Earth is a true water world, with oceans covering over 70% of its surface and water permeating its atmosphere, crust and even mantle. This abundance of water has made Earth habitable and allowed life to flourish for billions of years. But the origin of Earth’s water has long been a mystery. Did Earth form with its water already present, or was it delivered later by comets and asteroids? Recent research is providing new clues to this enduring question and shedding light on the complex processes that shaped our blue planet.
Earth’s Wet Start
The prevailing view has been that Earth formed dry, with high-energy impacts creating a molten surface on the infant planet. Water was thought to have arrived hundreds of millions of years later, delivered by icy comets and asteroids from the outer solar system.
However, this theory has been challenged by studies of ancient meteorites and models of planet formation. Chemical analysis of enstatite chondrite meteorites, which are thought to resemble the building blocks of Earth, found they contain sufficient hydrogen to have delivered at least three times the mass of water in Earth’s oceans. This suggests Earth’s ingredients were already wet from the beginning.
Other evidence comes from studies of lunar rocks returned by the Apollo missions. The deuterium-to-hydrogen ratio in these rocks closely matches that found in carbonaceous chondrite meteorites and Earth’s water. Since the Moon formed from the debris of a collision between early Earth and a Mars-sized impactor, this common water signature hints that Earth had a substantial water endowment before the Moon-forming impact.
Wet Asteroids
While comets were long favored as the source of Earth’s water, their deuterium-to-hydrogen ratios are generally much higher than oceanic water. In contrast, water-rich asteroids are a closer match. Meteorites linked to the asteroid Vesta have a similar hydrogen isotopic composition to Earth’s oceans, suggesting asteroids were a major water source.
The largest reservoir of water-rich asteroids is the outer main belt between Mars and Jupiter. Carbonaceous chondrite meteorites from this region, particularly rare CI and CM types, contain up to 20% water by weight bound in hydrated minerals. They also have a good match to Earth’s inventory of other volatile elements like carbon, nitrogen and noble gases.
Simulations show that gravitational perturbations from Jupiter could have scattered a large population of these wet asteroids into the inner solar system during Earth’s formation. Enough could have been delivered to account for several oceans worth of water. The heat of impact would have vaporized the water, creating a steam atmosphere that later cooled and condensed into oceans.
Magma Oceans and Atmospheric Ingassing
A novel theory proposes that Earth’s oceans formed from the planet itself through a process of ingassing. In this scenario, a dense atmosphere of hydrogen surrounded the molten early Earth. As the hydrogen interacted with the vast magma oceans, it became oxidized to form water.
Models show that this process could generate several oceans worth of water, even if the original building blocks were completely dry. The hydrogen dissolved into the magma would also explain why Earth’s core is less dense than expected – it may contain a significant fraction of the planet’s primordial hydrogen.
This magma ocean ingassing theory is based on observations of sub-Neptune sized exoplanets, which are the most common type in our galaxy. Many of these worlds are thought to have deep oceans of magma, topped by thick hydrogen atmospheres. If such conditions were present on the early Earth, substantial water could have been produced indigenously through chemical reactions.
Volcanic Outgassing
Since Earth’s formation, volcanic outgassing has also contributed to its water inventory. Water stored in hydrated minerals in the mantle is released during volcanic eruptions, adding to the surface water budget. The upper mantle contains an estimated 1-2 oceans worth of water.
Some of this mantle water may be primordial, dating back to Earth’s formation. However, plate tectonics also acts to recycle water back into the mantle. Hydrated oceanic crust is carried down subduction zones, transporting surface water into the deep Earth. Most of this water is returned to the surface via arc volcanism within a few million years, but some may be stored in the mantle for billions of years before resurfacing.
This ongoing cycle of outgassing and regassing helps to stabilize the amount of water at Earth’s surface. While some water is continually lost to space via hydrogen escape from the upper atmosphere, it is roughly balanced by water released from the mantle. This dynamic cycling has maintained Earth’s oceans at a nearly steady state over geologic time.
An Integrated Delivery Model
The emerging view is that Earth’s water came from multiple sources at different stages of the planet’s formation and evolution. The building blocks of Earth already contained some water, bound in minerals or as ice coatings on dust grains. This was supplemented by a late veneer of wet asteroids, possibly triggered by Jupiter’s migration, that delivered a surge of water during the final stages of accretion.
Magma ocean ingassing and volcanic outgassing further contributed to the water budget, helping to explain Earth’s total water inventory. Comets likely played a minor role, but may have provided some of the lighter hydrogen isotopes. Small amounts of water may also have been supplied directly from the solar nebula or by solar wind implantation.
This integrated model is not yet complete, and many details still need to be worked out. How much water was lost during the Moon-forming impact and subsequent collisions? How efficiently was water delivered by asteroids, and how much reached the surface versus being sequestered in the mantle? To what degree did ingassing and outgassing contribute to the water budget? Answering these questions will require further analysis of meteorites, experiments at extreme conditions, and sophisticated computer modeling.
Implications for Habitability
Understanding the origin of Earth’s water is important for evaluating the potential habitability of exoplanets. The classical notion of a circumstellar habitable zone based solely on distance from the host star is outdated. Other factors like size, atmospheric composition, and interior dynamics must also be considered.
The numerous sub-Neptune sized planets discovered by the Kepler mission may have the right conditions to form substantial water oceans through magma-atmosphere interactions, even if they lack a large icy reservoir. Larger super-Earths may retain primordial hydrogen envelopes that can generate water. This expands the possibilities for oceanic worlds beyond the ice line.
However, water alone does not guarantee habitability. Too much water can lead to a runaway greenhouse effect, as occurred on Venus. A deep global ocean may prevent the cycling of nutrients and hinder the development of complex life. A dynamic interior with plate tectonics and continent-ocean dichotomies may be necessary to create diverse habitats and stabilize the climate.
Earth’s water story is intimately connected to the larger tale of our planet’s origin and evolution. As we continue to explore the solar system and exoplanetary systems, we will gain a deeper understanding of the many factors that shape the habitability of worlds. The study of Earth’s oceans will guide us in the search for life elsewhere in the cosmos.
