Why Is Venus Hotter Than Mercury?

Mercury, the closest planet to the Sun, has virtually no atmosphere — so it cannot retain solar heat. Its surface temperature swings between -180 degrees Celsius at night and 430 degrees Celsius during the day. Venus, though 108 million km from the Sun (vs Mercury 58 million km), has the densest atmosphere of any rocky planet: 92 times Earth surface pressure, composed of 96.5% CO2 with thick sulfuric acid clouds. This massive CO2 blanket creates a runaway greenhouse effect: solar radiation passes through the clouds (which reflect 75% of sunlight, but enough gets through), heats the surface, and the re-radiated infrared energy is trapped by the CO2. The result is a nearly uniform surface temperature of about 465 degrees Celsius — hot enough to melt lead and zinc. Venus is a cautionary tale about runaway greenhouse warming. Scientists believe Venus may have had liquid water oceans early in its history, but solar brightening triggered a feedback loop that boiled the water, adding more greenhouse gas, until the planet became the inferno it is today.