Science Puzzle
Why Is Venus Hotter Than Mercury?
Mercury is the closest planet to the Sun, 58 million kilometres away. Venus is almost twice as far out, at 108 million kilometres, and gets less than a third as much sunlight on each square metre. Yet Venus is the hottest planet in the solar system, at about 465 °C day and night.
Why is Venus hotter than Mercury?
The Answer
Venus is wrapped in an enormously thick atmosphere, about 96% carbon dioxide, which traps heat. Mercury has almost no atmosphere at all.
The air pressure on Venus's surface is about 90 times that on Earth, like being nearly a kilometre under the sea. Thick clouds of sulphuric acid reflect much of the sunlight away, but the light that does get through warms the ground. The warm ground gives off heat as infrared radiation, and carbon dioxide is very good at absorbing infrared. The heat can't escape, so it builds up until the surface reaches about 465 °C, hot enough to melt lead, and it stays that hot all night too.
Mercury has nothing to hold its heat in. Its sunny side bakes at about 430 °C, but its night side, facing out into space, radiates its heat away and plunges to about -180 °C.
Venus shows the greenhouse effect at its most extreme. Earth has a much gentler version: our thin blanket of water vapour, carbon dioxide and other gases keeps the planet about 33 degrees warmer than it would otherwise be, which is what makes it comfortable for life.
The principle: The greenhouse effect. Gases such as carbon dioxide let sunlight in but absorb the infrared heat a planet gives off, so a thick carbon dioxide atmosphere can make a planet hotter than one much closer to the Sun.