From the microbes deep within hydrothermal vents to the jumping spiders of the Himalayas, life is what makes Earth incredibly unique within our solar system. Today, the most supported theory of life’s origins hypothesizes that all life came from deep under our ocean’s waves. While this theory is yet to be entirely proven, the assertion that water is essential to life on Earth is not one for dispute.
Currently, water covers over 70 percent of Earth’s surface — most people are familiar with the image of startling expansive blue in classic photographs of our planet from Apollo missions. However, unlike its current state, the early Earth was born from a protoplanetary disk that was supposedly relatively dry. Allegedly — as the early Earth’s environment and conditions are also theorized, most believe that the Earth’s surface was molten, too hot with magma to be capable of water. Therefore, the question of where Earth’s extraordinary amounts of water originated from is complex and puzzling. To this day, it remains a hotly debated topic within the scientific community.
In August 2020, a study published in the journal Science reported a particularly unexpected source of Earth’s water: enstatite chondrite (EC) meteorites. This newly presented data is surprising as EC meteorites were formed within the solar system’s inner region and are composed of similar materials to the Earth. ECs are usually brushed off within the debate of Earth water’s origins due to prior belief that they, much like early terrestrial planets with similar isotopic compositions and proximity to the Sun, simply lacked water. However, this study, authored by Laurette Piani et al., showcased that ECs contain enough hydrogen, essential to water’s composition, to drown the Earth’s surface with water amounting to at least three times the oceans’ current mass. With Earth’s mantle presenting matching compositions to EC meteorites, these researchers theorized that EC asteroids incorporated elements like hydrogen and nitrogen into Earth to create the planet’s water.
While enlightening with its unique position on Earth having been wet since its formation, this study is only one theory within the cascade of hypotheses comprising its realm of research. Much more common is the inference that the early Earth had a low quantity of water produced from volcanic degassing activity coupled with the idea that its current oceans of water were effectively delivered by extraterrestrial bodies — icy comets and asteroids far out in the ring of the Asteroid belt.
At first, icy comets seemed to be the obvious answer to where our oceans had originated from. Comets are known to contain ice water due to their origins from farther out in the solar system, that is, away from the Sun. These bodies come to Earth either from the Oort Cloud as long-period comets or the Kuiper Belt as short-period comets. Through an era known as the Late Heavy Bombardment — a period around 4 to 4.5 billion years ago where leftover planet-building material perpetually attacked the early Earth — comet and asteroid impacts were thought to have built up the Earth’s supply of water. However, in 2014, theories of Kuiper Belt comets being the main source of our oceans were undermined by European Space Agency’s (ESA) Rosetta spacecraft, which became the first spacecraft to orbit and land on a comet’s surface: Comet 67P/Churyumov-Gerasimenko. From its proximity to the comet, Rosetta retrieved data on the comet’s chemical composition and found that its deuterium-to-hydrogen (D/H) ratio was too high in comparison to Earth’s oceanic water. Likewise, in 1986, ESA found Halley’s Comet, a long-period comet from the Oort Cloud, to have twice the levels of heavy water — D/H ratio — as terrestrial water. From these probes, it could be safe to say that comets, from both the Kuiper Belt and the Oort Cloud, are most likely not our main sources of water.
However, comets cannot be ruled out entirely just yet. Although Oort Cloud comets seem unlikely to be the origins of our water, the 2018 inspection of comet 46P/Wirtanen revealed that particular Kuiper Belt objects had compositions that matched the Earth’s water. Therefore, while comets don’t seem to be our ultimate solution, “hyperactive comets,” comets that release more water than contained on its surface, such as 46P, show that they were the cause of at least some proportion of the Earth’s oceans.
Meanwhile, asteroids continue to enjoy a lead on hypotheses pertaining to Earth’s early supply of water. Once icy comets were partly discredited within this field of research, scientists turned to asteroids, which trap hydrogen and oxygen within instead of containing ice. Asteroids, particularly a subgroup of the oldest meteorites: carbonaceous chondrites, display D/H ratios and nitrogen isotopic compositions that are akin to those found on Earth.
This broad theory on asteroids continues to be researched upon, introducing new sub-theories that answer further questions such as how asteroids with water came to Earth and how water survived through the heat of such impacts. For example, a proposed association with Theia, the Mars-sized object whose impact upon Earth is hypothesized to have created the Moon. Additionally, new probes and experiments are needed for objects such as active asteroids: comet-like asteroids with trails, indicating sublimation, to be researched upon.
Just as the origin of life remains intensely debated through the scientific community, the origin of Earth’s water is a question that continues to be investigated. As humanity reaches out to greater progress in space, our understanding of Earth’s interactions with the cosmos, both in its past and present, continues to improve. Finding the answer to an inquiry that is based on such a fundamental aspect of the Earth, and therefore, our lives, could have substantial impacts on our vision and interaction with water as both a source of living and as a cosmological wonder.
Water is essential within the intricate balance of conditions that makes Earth just right for complicated life. However, despite its misleading abundant appearance, water is a limited resource. This is due to the reality that freshwater, the resource that sustains nearly all living species, only comprises 2.5% of Earth’s water. With the near exponential growth of humanity, concerns accumulate around freshwater depletion — the United Nations predicts that by 2025, 1.8 billion people will live in areas of water scarcity and “two-thirds of the world’s population living in water-stressed regions as a result of use, growth, and climate change.”
To combat the issues revolving around responsible use of clean water, a deeper look into not only limiting human impacts on climate change, but also new frontiers of water reservoirs is needed in both political and scientific discussion. Uncovering secrets around terrestrial water’s origins and cosmological associations may bring novel ideas to the increasing struggle for sustaining Earth’s most vital natural resource.