The solar system’s water supply was inherited as ice from interstellar space. The water included prebiotic matter that would later integrate into life on Earth. Similar ices are likely to be found around other protoplanetary disks. There’s a surprising amount of cosmic water.
Water is pervasive throughout the universe, even at the very earliest times. ~ American astrophysicist Matt Bradford
The solar system has a planetary snow line: the zone beyond which ice could have condensed on emergent planets.
Although water covers 70.9% of Earth’s surface, it accounts for far less than 1% of the planet’s mass. Uranus and Neptune, which formed well past the snow line, are loaded with tens of percents of water by mass.
During Earth’s birth, the inner solar system was hot enough to melt lead. The inner planets – those as far out as Mars – would have been born dry, had they started out where they are now; which was certainly not the case for Mercury. If Earth did not start out as a hot dry rock, then it either moved into its current orbit after formation or made much of its water itself.
Earth looks to have been born wet, not dry. The chemical signature for water found deep within the mantle suggests that much of the planet’s water was primordial. Water was generated within the mantle by combining fluid hydrogen and the silica in quartz, both of which would have been abundant in Earth’s early mantle. Earth’s crust is now 59% silica.
Fluid hydrogen and silica form water at 1700 K and pressure 20,000 times that of Earth’s atmosphere. These requirements were easily met in the planet’s mantle.
The swirling in the solar system that caused the planets to coalesce from bits of cosmic dust dragged an emerging Jupiter about before it settled into its current orbit. Earth’s primordial water supply suggests that Earth, like Jupiter, came toward the Sun during its formation, likely in tow of Jupiter (and Saturn).
A chondrite is a stony meteorite, formed by accretion from dust and small grains. Many chondrites in the early solar system acquired a coating of ice.
Jupiter’s promenade dragged chondrites into collision with Earth, seeding it and the Moon with water. Hence, the bombardment of Earth – that largely started and abated by Jupiter’s planetary evolution – supplemented Earth’s water supply. The leftovers which did not rain down form the asteroid belt that ranges between the orbits of Mars and Jupiter, where the solar system’s snow line is situated.
As Earth cooled, a crust formed, as well as an atmosphere bearing water; bringing rain, and in time, oceans. This allowed surface temperatures to drop to less than 102 °C as early as 4.4 bya.
Earth may have sported its first ocean at this time, when the planet was but 150 million years old. The Sun’s evaporative blaze was 30% less than now.
3 billion years ago, the ocean was 67 °C. By 1.5 bya, the ocean had cooled to 27 °C; a warm soup supporting life.
Sponged up in the Earth’s interior, 410–600 km down, is at least 25 times the water in the oceans. If not for this lubrication, there would be no plate tectonics and no continents. Without continents, there would be no transport of life-sustaining nutrients from rivers into the oceans.