Venus is hotter than Mercury primarily because of its very thick atmosphere composed mostly of carbon dioxide, which creates a strong greenhouse effect. This thick atmosphere traps heat from the Sun, preventing it from escaping back into space and causing surface temperatures on Venus to reach around 465 degrees Celsius (869 degrees Fahrenheit), hot enough to melt lead. In contrast, Mercury, despite being closer to the Sun, has almost no atmosphere to trap heat, so temperatures swing widely between extreme heat during the day and freezing cold at night. This makes Venus hotter overall than Mercury.

Key Reasons Venus Is Hotter Than Mercury

• Thick Carbon Dioxide Atmosphere : Venus has an extremely dense atmosphere (about 92 times Earth's surface pressure) dominated by CO2, a powerful greenhouse gas. This traps heat effectively in a runaway greenhouse effect, heating the planet's surface much more than solar distance alone would allow.

• Runaway Greenhouse Effect : The thick CO2 atmosphere absorbs and re-radiates infrared heat, causing temperatures to soar and remain consistently high throughout Venus's day and night, unlike Mercury's large temperature swings.

• Lack of Atmosphere on Mercury : Mercury has only a thin exosphere that cannot retain heat, leading to rapid cooling after sunset. This results in much lower average temperatures than Venus despite its proximity to the Sun.

• Reflectivity Differences : Venus reflects about 75% of sunlight due to sulfuric acid clouds but still traps the heat that penetrates. Mercury has less reflection but cannot hold heat because of its lack of atmosphere.

Therefore, it is the composition and density of Venus's atmosphere and the resulting greenhouse effect that make it hotter than Mercury, despite Mercury being closer to the Sun.