What is the main reason that Venus has a much higher temperature than Earth?

Venus has a much higher temperature than Earth primarily due to its thick atmosphere composed predominantly of carbon dioxide. This greenhouse gas plays a crucial role in trapping heat in the atmosphere. When sunlight reaches Venus, the surface absorbs that energy and subsequently radiates it as infrared radiation. However, the carbon dioxide in the atmosphere absorbs much of this radiation and re-emits it in all directions, including back to the surface. This process significantly enhances the greenhouse effect, causing surface temperatures on Venus to soar, making it the hottest planet in our solar system despite being further from the Sun than Mercury.

In contrast to the other options, while Venus is indeed closer to the Sun than Earth, the difference in distance alone does not account for the extreme temperatures observed. The composition of its atmosphere, particularly the presence of substantial carbon dioxide, is the defining factor in its ability to retain heat. The idea that Venus absorbs more sunlight due to being darker is misleading because its heavy cloud cover reflects much sunlight instead of absorbing it. Lastly, methane is not a significant component of Venus's atmosphere; the planet’s high temperatures and pressure arise mainly from carbon dioxide. Thus, the atmosphere's composition is the primary driver of Venus’s extreme temperatures.