Venus is the hottest planet in the solar system, with an average surface temperature of 867°F (464°C), hot enough to melt lead. That’s hotter than Mercury, even though Mercury orbits much closer to the sun. The reason comes down to one thing: a massive, suffocating atmosphere that traps heat and never lets it go.
The Atmosphere Is the Key
Venus has an atmosphere roughly 90 times heavier than Earth’s. Standing on the surface would feel like being 3,000 feet underwater. That atmosphere is over 96% carbon dioxide, which is the same gas driving climate change on Earth, just in staggeringly larger quantities. On Earth, CO2 makes up about 0.04% of the atmosphere. On Venus, it’s nearly the entire atmosphere, and there’s far more of it pressing down.
Carbon dioxide is effective at trapping heat because it absorbs infrared radiation, the kind of energy that a warm surface radiates back toward space. When sunlight warms the rocks and terrain on Venus, those surfaces re-emit that energy as infrared light. But instead of escaping into space, the infrared radiation gets absorbed by the thick blanket of CO2 overhead. The atmosphere heats up, radiates some of that energy back down to the surface, and the cycle continues. This is the greenhouse effect, and on Venus it adds roughly 500°C (900°F) of warming on top of what the planet would otherwise experience.
Why Clouds Make It Worse, Not Better
Venus is wrapped in dense clouds of sulfuric acid, likely produced by volcanic activity releasing sulfur into the atmosphere, where it combines with water and oxygen. These clouds reflect about 80% of incoming sunlight back into space, giving Venus one of the highest reflectivity ratings of any planet. You might expect all that reflection to keep Venus cool. It does the opposite.
The 20% of sunlight that does penetrate the clouds reaches the surface and warms it. The surface then radiates infrared energy upward, but those same clouds act as a lid. Infrared radiation cannot pass back through the cloud layers. It gets trapped below, bouncing between the surface and the atmosphere. The clouds also contain sulfuric acid droplets and water vapor, which absorb infrared at wavelengths where CO2 absorption is weak, plugging the gaps and making the heat trap nearly airtight.
Hotter Than Mercury, Despite Being Farther Away
Venus sits about 67 million miles from the sun, while Mercury orbits at roughly 36 million miles. Venus receives almost twice the solar energy that Earth does (about 2,643 watts per square meter compared to Earth’s 1,370), but Mercury gets even more. Yet Mercury’s average temperature is far lower than Venus’s because Mercury has essentially no atmosphere. Whatever heat Mercury’s surface absorbs during the day radiates freely back into space at night. Venus holds onto its heat permanently.
This also explains one of Venus’s strangest features: the temperature barely changes anywhere on the planet. The night side is almost as hot as the day side. Temperatures don’t drop significantly at the poles compared to the equator, either. Infrared measurements from Japan’s Akatsuki orbiter confirmed that the average nightside surface temperature hovers around 425°C (797°F). Only about 2.5% of solar energy actually reaches the surface directly, so the sun’s position overhead matters very little. The heat comes from the atmosphere itself, which distributes warmth uniformly across the entire planet like an oven set to a fixed temperature.
How Venus Got This Way
Venus wasn’t always a furnace. Scientists believe it once had liquid water on its surface, possibly for hundreds of millions of years. The turning point was the loss of that water, which set off a chain of events that made the planet’s heating permanent.
The process started with ultraviolet light from the sun breaking apart water molecules high in the atmosphere, splitting them into hydrogen and oxygen. Hydrogen is light enough to escape a planet’s gravity, and on Venus it did, steadily leaking into space over roughly a billion years. The evidence for this is still measurable today: Venus’s atmosphere contains an unusually high ratio of deuterium (a heavier form of hydrogen) to regular hydrogen. Because deuterium is too heavy to escape as easily, it got left behind while the lighter hydrogen drifted away. That imbalance is a chemical fingerprint of massive water loss.
Losing the ocean was catastrophic because liquid water is what keeps CO2 levels in check on a planet. On Earth, rain dissolves atmospheric CO2 and carries it into the oceans, where it eventually gets locked into rocks on the seafloor. This cycle pulls CO2 out of the air almost as fast as volcanoes put it in. Without an ocean, Venus had no way to recycle the CO2 that volcanoes kept pumping out. It simply accumulated in the atmosphere, layer upon layer, over billions of years. Each new addition of CO2 trapped more heat, which prevented any chance of water condensing again, which meant even more CO2 could build up. This self-reinforcing loop is what scientists call a runaway greenhouse effect.
Why It Can’t Cool Down
The reason Venus stays locked in this state is that every natural cooling mechanism has been shut off. There’s no ocean to absorb CO2. There’s no rainfall to weather rocks and pull carbon from the air. The atmosphere is so thick and opaque to infrared that even the small fraction of heat that might escape gets reabsorbed before it reaches space. The planet reached what climate scientists describe as a “baked crust” state, a permanent greenhouse with no exit.
On Earth, if you doubled or tripled the CO2 in the atmosphere, temperatures would rise significantly, but the ocean and rock weathering would eventually pull some of that carbon back out over thousands of years. Venus shows what happens when those safety valves disappear entirely. The planet became a one-way street: CO2 goes in, nothing takes it out, and the temperature ratchets up until it stabilizes at a point where the surface glows with heat and the rocks themselves are warm enough to soften certain metals.