Being closest to the sun, the planet Mercury was initially also thought to be the hottest planet in our solar system, receiving more energy from the sun than any other body in the solar system. However, it has since been confirmed that the planet Venus, although approximately twice the distance away from the sun, is even hotter than Mercury at its surface.
The reason for this unexpected phenomenon lies in Venus’s massive atmosphere and the gases that make it up. Mercury has little to no atmosphere, thus reducing its ability to hold on to any heat other than that which hits it directly from the sun. This, in effect, means that Mercury has scorching temperatures of up to 420oC during the day and absolutely frigid temperatures of down to -220oC during the night.
On the other hand, Venus has an atmospheric density 90 times that of Earth’s at the surface. Its surface landscape is shrouded from direct view from Earth by thick clouds of sulfuric acid, which are highly reflective to visible light. Its atmosphere, mainly composed of carbon dioxide, has led to a runaway greenhouse gas effect on the planet, with temperatures rising slowly over millions of years to today’s average 460oC.
Being a greenhouse gas, carbon dioxide has been trapping heat in Venus’s atmosphere, and any lakes of liquid water that are thought to have been prevalent on Venus’s surface in the past have been replaced by a dry, barren landscape. For this reason, Venus can hold on to most of its heat even during Venusian night time, with very limited change in overall temperatures at the surface.