity in the deuterium-hydrogen ratios among them. It was long thought that the variation in composition is mainly caused by complex processes, which took place on the parent bodies of meteorites.

It is not possible to separate hydrogen’s two main phases, water and organic matter, mechanically. Thus, our understanding of the history of water in meteorites comes from the total composition of meteorite fragments. To shed more light on this question, a group of researchers tried to measure the single components directly by hitting them with a high energy ion beam. The evaporated material was then analysed using mass spectrometry. The results of this new study indicate that the heterogeneity in hydrogen isotopes between the meteorites and also between the organic phase and the water cannot be explained by parent body processes alone.

Where is the deuterium enrichment coming from if not from the home worlds of meteorites? There is another source of deuterium, which lies beyond the borders of our solar system. Our Sun originated from a large gas cloud, mainly composed of hydrogen in its molecular form (H2). We know from the observation of present molecular clouds that they are very rich in deuterium. Although astronomers discovered the existence of molecular clouds and their role in star formation decades ago, it was generally thought that the isotopic signature of the Sun’s parental cloud was destroyed during its violent birth and ignition.

Nebula W51 is one of the largest molecular clouds in the milky way. Star nurseries like this can be active for several millions of years. Small parts of it collapse further and further until a new star is born. Image credit: NASA/JPL, Public Domain (CC0)

New findings from the work of Piani and colleagues indicate that material was exchanged between our solar system and its surrounding molecular cloud after the Sun formed. Over millions of years, deuterium rich gas from the molecular cloud fell into our solar system and was eventually incorporated as water or organic matter in the parent bodies of meteorites. The deuterium rich material, which reached the inner solar system, was heated up enough to exchange with deuterium poor gas. This led to a loss of the deuterium rich signature. However, temperatures in the outer solar system were low enough to preserve the deuterium-hydrogen ratio of the molecular cloud. This theory is supported by the higher deuterium fractions of meteorites, which are thought to originate from the outer solar system, compared to meteorites formed in the inner solar system. Moreover, it explains the low deuterium concentration in organic matter compared to water, observed in some meteorites: organic matter condenses at much higher temperatures than water, closer to the Sun where most of the gas from the molecular cloud was already equilibrated with the local gas.

While this study has helped us to better understand the origin of our solar system’s water, the researchers point out that more work has to be done to uncover the complex processes involved in the formation of our very own oasis in space.

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‘Strange water — the source of water in our solar system’ by Max Winkler is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

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