The Solar System is home to four terrestrial, or rocky, planets: Mercury, Venus, Earth, and Mars. The hottest planet is determined not by distance alone, but by atmospheric composition. The hottest terrestrial planet, and indeed the hottest planet in the entire solar system, is Venus.
Venus and the Runaway Greenhouse Effect
Venus maintains an average surface temperature of approximately 867°F (464°C), which is hot enough to melt lead. This intense, uniform heat results from the runaway greenhouse effect. The atmosphere is overwhelmingly composed of carbon dioxide (about 96.5%). This dense layer acts like a planet-wide blanket, efficiently trapping solar energy.
The planet’s atmosphere is so thick that the pressure at the surface is about 92 times greater than Earth’s sea-level pressure. This immense atmospheric mass prevents heat from escaping back into space. The carbon dioxide absorbs infrared radiation emitted from the surface, constantly re-radiating the heat back down.
Because of this constant heat trapping, the surface temperature remains nearly identical across the entire planet, day and night. Venus rotates very slowly, with a solar day lasting longer than its year. Despite this slow rotation, there is no significant temperature difference between the sunlit side and the dark side.
The Proximity Paradox of Mercury
Mercury, the planet closest to the Sun, receives the most intense solar radiation, yet its average surface temperature is lower than Venus. Mercury reaches a peak temperature of about 800°F (430°C) on its sunlit side. The primary reason Mercury fails to retain its heat is its almost complete lack of an atmosphere.
Instead of a substantial atmosphere, Mercury possesses an extremely tenuous exosphere incapable of retaining heat. Solar energy that strikes the surface immediately radiates back into space once the surface rotates away from the Sun. This lack of a thermal blanket results in the most extreme temperature fluctuations in the Solar System.
When the surface rotates into the long night, the temperature plummets rapidly to lows of approximately -290°F (-180°C). This swing of over 1,000°F highlights the difference between a high peak temperature and a high average temperature. The average temperature of Mercury is only around 333°F (167°C), far below the consistent heat of Venus.
Temperature Differences on Earth and Mars
Earth and Mars provide a context of moderation and cold compared to the extremes of Venus and Mercury. Earth maintains a mild global average surface temperature of approximately 59°F (15°C). This moderate temperature is a consequence of Earth’s balanced atmospheric composition and its favorable orbital distance from the Sun.
Earth’s atmosphere, which is mostly nitrogen and oxygen, provides a sufficient greenhouse effect to keep the surface warm without becoming superheated. This balance allows for the stable existence of liquid water, a condition unique among the terrestrial planets. Mars presents the coldest environment of the four rocky worlds.
Mars has a very thin atmosphere, composed primarily of carbon dioxide, much like Venus. However, the Martian atmosphere is so sparse that it fails to create a significant heat-trapping effect. The average surface temperature on Mars is -85°F (-65°C), though temperatures can briefly rise to about 70°F (20°C) near the equator during the day.