4.6 billion years ago, our Sun was a nascent star, surrounded by a vast disk of gas and dust. Over a span of millions of years, this disk condensed to form the planets. In the outer reaches of the solar system, where temperatures were cold enough, planets like Jupiter developed icy cores. Closer to the Sun, where temperatures exceeded 1000 degrees Fahrenheit, Earth formed from materials that could condense at such high temperatures, such as rock and iron. Given the intense heat, Earth initially formed dry, devoid of water. Scientists propose that water might have been delivered to Earth by comets, which are known to be rich in water. The hypothesis suggests that cometary impacts brought ice to Earth, which, upon melting, formed the planet's oceans.
Comets are considered very primitive celestial bodies, implying that Earth's water predates the planet and even the solar system itself. Earth's water molecules consist of one oxygen atom and two hydrogen atoms, primarily light hydrogen but with 6% heavy hydrogen. Observations of Comet Halley in March 1986 by the Giotto probe revealed that its water contains 12% heavy hydrogen, indicating that comets did not contribute all of Earth's water; other sources must also exist.
Earth formed approximately 4.5 billion years ago. During the first 500 million years, known as the Hadean Eon, Earth was a molten mass. However, zircon crystals dating back 4.4 billion years (making them roughly a third the age of the universe) contain water, confirming that early Earth had surface water. This finding contradicts the theory that Earth initially formed dry, suggesting that molten rock also contained water. Water can be trapped within rocks, not just in the form of ice. The primordial disk of gas and dust that formed the solar system contained water, which had a significantly lower proportion of heavy hydrogen compared to Earth's water.
The asteroid belt between Mars and Jupiter, spanning 200 million miles, contains numerous asteroids. These remnants from the early solar system formation processes also contain water. When asteroids collide, they produce fragments known as meteorites, which can contain about 3% water. Some of these meteorites, potentially ejected from the asteroid belt, may have impacted the early Earth, contributing to its water content. Notably, some of these meteorites have water with 6% heavy hydrogen, matching the composition of Earth's water, suggesting that the primary source of Earth's water is from the asteroid belt.
All planets in the solar system are connected by the force of gravity. To understand planetary formation, it is crucial to consider the entire solar system rather than just the four innermost planets (Mercury, Venus, Earth, and Mars). Recent studies emphasize that examining the formation of all planets is essential due to their gravitational interactions.
As planets formed and grew, they interacted gravitationally. About a million years after the Sun's formation, 15 mini-planets emerged from the disk of gas and dust. Those closer to the Sun were dry and hot, lacking trapped water, while those about 230 million miles from the Sun were cool enough to contain water-bearing rocks. Earth, initially dry, likely accumulated water through collisions with these water-rich mini-planets.
Comets from the Oort Cloud periodically enter the solar system, some coming close to Earth. These comets, with water containing 6% heavy hydrogen, also contributed to Earth's water reserves.
Reference: Spark, Where Earth's Water Originally Comes From
