Hotter than the planet closest to the Sun
Mercury is the planet closest to the Sun, so it seems like it should be the hottest. The surprise is that Venus, about twice as far away, is hotter. The surface of Venus is commonly given as roughly 465 degrees Celsius (around 870 degrees Fahrenheit), hot enough to melt lead. It stays about that warm day and night and across most of the globe.
Mercury has almost no atmosphere. Its sunlit side gets very hot, but with no air to hold the warmth, the night side drops to extreme cold. Venus has the opposite situation: a very thick atmosphere that acts like a heavy blanket. The difference between the two worlds comes down almost entirely to the air.
What the atmosphere is made of
Venus’s atmosphere is roughly 96 percent carbon dioxide, with most of the remainder nitrogen. The pressure at the surface is about 90 times Earth’s sea-level pressure, comparable to being almost a kilometer down in an ocean on Earth. This is why landers sent to the surface had to be built like deep-sea submersibles.
High above the ground, a thick deck of clouds hides the surface from view. These clouds are made largely of droplets containing sulfuric acid, a corrosive chemical. They are highly reflective, which is why Venus looks so bright in the sky. Winds at cloud level are very fast, circling the planet in a few Earth days, even though the planet itself rotates extraordinarily slowly.
How the heat gets trapped
Heat on Venus works through the same basic process as the greenhouse effect on Earth, only far stronger.
- Sunlight passes through the atmosphere and clouds, and some of it reaches the surface and warms the rock.
- The warm surface gives off heat as infrared radiation.
- Carbon dioxide absorbs much of that infrared radiation and radiates some of it back down, so the heat has trouble escaping to space.
Because Venus’s atmosphere holds so much carbon dioxide at such a high density, the effect is enormous. Cloud droplets and small amounts of other gases also absorb infrared. Interestingly, the bright clouds reflect a large share of incoming sunlight, so the surface receives less direct light than you might expect. The heat that does arrive is trapped efficiently, and the temperature climbs until the energy leaving matches the energy coming in.
The result is also very stable. Deep in the thick atmosphere, there is little difference between day and night or between the equator and the poles, since the heavy gas moves heat around.
A runaway greenhouse in the past?
Venus is close to Earth in size and mass, and probably formed from similar material, so its different outcome has long puzzled scientists. A leading idea is that Venus once had more water, possibly even oceans, and then suffered a runaway greenhouse effect.
In this picture, the young planet warmed enough for water to evaporate. Water vapor is itself a greenhouse gas, so more evaporation caused more warming in a feedback loop. Eventually water vapor reached the upper atmosphere, where ultraviolet light from the Sun broke it apart. The light hydrogen could then escape into space. Measurements show that Venus’s remaining hydrogen has an unusually high share of the heavy form called deuterium, which is consistent with a lot of hydrogen having been lost, though it does not prove how much water there was or for how long.
With the water gone, carbon dioxide that on Earth is locked away in rocks, and dissolved in oceans, could have remained in the air. Volcanic activity would keep adding more over time. Details are uncertain, including how wet early Venus really was, and some models suggest it may never have had long-lived oceans. Future missions are planned to look for more clues.
Slow spin and other oddities
Venus rotates backward compared with most planets, so the Sun would rise in the west if you could see it. A day there, meaning one spin, takes about 243 Earth days, which is longer than its year of roughly 225 Earth days. Its lack of a strong global magnetic field means the solar wind interacts directly with the upper atmosphere.
The surface is shaped by volcanism. Radar maps made through the clouds show vast lava plains, volcanoes and highland regions. Whether volcanoes are erupting today is still being investigated, and some evidence suggests recent activity. Venus is also not known to have plate tectonics like Earth’s, and this may be part of why heat and gases behave differently.
Visiting a hostile place
Several Soviet landers reached the surface in the 1970s and 1980s, and they sent back data and a handful of images before the heat and pressure disabled them, typically within about an hour or two. Orbiters have used radar to map the planet. Because the environment is so extreme, long-lived surface missions are very difficult, and many ideas focus on balloons or instruments that float in the cooler cloud layers instead.
What Venus teaches about Earth
Venus is sometimes described as a warning, but the comparison needs care. The conditions on Venus come from its history, its distance from the Sun and its unique atmosphere, and Earth is not on track to turn into Venus in any short timescale. What the planet does show is how powerful a greenhouse effect can be, which helps scientists test models of planetary atmospheres, including those used to study the climate change drivers on Earth.
The planet also gives a sense of how fragile habitable conditions can be. Temperature, atmosphere and water interact in ways that can lead similar worlds down very different paths, and Venus is the nearest example in our own neighborhood.