Where did Earth’s water come from – a new, surprising theory

Until now, it was believed that the hydrogen needed to form H₂O arrived on our planet aboard bombarding asteroids. However, scientists from the University of Oxford have turned this theory on its head, shedding new light on the mystery of where Earth’s water came from and what we truly owe our existence to.

Surprising experiment results

Published on April 16 in the scientific journal Icarus, the study is in fact an analysis of the composition of the enstatite chondrite LAR 12252 – a rare meteorite discovered in 2012 in Antarctica. Its structure closely resembles that of early Earth, from around 4.55 billion years ago. The research team, led by Tom Barrett, used a synchrotron – an advanced cyclic accelerator – located in Harwell, Oxfordshire, to scan the meteorite’s structure with X-rays and identify its chemical composition.

What did they find? To the researchers’ surprise, instead of hydrogen atoms in the chondrules – the non-crystalline grains within the meteorite – large amounts of hydrogen sulfide were discovered in the surrounding material. This suggests that LAR 12252 was rich in hydrogen from the beginning, which, together with sulfur, was trapped in the meteorite’s crystalline matrix. The conclusions drawn from this study were then applied to theories about Earth’s composition.

Where did Earth’s water come from?

For many years, the dominant theory stated that proto-Earth was poor in hydrogen, and only about 100 million years later did hydrogen-rich asteroids collide with its surface, enabling water to form. However, the Oxford scientists’ study challenges this view, suggesting that enstatite chondrites – known to have been the building blocks of early Earth – already contained significant amounts of hydrogen. By the time of the asteroid bombardment, Earth was likely already large enough to have had plenty of hydrogen.

The answer to the question of where Earth’s water came from may therefore have nothing to do with a cosmic coincidence such as a meteor shower. Hydrogen is likely not, as previously thought, an external contaminant, but an atom present in the planet’s structure from the very beginning.

Life on Earth is no accident

Summing up the experiment’s results, Tom Barrett expressed great excitement: This study provides key evidence supporting the theory that Earth’s water is native. Co-author Professor James Bryson added: We believe the material that formed Earth – which we examine by studying rare meteorites – was much richer in hydrogen than previously thought. As a result, the development of life on our planet seems to be a natural process, rather than a matter of chance.

Of course, we still do not know where and how the first H₂O molecules formed, nor how the asteroid bombardments – which certainly did occur – influenced this process. It is worth recalling that as recently as 2020, French scientists from the University of Lorraine, after studying the Sahara 97096 meteorite, concluded that the hydrogen in enstatite chondrites was a result of external contamination, supporting the theory of the cosmic origin of hydrogen. It is difficult to predict what future studies of chondritic meteorites will reveal – unfortunately, very few of them have been found on Earth.