Scientists may have learned how Earth got its water

A breakthrough study from researchers at Carnegie Science and UCLA suggests that the origins of Earth’s water may be linked to the interactions between hydrogen-rich atmospheres and magma oceans of the planetary embryos during Earth’s formative years. 

The research, published in the journal Nature, offers new insights into the development of Earth’s signature features and has significant implications for our understanding of the planet’s evolution.

For many years, the understanding of planet formation was primarily based on observations of our own Solar System. While there are active debates surrounding the formation of gas giants like Jupiter and Saturn, it is widely accepted that Earth and other rocky planets accreted from the disk of dust and gas surrounding the Sun during its infancy.

The planetesimals that eventually formed Earth grew larger and hotter as they collided with increasingly larger objects, creating a vast magma ocean due to the heat generated from these impacts and the presence of radioactive elements. Over time, as the planet cooled, the densest material sank inward, separating Earth into three distinct layers: the metallic core, and the rocky, silicate mantle and crust.

Recent advancements in exoplanet research have led to a new approach to modeling Earth’s early stages. “Exoplanet discoveries have given us a much greater appreciation of how common it is for just-formed planets to be surrounded by atmospheres that are rich in molecular hydrogen, H2, during their first several million years of growth,” explained Anat Shahar, one of the researchers involved in the study. “Eventually these hydrogen envelopes dissipate, but they leave their fingerprints on the young planet’s composition.”

How the study was done

Armed with this new perspective, Shahar and her colleagues at Carnegie Science and UCLA developed a model to simulate Earth’s formation and evolution, examining whether the planet’s unique chemical traits could be reproduced. 

Their findings reveal that early interactions between Earth’s magma ocean and a molecular hydrogen proto-atmosphere could have led to the formation of its abundance of water and its overall oxidized state.

The research team utilized mathematical modeling to explore the exchange of materials between molecular hydrogen atmospheres and magma oceans, considering 25 different compounds and 18 different types of reactions.

The results from these simulations revealed that the interactions between the magma ocean and the atmosphere caused large masses of hydrogen to move into the metallic core, oxidized the mantle, and produced significant amounts of water.

The study suggests that even if all the rocky material that combined to form Earth was completely dry, the interactions between the molecular hydrogen atmosphere and the magma ocean would still generate substantial amounts of water. While other water sources may exist, they are not necessary to explain the current state of our planet.

Shahar emphasizes that this is just one possible explanation for Earth’s evolution, but one that could establish an important connection between Earth’s formation history and the most common exoplanets discovered orbiting distant stars, known as Super-Earths and sub-Neptunes.

This research was conducted as part of the multi-institution, interdisciplinary AEThER project, led by Shahar, which aims to understand the chemical makeup of the Milky Way galaxy’s most common planets and develop a framework for detecting signs of life on distant worlds. 

Funded by the Alfred P. Sloan Foundation, the project seeks to understand how the formation and evolution of these planets shape their atmospheres, which could, in turn, help scientists differentiate true biosignatures from atmospheric molecules of non-biological origin.

“Increasingly powerful telescopes are enabling astronomers to understand the compositions of exoplanet atmospheres in never-before-seen detail,” Shahar said. “AEThER’s work will inform their observations with experimental and modeling data that, we hope, will lead to a foolproof method for detecting signs of life on other worlds.”

Prevalence of water in the universe

Water is indeed quite common throughout the universe. It is primarily composed of two of the most abundant elements in the cosmos: hydrogen and oxygen. Hydrogen is the most abundant element in the universe, making up about 75% of its elemental mass, while oxygen is the third most abundant element.

Water has been detected in various forms in our Solar System, such as ice on planets and moons, as well as in the form of water vapor in the atmosphere of some planets. Additionally, water has been found in the form of ice in comets and asteroids, which are remnants from the early Solar System.

Beyond our Solar System, water has been detected in interstellar clouds and the disks around young stars, where new planets are forming. Observations of exoplanets, planets orbiting stars outside our Solar System, have also revealed the presence of water vapor in the atmospheres of some of these distant worlds.

Although water is common throughout the universe, it is important to note that the conditions required for liquid water to exist, which is essential for life as we know it, are more specific and depend on factors such as temperature, pressure, and the presence of a suitable atmosphere. 

The search for planets within the “habitable zone” around their host stars, where conditions might allow for liquid water to exist, is a key aspect of the ongoing quest to find potentially habitable worlds and evidence of extraterrestrial life.

Liquid water in our solar system

Apart from Earth, where liquid water is abundant, there is evidence of liquid water on other celestial bodies as well, although it is often hidden beneath the surface or mixed with other materials. Some examples include:

1. Mars: There is evidence of liquid water beneath the surface of Mars, with several underground lakes discovered through radar data from the Mars Express orbiter. Additionally, scientists believe that water may exist as transient, thin layers of briny (salty) water on the Martian surface due to the presence of hydrated salts.
2. Europa: Europa, one of Jupiter’s largest moons, is believed to have a subsurface liquid water ocean beneath its icy crust. This ocean might contain more than twice the amount of water found on Earth. The presence of this subsurface ocean is inferred from observations made by the Galileo spacecraft and the Hubble Space Telescope, such as evidence of geysers and surface features indicating a liquid layer below the ice.
3. Enceladus: Enceladus, a moon of Saturn, is also believed to have a subsurface ocean of liquid water beneath its icy shell. This has been supported by data from the Cassini spacecraft, which observed geysers spouting water vapor and ice particles from the moon’s south polar region. These geysers are believed to be connected to the subsurface ocean, which could contain hydrothermal vents, making Enceladus a potential target in the search for extraterrestrial life.
4. Ganymede: Ganymede, another of Jupiter’s large moons, is also suspected to have a subsurface ocean of liquid water beneath its icy surface. Evidence from the Galileo and Hubble missions, as well as from Earth-based telescopes, suggests the existence of this subsurface ocean.

These examples show that liquid water exists in our Solar System beyond Earth, primarily in the form of subsurface oceans on icy moons. This has led to increased interest in these celestial bodies as potential habitats for life, as water is a crucial ingredient for life as we know it.

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