Mercury and Venus offer a compelling comparison as the two planets closest to the Sun. Mercury orbits at about 36 million miles, while Venus is farther at roughly 67 million miles. Based on proximity, one might assume Mercury is hotter due to receiving more solar radiation. However, a planet’s temperature is not determined by distance alone. The complex interplay of atmospheric composition and rotation rate dramatically alters this expectation, requiring a look at the unique thermal environments of both worlds.
The Counter-Intuitive Answer
Despite being the closest planet to the Sun, Mercury is not the hottest world. Venus is significantly hotter, holding the title for the hottest planet. Venus maintains a searing average surface temperature of approximately 867°F (464°C). In contrast, Mercury’s average surface temperature is much cooler, around 333°F (167°C). This difference highlights that atmospheric presence and composition are far more influential in determining a planet’s thermal state than solar distance alone.
Mercury’s Extreme Temperature Swings
Mercury’s thermal environment is characterized by a colossal temperature range, preventing it from retaining heat. The sunlit side can reach blistering highs of about 800°F (430°C). This intense heating results directly from its close proximity to the Sun, which delivers a high concentration of solar energy to the surface.
Mercury lacks any substantial atmosphere to regulate or trap this heat. It possesses only an extremely thin exosphere, which is ineffective at insulating the planet. Consequently, when a part of the planet rotates away from the Sun, absorbed thermal energy is rapidly radiated back into space.
The planet’s slow rotation also contributes to this dramatic thermal fluctuation. One solar day on Mercury lasts about 176 Earth days. This lengthy night allows temperatures to plummet drastically, falling to lows of approximately -290°F (-180°C). The difference between its day and night temperatures, exceeding 1,000°F (550°C), is the largest of any planet in the Solar System.
Venus and the Runaway Greenhouse Effect
Venus’s extreme heat is a direct consequence of its incredibly dense atmosphere. The atmosphere is composed primarily of carbon dioxide, accounting for about 96% of its gaseous makeup. Carbon dioxide is a highly effective greenhouse gas, meaning it readily traps infrared radiation.
This dense gaseous layer is approximately 92 times heavier than Earth’s atmosphere, creating immense surface pressure. The mass and composition of this atmosphere prevent heat from escaping into space, initiating a runaway greenhouse effect. Solar energy warms the surface, but the resulting infrared radiation is efficiently absorbed by the carbon dioxide.
The trapped heat continuously warms the planet, leading to an exceptionally high, uniform surface temperature across the entire world. This process ensures that the night side of Venus remains just as hot as the day side, with no significant temperature variation. This sustained, planet-wide thermal blanket is why Venus’s average temperature far exceeds Mercury’s, solidifying its status as the hottest planet.