The amount of water on Earth greatly exceeds that known to be on or within any other planet in the solar system. Liquid water, which is essential for life, has unique and amazing properties. Water covers 70% of Earth’s surface. Where did all Earth’s water come from?
If the Earth and solar system evolved from a swirling cloud of dust and gas, almost no water should reside near Earth—or within 5 astronomical units (AU) of the Sun. (1 AU is the average Earth-Sun distance.) Any water (liquid or ice) that close to the Sun would vaporize and be blown by the solar wind to the outer reaches of the solar system,a as we see happening with water vapor in the tails of comets.
Had comets, asteroids, or meteorites delivered Earth’s water, the energy of such impacting bodies would have vaporized the transported water, leading to a runaway greenhouse on Earth, that would have made life on Earth permanently impossible. Although comets contain much water,b the water in comets did not provide much of Earth’s water, because the water contains too much heavy hydrogen, relatively rare in Earth’s oceans. Comets also contain too much argon. If comets provided even just 1% of Earth’s water, our atmosphere should have much more argon than it does.c Meteorites that contain water also have too much heavy hydrogen.d [Pages 310– 374 explain why comets, asteroids, and some types of meteorites contain so much water and heavy hydrogen. Pages 391– 429 explain why comets have so much argon. Heavy hydrogen is described on page 321.]
These observations cause some to conclude that water was transported from the outer solar system to Earth by objects that no longer exist. If so, many of these “water tankers” should have also collided with the other inner planets (Mercury, Venus, and Mars) as well. Actually, their water characteristics are not like those of Earth.e Instead of imagining “water tankers” that conveniently disappeared, perhaps we should ask if the Earth was created with its water already present.