I was surprised to see no mention of small comets hitting Earth’s atmosphere over the eons as a source of surface water. This explanation, based on atmospheric observations, has gained growing acceptance over the past several years.

Kenneth Saum
Cotuit, Mass.

Jonathan I. Lunine says that if local bodies provided water to Earth, then “Mars [too] should have been swimming in water.” Many years ago, Immanuel Velikovsky (whose theories are generally sneered at by scientists) posited that Earth received a great deal of its water from Mars in a near-collision of the two planets. His theory would explain the relative wet-dry states of the sister planets.

Christina Russell
Sugar Land, Texas

It seems premature to abandon comets as the source of Earth’s water based on the higher percentage of deuterium found in comets Halley, Hyakutake, and Hale-Bopp as compared with present-day terrestrial water. First, a three-comet sample is incredibly small. Second, perhaps earlier cometary water was made up of the lighter hydrogen form.

William Britton
Bayville, N.Y.