Where did Earth’s water come from?

By Fred Watson, Australia’s Astronomer-at-Large 19 July 2023
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Our planet’s oceans are something we take for granted. But for planetary scientists seeking to understand their origin, there are questions to be answered.

Water, we know, is common in the universe. Indeed, with its hydrogen and oxygen makeup, it’s the most abundant of all two‑element compounds. Frozen water is plentiful in planetary systems in the form of comets – mountain-sized icebergs in space.

Astronomers believe comets are fossils from the outlying regions of the gas-and-dust clouds from which stars and planets formed, some 4.57 billion years ago in the case of our solar system. So, can we draw the conclusion that Earth’s water came from ancient comets impacting our planet in its infancy? There’s a way to find out, because water comes in two kinds – the normal kind and a “heavy” version, in which each hydrogen atom contains an additional neutron. The ratio of these two in any given sample provides a signature that allows astronomers to track the cosmic pathway of water to planets such as Earth.

It turns out that, yes, the normal-to-heavy water ratio in some comets matches that in Earth’s oceans, confirming that a proportion, at least, of our oceans came from comets. But there is still a missing link in the argument. Did comets themselves really come from primordial water that existed before the solar system formed, permeating the gas-and-dust cloud that later collapsed to form the Sun and planets?

New research confirms that this is, indeed, the case. An international team of scientists has used the powerful Atacama Large Millimeter/submillimeter Array (ALMA) in northern Chile to investigate the water content in a planet‑forming disc of material swirling around a young star some 1300 light-years away – a disc that is a remnant of the gas cloud in which the star formed. This radio telescope is sensitive to water in gaseous rather than icy form, and the particular star, known as V883 Orionis, is hot enough to prevent water freezing in the inner part of the disc. The astronomers have confirmed the water around V883 Orionis has a very similar normal-to-heavy ratio to that of comets in our own solar system. 

Their work provides strong support for the idea that Earth’s water formed in the cold of space, long before our planet was born.