Dear Tom,
I have heard that more than 65 percent of the Earth is covered with water. Where did all that water come from?

Steven Corser, Davenport, Iowa

Dear Steven,
Water is the most abundant material on the surface of the Earth. The actual percentage of water coverage is about 71 percent. It has been suggested that “Water” might be a more appropriate name for our planet, and not Earth.

The source of our planet’s water is still an open question, but there is widespread belief that the majority of it has come from the Earth itself. It has arrived as water vapor from the exhalations of volcanoes. It is estimated that volcanoes have provided from 70 to 90 percent of the Earth’s water.

The remaining 10 to 30 percent of the world’s water is believed to have arrived from collisions with comets (which contain a good deal of ice).