The surface of Venus averages a blistering 464°C (867°F), making it the hottest world in the solar system. Mercury, the closest planet to the Sun, receives a far greater amount of solar energy, yet its average surface temperature is significantly lower at approximately 167°C (333°F). This counter-intuitive difference highlights that a planet’s distance from the Sun is not the sole factor determining its surface temperature. The true explanation lies in the fundamental differences in how these two worlds manage the heat they receive.
Solar Energy Received by Mercury and Venus
Mercury orbits the Sun at an average distance of about 57.9 million kilometers, while Venus is nearly twice as far away, orbiting at around 108.2 million kilometers. Because the intensity of sunlight decreases with the square of the distance from the source, Mercury receives a significantly greater amount of solar energy per unit area than Venus. The solar irradiance at Mercury is approximately 9,116 watts per square meter, compared to Venus’s 2,611 watts per square meter.
This energy comparison establishes the baseline expectation that Mercury should be the hotter planet. The initial input of energy is higher at Mercury, but the planet’s characteristics prevent it from converting that input into consistently high temperatures.
Mercury’s Heat Loss Problem
Mercury’s struggle to retain heat stems from its near-total lack of a substantial atmosphere. The planet is too small and too hot to hold onto a dense envelope of gas, resulting in only a trace amount of gas referred to as an exosphere. This extremely tenuous exosphere is ineffective at insulating the planet’s surface.
Without a thick layer of atmosphere to trap heat, the solar radiation that strikes Mercury’s surface is quickly radiated back into space as infrared energy. This rapid loss of heat causes enormous temperature swings between the planet’s day and night sides. While the sunlit side can reach a scorching 430°C (800°F), the side facing away from the Sun plummets to an extremely cold -173°C (-280°F).
The planet’s slow rotation, with one solar day lasting 176 Earth days, exacerbates this problem by allowing the night side to cool for a prolonged period. Mercury’s average temperature is calculated by combining these extreme day and night temperatures, which results in a value much lower than Venus’s constant heat.
The Runaway Greenhouse Effect on Venus
The reason for Venus’s extreme heat is its extraordinarily dense and thick atmosphere, which creates the most powerful greenhouse effect in the solar system. This atmosphere is composed overwhelmingly of carbon dioxide, which makes up about 96.5% of the total gas volume. The atmospheric pressure at the surface is about 93 times that of Earth’s.
The mechanism for this extreme heating begins when visible sunlight penetrates the atmosphere and heats the surface. The planet’s surface then re-radiates this energy as infrared radiation. However, the massive quantity of carbon dioxide acts as a thick, insulating blanket, effectively blocking this infrared radiation from escaping back into space.
This process is so intense that it is described as a “runaway” greenhouse effect, a self-sustaining cycle where rising temperatures cause more heat to be trapped. The atmosphere traps the heat so efficiently that the surface temperature reaches a constant high of 464°C (867°F). The sheer density of Venus’s atmosphere also ensures that powerful winds circulate the heat uniformly across the entire planet, preventing any significant temperature difference between the day and night sides.