Lessons from Extremes

Mercury and Venus bracket two extremes of planetary failure. Mercury is too small to hold an atmosphere, so it cannot regulate temperature. Venus held onto its atmosphere too well, trapping so much heat that any oceans it may have had boiled away billions of years ago.

Venus is a warning about runaway greenhouse effects. Scientists believe Venus once had liquid water and moderate temperatures. But as the Sun slowly brightened, surface water evaporated, adding more greenhouse gas, which trapped more heat, which evaporated still more water — a feedback loop that boiled the oceans dry.