Mercury orbits closer to the Sun than any other planet, yet Venus consistently holds the title of the hottest planet in the Solar System. This presents a paradox, as proximity to the Sun suggests Mercury should be hotter. Venus, the second planet, maintains an average surface temperature of approximately 464 degrees Celsius (867 degrees Fahrenheit). Mercury’s average temperature, by contrast, is significantly lower, at about 167 degrees Celsius (333 degrees Fahrenheit), despite receiving intense solar radiation. The explanation for this difference lies not in the distance from the Sun, but in how each planet manages and retains absorbed heat.
Temperature Dynamics on Mercury
Mercury experiences the greatest temperature swings of any planet, directly related to its lack of a substantial atmosphere. It possesses only an extremely thin layer of gas, called an exosphere, which is too tenuous to trap or distribute heat. Solar energy striking the sunlit side is absorbed by the rocky surface, raising temperatures dramatically.
During the lengthy day, which lasts about 59 Earth days, equatorial temperatures can soar to nearly 430 degrees Celsius (806 degrees Fahrenheit). However, the absence of an insulating atmospheric blanket means that when the surface is plunged into darkness, stored heat radiates rapidly back into space. This fast thermal emission causes nighttime temperatures to plummet to approximately -180 degrees Celsius (-292 degrees Fahrenheit).
This massive thermal fluctuation between day and night pulls Mercury’s overall average temperature down considerably. Since the planet cannot retain the heat it receives, the cold temperatures of the night side significantly influence the planetary average. Mercury’s slow rotation contributes to this extreme contrast by allowing the day side to heat and the night side to cool over long cycles.
The Runaway Greenhouse Effect on Venus
Venus’s consistently scorching temperatures are a consequence of the runaway greenhouse effect. The planet is shrouded by an extraordinarily dense atmosphere, creating an insulating layer that traps thermal energy with exceptional efficiency. This atmosphere is composed of over 96 percent carbon dioxide, a potent heat-trapping gas.
The density of this gaseous envelope is extreme, resulting in surface atmospheric pressure more than 90 times greater than Earth’s sea level pressure. Solar radiation penetrates the thick cloud layers, which are primarily sulfuric acid droplets, warming the planetary surface. The surface then re-emits this energy as infrared radiation.
The immense concentration of carbon dioxide in the lower atmosphere absorbs this outgoing infrared energy almost completely, preventing it from escaping into space. This trapping mechanism continually raises the temperature of the surface and lower atmosphere. The process maintains a uniformly high temperature of about 464 degrees Celsius across the entire globe, day and night. This constant, intense heat makes Venus the hottest world in the Solar System, exceeding even the maximum temperatures reached on Mercury’s day side.
Comparing the Heat Retention Mechanisms
The fundamental difference between the two planets is the presence of an atmosphere capable of effective heat retention. Mercury receives the highest solar energy input per unit area due to its proximity to the Sun. However, its virtually non-existent atmosphere means this energy is not retained, leading to vast temperature differences and a low planetary average.
Venus, although farther from the Sun and receiving only about 25 percent of Mercury’s solar energy, possesses a dense carbon dioxide atmosphere that functions like a thick thermal blanket. This insulation overrides the advantage Mercury gains from its closer orbit.
Venus’s dense atmosphere prevents thermal energy from radiating back into space, maintaining a high, stable temperature. This comparison reveals that atmospheric composition and density are far more influential in determining a planet’s overall surface temperature than its distance from the Sun. Mercury’s high daytime temperatures are peaks that cannot be sustained, while Venus’s temperature represents a constant, stable baseline. The runaway greenhouse effect on Venus is the determining factor, establishing the planet as the hottest despite its greater distance.