Dear Tom,

Where did the Earth’s water come from?

Jimmy Carver, Rockford

Dear Jimmy,

Water is the most abundant substance on the surface of the Earth, covering 71 percent of it. The source of all that water is still an unanswered question, but there is widespread belief that the majority of it has come from volcanoes. The water is the accumulated result of a few billion years of volcanic activity. Gases exhaled from volcanoes contain a great deal of vapor, among other things.

Another source of the Earth’s water is thought to be from space: from collisions of the Earth with asteroids and comets. Asteroids and comets, especially “old” ones that were around a few billion years ago, contain a great deal of ice. Collisions between the Earth and these asteroids and comets are thought to have delivered water (in the form of ice) to the young Earth.

Best estimate is that up to 30 percent of the Earth’s water has been delivered to the planet by comets and asteroids and that the remainder has come from the Earth itself, from volcanoes.