Why is Venus hotter than Mercury, though farther from the Sun?

Distance from the Sun is only one factor in a planet's temperature. Venus has a very thick atmosphere made mostly of carbon dioxide that traps heat, while Mercury has almost no atmosphere to hold heat in. Venus's surface averages about 464 to 467 degrees Celsius, hotter than the hottest surface temperatures on Mercury. [1] [2] [3]

NASA describes the atmosphere of Venus as mostly carbon dioxide, with dense clouds of sulfuric acid. Surface pressure is about 93 times that at sea level on Earth, and NASA says the thick atmosphere traps heat in a runaway greenhouse effect. The BBC Sky at Night Magazine adds that carbon dioxide absorbs infrared radiation, both from incoming sunlight and from the heated planet, which stops heat escaping to space. [2] [1]

Astrophysicist Stephen Kane told Live Science that infrared energy leaving the surface is absorbed and re-emitted many times before any of it escapes, and some is sent back down, further warming the surface. Because of this, the temperature barely changes between day and night or from place to place. [3] [1]

Mercury has only a very thin exosphere, in which atoms are so sparse that they rarely collide. Such a layer cannot retain heat. Sunlight heats bare rock to roughly 430 degrees Celsius by day, but after sunset the surface cools quickly, and night temperatures fall to about minus 180 degrees Celsius. [1] [3]

One explainer from University College London says Mercury lacks an atmosphere because it is small and its gravity is too weak to hold one. [4]

Venus does not take in more sunlight than Mercury. According to Kane, its clouds reflect roughly three-quarters of incoming sunlight, and only about 3 percent reaches the surface. He said a bare-rock Venus with no atmosphere or clouds would be cooler than Mercury, because it receives only about 29 percent as much sunlight to begin with. A UC Santa Barbara answer makes a similar point, saying that without an atmosphere Venus would be colder than Mercury's average temperature. In Kane's phrasing, it is the blanket, not the sunbathing, that makes Venus hotter. [3] [5]

How Venus acquired its atmosphere is described by Live Science as an open question. Planetary scientist Paul Byrne points to evidence of past and present volcanic and tectonic activity adding greenhouse gases, and describes modern Venus as in a post-runaway greenhouse state, where the heat sustains itself because of the atmosphere already in place rather than heat from the interior. [3]