If oceans existed earlier than this, any geological evidence has yet to be discovered (which may be because such potential evidence has been destroyed by geological processes like crustal recycling). In the Nuvvuagittuq Greenstone Belt, Quebec, Canada, rocks dated at 3.8 billion years old by one study and 4.28 billion years old by another show evidence of the presence of water at these ages. A sample of pillow basalt (a type of rock formed during an underwater eruption) was recovered from the Isua Greenstone Belt and provides evidence that water existed on Earth 3.8 billion years ago. Geological evidence also helps constrain the time frame for liquid water existing on Earth. Other, much older pillow basalt formations provide evidence for large bodies of water long ago in Earth's history. This pillow basalt on the seafloor near Hawaii was formed when magma extruded underwater. As the cooling continued, most CO 2 was removed from the atmosphere by subduction and dissolution in ocean water, but levels oscillated wildly as new surface and mantle cycles appeared. Afterward, liquid water oceans may have existed despite the surface temperature of 230 ☌ (446 ☏) due to the increased atmospheric pressure of the CO 2 atmosphere. The rock vapor would have condensed within two thousand years, leaving behind hot volatiles which probably resulted in a majority carbon dioxide atmosphere with hydrogen and water vapor. Īny water on Earth during the latter part of its accretion would have been disrupted by the Moon-forming impact (~4.5 billion years ago), which likely vaporized much of Earth's crust and upper mantle and created a rock-vapor atmosphere around the young planet. Not only is it a noble gas (and therefore is not removed from the atmosphere through chemical reactions with other elements), but comparisons between the abundances of its nine stable isotopes in the modern atmosphere reveal that the Earth lost at least one ocean of water early in its history, between the Hadean and Archean eons. In particular, xenon is useful for calculations of water loss over time. Lighter elements like hydrogen and helium are expected to leak from the atmosphere continually, but isotopic ratios of heavier noble gases in the modern atmosphere suggest that even the heavier elements in the early atmosphere were subject to significant losses. When the Earth was younger and less massive, water would have been lost to space more easily. H 2O molecules in the atmosphere are broken up by photolysis, and the resulting free hydrogen atoms can sometimes escape Earth's gravitational pull (see: Atmospheric escape). One factor in estimating when water appeared on Earth is that water is continually being lost to space. The two ideas are not mutually exclusive, as there is also evidence that water was delivered to Earth by impacts from icy planetesimals similar in composition to asteroids in the outer edges of the asteroid belt. Recent research, however, indicates that hydrogen inside the Earth played a role in the formation of the ocean. Instead, it was hypothesized water and other volatiles must have been delivered to Earth from the outer Solar System later in its history. It was long thought that Earth’s water did not originate from the planet’s region of the protoplanetary disk. Liquid water, which is necessary for all known forms of life, continues to exist on the surface of Earth because the planet is at a far enough distance (known as the habitable zone) from the Sun that it does not lose its water, but not so far that low temperatures cause all water on the planet to freeze. Earth is unique among the rocky planets in the Solar System in having oceans of liquid water on its surface. The origin of water on Earth is the subject of a body of research in the fields of planetary science, astronomy, and astrobiology. Water covers about 71% of Earth's surface
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