Earth’s Dechlorination

That complex life could evolve on Earth involved a set of fantastic coincidences. One of them was the prolonged pummeling of the planet for 800 million years, starting over 4.5 billion years ago. Seeding the planet with cosmic debris was essential to its evolution, as was what was taken off in the process. One benefit of bombardment was ridding Earth of most of its chlorine.

The composition of ancient meteorites indicates that Earth should have ten times the chlorine that it does. Mars, which suffered much less assault, has more than twice the chlorine of Earth.

The four halogen elements, including chlorine, do not readily dissolve in metals. Nor do they often combine with other elements to form rock minerals.

Thus chlorine is concentrated on the surface. Much of Earth’s chlorine that is not in the ocean lies in salt deposits and brines.

The relentless bombardment of early Earth engendered life later, by scouring much of the chlorine off the planet. If not, the world’s oceans would have been too salty for complex life to evolve.

Additionally, chlorine-rich seas would have reduced precipitation. With less rain there would have been less erosion, and fewer nutrients washing into the ocean to foster life.


Ishi Nobu, Spokes of the Wheel, Book 1: The Science of Existence, 2nd edition (pre-publication) (2019).

Erin Wayman, “Maybe Earth’s chlorine blew away,” Science News 183(11): 14 (1 June 2013).