Why Is Venus So Hot? The Runaway Greenhouse Effect on Earth’s Twin

At first glance, Venus seems like Earth’s twin. It is almost the same size, has a similar mass and a rocky surface, and it comes closer to Earth than any other planet. In the night sky, it shines brighter than any star. But beneath its bright, creamy clouds lies one of the most hostile places in the solar system: a scorching, crushing world where lead would melt and spacecraft cook within hours. NASA calls it Earth’s “evil twin”. So what went wrong?

Hotter than the planet closest to the Sun

Here is the puzzle. Venus is about twice as far from the Sun as Mercury, so it receives only about a quarter as much sunlight. Yet according to NASA, the surface of Venus averages about 467 °C, making it the hottest planet in the solar system.

Mercury’s daytime side can reach about 430 °C, but at night it plunges to around −180 °C, because it has almost no atmosphere to hold heat. Venus, by contrast, is just as hot at the poles as at the equator, and at midnight as at noon. The difference is not distance from the Sun. It is air.

A crushing blanket of carbon dioxide

Venus’s atmosphere is extraordinarily thick. It is made of about 96.5% carbon dioxide, with some nitrogen, and the pressure at the surface is around 93 times that at sea level on Earth. NASA compares it to being about 900 metres deep in the ocean.

High above, between roughly 45 and 70 kilometres, lie dense layers of sulfuric acid clouds. These clouds reflect most of the sunlight that hits them, which is why Venus looks so bright. Only a small part of the sunlight reaches the ground, and yet that is enough to cook the surface.

The runaway greenhouse effect

The greenhouse effect works the same way on every planet with an atmosphere:

1. Visible sunlight passes through the air and warms the ground.
2. The warm ground gives off energy as invisible infrared radiation (heat).
3. Greenhouse gases such as carbon dioxide and water vapour absorb that infrared radiation and send part of it back down.
4. The planet ends up warmer than it would be without an atmosphere.

On Earth, a moderate greenhouse effect is good news: without it, our average temperature would be well below freezing. On Venus, the effect has run out of control. There is so much carbon dioxide that almost no heat can escape to space. The surface keeps warming until it reaches about 467 °C, far above the melting point of lead, which is 327 °C.

How did Venus get this way?

Venus and Earth probably started out with similar ingredients, including water and carbon. Scientists think the main differences came from Venus being closer to the Sun.

One widely discussed scenario goes like this:

• Venus may once have had liquid water, perhaps even shallow oceans.
• As the young Sun grew brighter, water evaporated. Water vapour is itself a powerful greenhouse gas, so warming caused more evaporation and even more warming, a feedback loop.
• High in the atmosphere, sunlight split water molecules apart, and the light hydrogen escaped to space. The water was lost for good.
• Without water, carbon dioxide could no longer be locked away in rocks and oceans as it is on Earth. Volcanoes kept adding it to the air.

Whether Venus ever had oceans is still debated. ESA’s Venus Express mission, which orbited Venus from 2006 to 2014, found evidence that the planet is still losing hydrogen and oxygen to space today, a clue that it once had much more water.

A world of extremes

Heat is not the only strange thing about Venus:

Because Venus spins backwards, the Sun rises in the west and sets in the east. And because it spins so slowly, a single sunrise-to-sunset period lasts 117 Earth days.

Landing in an oven

Between 1961 and 1984, the Soviet Union sent a series of Venera probes to Venus. Ten reached the surface. They were built like armoured diving bells, yet the heat and pressure overwhelmed them. According to NASA, the longest-lasting lander survived about two hours, and the shortest only 23 minutes. Before they were cooked, some sent back photographs of a barren, rocky landscape under a dim, yellowish sky.

Could life exist in the clouds?

Surprisingly, about 50 kilometres above the surface, conditions are much milder. NASA notes that temperatures there are around 30 to 70 °C and the pressure is similar to Earth’s at sea level. Some scientists have speculated that hardy microbes could live in the cloud droplets, although the clouds are extremely acidic and no life has been found. Proposals for future crewed exploration often imagine floating stations at this height.

What Venus teaches us

Venus is a natural laboratory for understanding climate. It shows how strongly a planet’s temperature depends on its atmosphere, and how feedback loops can push a planet into a completely different state. Several new missions, including NASA’s VERITAS and DAVINCI and ESA’s EnVision, are planned to study Venus in the coming years and answer whether it was once a habitable world like ours.

In short

• Venus is the hottest planet, at about 467 °C, hotter than Mercury.
• Its air is 96.5% carbon dioxide, with surface pressure about 93 times Earth’s.
• A runaway greenhouse effect traps heat, so Venus is as hot at night as by day.
• Venus probably lost its water to space as the young Sun grew brighter.
• A day on Venus (243 days) is longer than its year (225 days), and it spins backwards.
• About 50 km up, the clouds are surprisingly mild, and future missions will study them.