Four billion years ago, when the sun was young and dim, Earth may have stayed warm in part because it was spinning faster.
The sun, astronomers believe, wasn’t always as bright as it is today. Four billion years ago it was probably about 30 percent dimmer. Although the sun had the same mass then as it does now, it was a little larger and less dense. Over the eons, as it contracted under its own gravity, it grew hotter and brighter. Astronomers have no problem with this picture--it seems to be the typical way stars evolve--but geologists do. If the young sun was so feeble, Earth should have been a frozen wasteland. And yet the oldest sedimentary rocks--which were deposited in water--are themselves 4 billion years old. Somehow Earth’s surface was warm enough for liquid water despite a wan sun.
Geologists have weaseled out of this faint young sun paradox ...
