Closer does not always mean hotter
Venus is hotter than Mercury because its thick, carbon-dioxide-rich atmosphere holds in heat through a powerful greenhouse effect. Mercury is closer to the Sun, but it lacks a thick atmosphere to retain heat.
Picture the planets in order: Mercury first, Venus second. Distance tells you something about the sunlight reaching them, but it does not tell you the whole story about their surfaces.
How an atmosphere holds in heat
Sunlight warms a planet, and the planet releases energy as heat. Greenhouse gases absorb some of the heat that would otherwise escape into space, making the surface warmer.
NASA explains that carbon dioxide produces this effect on both Earth and Venus. Venus has a much thicker atmosphere, made mostly of carbon dioxide, so the effect there is far stronger.
The clouds in a picture of Venus hide an intensely hot world below. Its temperature reflects what its atmosphere does with heat, as well as its distance from the Sun.
A hot day and a cold night
NASA’s temperature comparison gives Venus a mean surface temperature of about 464°C. Mercury can reach roughly 430°C during the day, then fall to around −180°C at night.
Those figures describe different things: an average for Venus, and daytime and night-time extremes for Mercury. Even Mercury’s scorching daytime high is below that average for Venus.
There is a useful question to ask when comparing worlds: what happens to the heat after it arrives?
Another world, another question
Temperature is only one planetary puzzle. Discover why Mars looks red, or explore why the Moon keeps the same face towards Earth.