Venus is hotter than Mercury despite being farther from the Sun due to its dense atmosphere, which is primarily composed of carbon dioxide. This thick atmosphere creates a strong greenhouse effect, trapping heat and raising surface temperatures to extreme levels. While Mercury is closer to the Sun and receives more solar radiation, it lacks a significant atmosphere to retain heat, leading to cooler temperatures. In contrast, Venus's atmosphere is about 90 times denser than Earth's, making it the hottest planet in our solar system with surface temperatures hot enough to melt lead.