Scientists think they’ve solved the mystery of how water arrived on Earth

The study of meteorites has led to a breakthrough, Quentin Kral writes. When Earth first formed, it was too hot to retain ice. This means all the water on our planet must have originated from extraterrestrial sources. Studies of ancient terrestrial rocks suggest liquid water existed on Earth as early as 100 million years after the Sun’s formation – practically “immediately” on an astrophysical timescale. This water, now over 4.5 billion years old, has been perpetually renewed through Earth’s water cycle. My research team has recently proposed a new theory to explain how water first arrived on Earth.

Astrophysicists have been grappling with the question of how water arrived on our young planet for decades. One of the earliest hypotheses suggested that Earth’s water was a direct byproduct of the planet’s formation, released via magma during volcanic eruptions, in which most of the emitted gas is water vapour. However, this hypothesis evolved in the 1990s following analysis of Earth’s water composition and the discovery of the potential role of icy comets, pointing to an extraterrestrial origin. Comets, which are mixtures of ice and rock formed in the distant reaches of the solar system, are sometimes ejected toward the Sun. When warmed by the Sun, they develop striking tails of dust and gas that are visible from Earth. Asteroids, located in the asteroid belt between Mars and Jupiter, were also proposed as potential progenitors of Earth’s water.

The study of cometary and asteroid rocks via meteorites – small fragments of these bodies that have fallen to Earth – has provided key insights. By analysing the D/H ratio – the proportion of heavy hydrogen (deuterium) to standard hydrogen – scientists found that Earth’s water more closely matches that of “carbonaceous” asteroids, which bear traces of past water. This shifted the focus of research toward these asteroids.

Recent studies have centered on identifying the celestial mechanisms that could have delivered these water-rich asteroids to the dry surface of early Earth. Numerous theories have emerged to explain the “perturbation” of planetesimals – large, icy bodies in the asteroid and Kuiper belts. These scenarios propose gravitational interactions that dislodged these objects, sending them hurtling toward Earth. Such events would have required a complex “gravitational billiards” process, suggesting a tumultuous history of the solar system.

While it is evident that planetary formation involved significant upheavals and impacts, it is possible that Earth’s water delivery occurred in a more natural and less dramatic manner. I started with the assumption that asteroids emerge icy from their formation cocoon, also known as the protoplanetary disk. This cocoon is a massive, hydrogen-rich disk filled with dust, where planets and initial belts form. It envelops the entire nascent planetary system. Once this protective cocoon dissipates – after a few million years – the asteroids warm up, causing their ice to melt or, more precisely, to sublimate. In space, where pressure is nearly zero, the water remains in vapour form after this process.

A disk of water vapour is then superimposed on the asteroid belt orbiting the Sun. As the ice sublimates, the disk fills with vapour, which spreads inward toward the Sun due to complex dynamic processes. Along the way, this vapor disk encounters the inner planets, immersing them in a kind of “bath”. In a way, the disk “waters” the terrestrial planets: Mars, Earth, Venus and Mercury. Most of this water capture occurred 20 to 30 million years after the Sun’s formation, during a period when the Sun’s luminosity increased dramatically over a brief period of time, increasing the degassing rate of asteroids.

Once water is captured by a planet’s gravitational pull, many processes can occur. On Earth, however, a protective mechanism ensures the total mass of water has remained relatively constant from the end of the capture period until today. If water rises too high into the atmosphere, it condenses into clouds, which eventually return to the surface as rain – a process known as the water cycle. The quantities of water on Earth, both past and present, are well documented. Our model, which begins with the degassing of ice from the original asteroid belt, successfully accounts for the amount of water needed to form oceans, rivers and lakes, and even the water buried deep within Earth’s mantle. Precise measurements of the D/H ratio of water in the oceans also align with our model. Moreover, the model explains the quantities of water present in the past on other planets – and even on the Moon.

You might wonder how I arrived at this new theory. It stems from recent observations, particularly those made with ALMA, a radio telescope array of over 60 antennae located in Chile, on a plateau five kilometres above sea level. Observations of extrasolar systems with belts similar to the Kuiper Belt reveal that planetesimals in these belts sublimate carbon monoxide (CO). For belts closer to their star, such as the asteroid belt, CO is too volatile to be present, and water is more likely to be released.