In Earth’s early days, the planet was far too hot to support ice, indicating that all water on Earth likely came from sources beyond our planet. Research on ancient terrestrial rocks reveals that liquid water may have existed as soon as 100 million years after the sun was formed—essentially “immediately” when viewed from an astrophysical lens. This primordial water, dating back over 4.5 billion years, has continuously cycled through Earth’s systems.

Our research team has recently put forward a novel theory regarding the origins of water on Earth, with our findings published in the journal Astronomy & Astrophysics.

A Time-Worn Enigma

For decades, astrophysicists have wrestled with the puzzle of how water ended up on our young planet. An early theory proposed that Earth’s water resulted from volcanic activity during its formation, with much of the gases released being water vapor.

However, this view shifted in the 1990s as scientists examined Earth’s water composition and recognized the potential influence of icy comets, suggesting a more extraterrestrial origin. These comets, made of ice and rock and originating from the outskirts of the solar system, sometimes venture close to the sun. As they get warmed, they develop stunning tails of dust and gas, visible from our planet. Additionally, asteroids from the belt between Mars and Jupiter have also been identified as possible sources of Earth’s water.

Research on meteorites—small fragments from these celestial bodies that have landed on Earth—has yielded crucial insights. By studying the D/H ratio (the ratio of heavy hydrogen, or deuterium, to regular hydrogen), scientists discovered that Earth’s water resembles that found in “carbonaceous” asteroids, which contain remnants of ancient water. This revelation shifted the focus back to these asteroids.

Recent investigations have aimed at uncovering how such water-rich asteroids made their way to the parched surface of early Earth. Various theories explain the “perturbation” of planetesimals—large icy bodies residing in the asteroid and Kuiper belts. These theories suggest that gravitational interactions could have displaced these objects, propelling them towards our planet. Such scenarios would require a complicated gravitational “billiards” process, highlighting a chaotic history for our solar system.

While the evidence indicates that planetary development involved significant upheavals and collisions, it is conceivable that the arrival of water on Earth occurred in a more serene and less cataclysmic way.

A Streamlined Theory

My hypothesis began with the premise that asteroids remain icy as they form within their protective environment, known as the protoplanetary disk. This substantial disk, rich in hydrogen and composed of dust, envelops the developing solar system. After several million years, once this cocoon dissolves, the asteroids heat up, resulting in the melting and sublimation of the ice. Given the near-zero pressure in space, the water remains vaporized during this process.

This vapor then coalesces into a disk surrounding the asteroid belt, with the created vapor moving closer to the sun through various complex processes. As the vapor disk interacts with the inner planets, it effectively “waters” them—infusing Mars, Earth, Venus, and Mercury with H2O. Most of this water delivery likely occurred 20 to 30 million years post-sun formation, coinciding with a period of heightened solar luminosity which increased the release of gases from asteroids.

Once the gravitational pull of a planet captures the water, diverse processes can follow. On Earth, a protective mechanism ensures that the total water mass has remained relatively stable from the end of the initial capture period until now. If the water rises excessively into the atmosphere, it condenses into clouds, eventually falling back as precipitation—a process referred to as the water cycle.

The volumes of water found on Earth, both historically and presently, are well-documented. Our model, which is based on the degassing of ice from the original asteroid belt, successfully correlates with the amount of water required to form our oceans, rivers, lakes, and even the water trapped deep within Earth’s mantle. Accurate measurements of the D/H ratio in ocean water also support our framework. Furthermore, the model accounts for water quantities on other planets and the moon in earlier epochs.

You may wonder how I developed this theory. Its inception came from recent observations, particularly utilizing the ALMA radio telescope facility in Chile, located at an altitude of five kilometers. Observations of extrasolar systems with configurations resembling the Kuiper Belt have indicated that planetesimals in these regions can release carbon monoxide (CO) during sublimation. In areas closer to their stars, such as the asteroid belt, CO is too unstable, making water more likely to be emitted.

Establishing the Model

This theory began to take shape based on these emergent findings. Additionally, data from the Hayabusa 2 and OSIRIS-REx missions, which studied asteroids similar to those that likely contributed to the initial water vapor disk, have provided vital corroboration.

These missions, alongside ongoing research using ground-based telescopes, have uncovered significant quantities of hydrated minerals on these asteroids—minerals that can only form when there is contact with water. This supports the notion that these asteroids were originally icy, even if most have since lost their ice content (with exceptions like Ceres).

With the initial groundwork of the model established, the next step involved creating a numerical simulation to track ice degassing, water vapor dispersion, and its capture by planets. These simulations quickly indicated that the model could adequately explain Earth’s water supply. Ongoing investigations into historical water levels on Mars and other terrestrial planets confirmed its wider applicability. Everything lined up, and we prepared the results for publication.

As researchers, developing a model that fits and explains all variables is only part of the process. The theory must undergo verification at a broader scale. Although it is not feasible to detect the original water vapor disk that once “watered” the terrestrial planets, we can examine young extrasolar systems with asteroid belts to determine the existence of such water vapor disks. Our calculations suggest that these disks, albeit faint, should be discernible using ALMA. Our team has recently obtained time on ALMA to investigate specific systems for signs of these disks.

We may be on the brink of a groundbreaking revelation in understanding the origins of Earth’s water.

This article is republished from The Conversation under a Creative Commons license. Read the original article.