Why Venus Stays Hotter Than Mercury
Mercury is closest to the Sun, but distance alone does not determine a planet’s surface temperature. A world also needs to shed the energy it absorbs. Venus’s atmosphere makes that difficult: it is extremely thick and composed mostly of carbon dioxide, a gas that absorbs heat radiated upward from the ground.
Sunlight In, Heat Trapped
Sunlight passes through the atmosphere and warms Venus’s surface. The ground then gives off energy as infrared radiation—the kind of radiation we experience as heat. Carbon dioxide absorbs much of that outgoing energy and sends it in different directions, including back toward the surface. This slows the planet’s loss of heat to space.
The process is called the greenhouse effect. It occurs on Earth too, but Venus’s dense atmosphere makes it extraordinarily powerful. Heat is repeatedly absorbed and redistributed through the atmosphere rather than escaping efficiently.
A Planet-Wide Furnace
Venus’s thick blanket also spreads heat around the planet, so its surface stays scorching even far from the point receiving the most direct sunlight. The average temperature at the ground is about 880°F (470°C)—hot enough to melt lead. Mercury, by contrast, has only a very thin atmosphere, so it cannot hold heat in the same way; its surface temperatures swing sharply between day and night.
That contrast is the twist behind Venus’s record: the nearest planet gets more sunlight, but Venus’s atmosphere keeps the absorbed energy from escaping. Its persistent heat is not simply a matter of how much sunlight arrives, but of how effectively the planet can let that energy go.
