Mercury is the closest planet to the sun, orbiting at an average distance of 58 million kilometres from the sun, compared to Venus’s 108 million kilometres and the earth’s 150 million kilometres. It is thus reasonable to assume that Mercury would be hotter than Venus and earth, given its close proximity to our parent star. However, average surface temperature on Mercury is actually lower than Venus!

An image of the surface of Venus, extrapolated to the horizon from images taken by the Soviet Venera 13 probe and processed to remove the fish-eye effect.

The reason for this lies in the fact that Venus has an extensive atmosphere, as opposed to Mercury, which has practically no atmosphere at all. With its thick carbon dioxide atmosphere, Venus has been the subject of a runaway greenhouse effect, with surface temperatures soaring over billions of years to a present day scorching average temperature of over 450 degrees Celsius! At a surface pressure over 90 times that experienced on earth, Venus’s surface is one of the most inhospitable regions known in the solar system.

While Mercury experiences similar surface temperatures in regions directly exposed to the sun, the night-time side of Mercury experiences extremely cold temperatures, as low as -200 degrees Celsius. Without any atmosphere to trap incoming heat from the sun, Mercury has no way to retain heat at the surface over extended periods of time.