Why is Venus so hot? A runaway greenhouse, explained

Venus is so hot because its atmosphere, almost all carbon dioxide and about 90 times as heavy as Earth's, traps the Sun's heat in a runaway greenhouse effect. Its surface sits near 465°C, day and night, hot enough to melt lead. That is why Venus is hotter than Mercury, though it is almost twice as far from the Sun.

In short

• Venus is the hottest planet because of its air, not its distance: a thick carbon-dioxide atmosphere lets sunlight in and holds heat back.
• The greenhouse ran away: as the planet warmed, more heat-trapping gas entered the air, which warmed it further, until any oceans were gone.
• Venus also spins slowly backwards, so a single day there, sunrise to sunrise, lasts about 117 Earth days.

How hot is Venus, and how does it compare with Mercury?

According to NASA's Venus fact sheet, the average surface temperature is 737 kelvin, about 464°C. The surface pressure is 92 bars, 92 times the air pressure at sea level on Earth, which NASA compares to being more than 900 meters underwater. The air near the ground is 96.5% carbon dioxide and 3.5% nitrogen, and above it hang thick clouds of sulfuric acid droplets.

Mercury is about 58 million kilometers from the Sun, Venus about 108 million. Mercury's sunny side reaches about 430°C, but it has almost no atmosphere, so its night side falls to about −180°C. Venus is close to 465°C everywhere: on the day side and the night side, at the equator and near the poles. Its dense air carries heat around the whole planet.

How does a greenhouse effect make a planet hot?

The greenhouse effect works the same way on any planet. Sunlight, mostly visible light, passes through the air and warms the ground. The warm ground gives off infrared radiation, heat, and some gases, carbon dioxide and water vapor above all, absorb infrared and send part of it back down. The surface has to get warmer until enough heat escapes to balance the sunlight coming in.

On Venus there is so much carbon dioxide that very little infrared escapes straight from the surface to space. Heat gets out only from high in the atmosphere, so the ground below has to be extremely hot to push it there.

Worked example: Venus without its greenhouse would be cold

A planet's temperature with no greenhouse effect at all can be worked out from two numbers: how much sunlight reaches it, and what fraction it reflects, called its albedo. The rule balances sunlight absorbed against heat radiated: temperature = (sunlight × (1 − albedo) ÷ (4 × σ)) to the power ¼, where σ is the Stefan–Boltzmann constant, 5.67 × 10⁻⁸ in watts per square meter per kelvin⁴. The 4 spreads the sunlight over the whole sphere.

• Venus receives about 2,601 watts per square meter of sunlight, nearly twice Earth's 1,361. But its bright clouds reflect about 77% of it.
• Absorbed: 2,601 × 0.23 = about 598 watts per square meter, or about 150 averaged over the sphere.
• Temperature: (150 ÷ 0.0000000567) to the power ¼ = about 227 kelvin, or −46°C.

Earth, doing the same sum with its 29% albedo, comes out at about 255 kelvin, −18°C. So without a greenhouse effect, Venus would be colder than Earth. Its real surface is 737 kelvin. The greenhouse adds about 510 degrees on Venus, against about 33 on Earth.

	Mercury	Venus	Earth
Distance from the Sun	about 58 million km	about 108 million km	about 150 million km
Atmosphere	almost none	92 bars, 96.5% carbon dioxide	1 bar, about 0.04% carbon dioxide
Surface temperature	about 430°C by day, −180°C at night	about 465°C day and night	about 15°C average
Warming from the greenhouse effect	almost none	about 510°C	about 33°C

How did Venus's greenhouse run away?

Venus and Earth formed at about the same time, from similar material, at about the same size. The difference is where the carbon dioxide ended up. On Earth, most of it is locked in rocks such as limestone, pulled out of the air by rain and oceans over billions of years. On Venus it is in the air.

The usual account runs like this. If Venus started with water, the stronger sunlight at its distance warmed it, and warmer oceans put more water vapor, itself a greenhouse gas, into the air. That trapped more heat, which evaporated more water: a feedback that runs away instead of settling. High in the atmosphere, ultraviolet sunlight split the water molecules, and the light hydrogen escaped to space. With no oceans left, nothing pulled carbon dioxide out of the air, while volcanoes kept adding more. Today Venus holds only about 20 parts per million of water vapor near its surface.

Still argued

Whether Venus ever had oceans is open. A 2016 NASA climate model led by Michael Way at the Goddard Institute for Space Studies suggested early Venus could have kept a shallow ocean and mild temperatures for up to 2 billion years. A 2021 Nature paper by Martin Turbet and colleagues modeled the clouds differently and found that steam on young Venus would never have condensed into oceans at all. Missions selected in 2021, NASA's DAVINCI and VERITAS and ESA's EnVision, were designed in part to test this. Separately, a 2020 report of phosphine gas in Venus's clouds, a possible sign of life, was challenged by reanalyses soon after, and remains disputed.

Why does Venus spin backwards?

Venus turns on its axis once every 243 Earth days, in the opposite direction to most planets, so the Sun rises in the west. Its year is only about 225 Earth days, so it spins more slowly than it orbits. Combined, the two motions make a solar day, from one sunrise to the next, of about 117 Earth days.

Why it rotates this way is not settled. One idea is a giant collision early in its history. Another is that the Sun's pull on its heavy atmosphere, along with friction inside the planet, slowed and reversed its spin over billions of years. Venus also has no global magnetic field of its own, and its slow spin is one suggested reason.

What does Venus tell us about other planets?

Venus is the clearest example in the solar system of how much a planet's air decides its climate. Mars went the other way, losing most of its atmosphere, which is why Mars is a frozen desert. Astronomers searching for planets that might hold life around other stars in the universe use Venus as the warning case: a planet can sit at a reasonable distance from its star and still be uninhabitable.

Check yourself

• Without looking back: why is Venus hotter than Mercury, when Mercury is closer to the Sun?
• Using the worked example, explain why Venus would be colder than Earth if it had no greenhouse effect.
• What makes a greenhouse effect run away rather than settle at a new, warmer temperature?

Sources

• Venus fact sheet — NASA Goddard Space Flight Center
• Venus facts — NASA Science
• Mercury facts — NASA Science
• NASA climate modeling suggests Venus may have been habitable — NASA, 2016
• Day–night cloud asymmetry prevents early oceans on Venus but not on Earth — Turbet and others, Nature, 2021
• Phosphine gas in the cloud decks of Venus — Greaves and others, Nature Astronomy, 2020