Closer to the Sun doesn't mean hotter
Mercury is the first planet from the Sun, Venus the second, so you might expect Mercury to hold the heat record. But it has no atmosphere: by day, its rocks heat up to about +430 °C, and at night they cool to about -170 °C. Heat arrives with sunlight and immediately escapes again.
Venus is different: it's about +460 °C by day and by night, even on mountaintops. Lead would melt there. The planet rotates slowly, with night lasting about 2 Earth months, but the surface barely cools during that time.
Nearly twice as close to the Sun, with no atmosphere. About +430 °C by day and about -170 °C at night: heat escapes at once.
Farther from the Sun, with a dense atmosphere. About +460 °C around the clock, with no noticeable day-night difference.
Venus's atmosphere lets in light but traps heat
The planet is completely covered by clouds of sulfuric acid. They reflect about three-quarters of sunlight back into space: Earth reflects about a third, Mercury about a tenth. Even less light reaches Venus's surface than Earth's, so sunlight alone can't explain this heat.
The light that makes it down warms the rocks. They give off heat not as visible light but as infrared radiation - and carbon dioxide eagerly absorbs that. There is a lot of gas: pressure at the surface is about 92 times Earth's, like being 900 meters underwater. Radiation gets trapped in this thick layer, and only a tiny fraction escapes.
Without the greenhouse effect, but with clouds reflecting just as much light, the surface would be about -40 °C. Everything between that and the actual +460 °C is the work of carbon dioxide. Earth has a greenhouse effect too, adding about 33 degrees: the difference is not the phenomenon but its scale. And the heat is held near the ground - higher up, pressure is lower and it gets cooler.
Where Venus got so much carbon dioxide
It has about as much carbon as Earth - it's just in a different place. On Earth, almost all carbon is locked up in limestone: for millions of years, rain and oceans have carried it from the air into rock. Release it all back, and Earth would have an atmosphere comparable to Venus's.
On Venus, this process broke down. Water is thought to have once existed there. Water vapor is also a greenhouse gas: the hotter it got, the more evaporated. High up, sunlight split water molecules, and light hydrogen escaped into space - the water was lost forever, leaving carbon dioxide with nowhere to go. Astronomers still debate whether Venus had real oceans or only a moist atmosphere.
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Sources
- Temperature, pressure, and atmospheric composition: NASA Planetary Fact Sheet. The lost-water hypothesis is based on the hydrogen-to-deuterium ratio measured by the Pioneer Venus and Venus Express probes