Why is Venus hotter than Mercury, which is closer to the Sun?

Venus is hotter than Mercury because of its atmosphere. Venus is nearly twice as far from the Sun as Mercury and receives only about a quarter as much sunlight, yet its surface averages about 464 °C, hot enough to melt lead. The reason is a thick atmosphere made mostly of carbon dioxide, which traps heat in the strongest greenhouse effect in the Solar System. Sunlight gets in, but the heat can't get back out.

Mercury has the opposite problem. It lacks an atmosphere to trap heat, so there is nothing to hold the warmth in. Its surface swings between about 427 °C and about −173 °C. Being closest to the Sun only makes it the second hottest planet.

Venus's air is also crushingly heavy: the pressure at the surface is about 93 times Earth's, like being nearly a kilometre under the sea. Scientists think Venus may once have had liquid water before a runaway greenhouse effect boiled it away. That's why researchers like Carl Sagan pointed to Venus as a warning about climate change on Earth. In the early 1960s, when some imagined Venus as a balmy paradise, Sagan argued from its radio waves that it was dry and extremely hot, and the Mariner 2 probe proved him right in 1962.