Why Is Venus So Hot? The Runaway Greenhouse Next Door
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Venus is the brightest object in the night sky after the Moon, the nearest planet to Earth, and almost the same size and mass, and for most of the twentieth century it was imagined as a warmer Earth under its permanent clouds, with jungles and swamps. The first spacecraft to reach it found a surface at 465 degrees Celsius, hotter than Mercury though twice as far from the Sun, under an atmosphere ninety times as heavy as ours and clouds of sulphuric acid. The difference between the two planets is not distance from the Sun. It is what happened to the water.
The measurements
The clouds hide everything, so what is known was got by radar, by radio, and by landers that lasted minutes. The Soviet Venera probes reached the surface from 1970, the first ones crushed on the way down and the later ones built like submarines, and the longest survived 127 minutes and sent back pictures of flat orange rock under a yellow sky. The surface pressure is 92 times Earth's, equivalent to a kilometre under the sea; the temperature is 465 degrees everywhere, day and night, pole and equator, because the thick atmosphere spreads the heat; and the air is 96 percent carbon dioxide, with clouds of concentrated sulphuric acid 50 to 70 kilometres up that reflect three quarters of the sunlight back to space. Less sunlight reaches Venus's surface than reaches Earth's. The heat is not arriving; it is being kept.
The greenhouse turned up
Carbon dioxide lets sunlight in and absorbs the infrared that a warm surface gives off, so that heat leaves a planet more slowly than it arrives until the surface is hot enough to force it through. On Earth, with carbon dioxide at 0.04 percent of a thin atmosphere, the effect raises the surface by about 33 degrees, from an airless minus 18 to a habitable 15. On Venus the same effect operates through a column of carbon dioxide two hundred thousand times as massive, and the surface must reach 465 degrees before the infrared escaping at the top balances the sunlight absorbed. The physics is identical; only the quantity differs. Venus is the demonstration that the greenhouse effect has no natural ceiling.
Where the water went
Earth's carbon dioxide is mostly not in the air. It is in limestone, hundreds of atmospheres' worth, locked there because rain dissolves the gas, carries it to the sea, and marine organisms and chemistry turn it into rock, a cycle that needs liquid water. Venus, being closer to the Sun and receiving about twice the sunlight, probably started with oceans and lost them:
- •The young Sun's warmth evaporated more water into the air, and water vapour is itself a greenhouse gas, which warmed the surface and evaporated more, a feedback that ran until the oceans were in the sky
- •High in the atmosphere, ultraviolet light split the water molecules, and the hydrogen, too light to hold, escaped to space, which is why the deuterium left behind on Venus is enriched a hundredfold over Earth's
- •With no water there was no rain, no weathering and no way to make limestone, so every tonne of carbon dioxide the volcanoes released stayed in the air
- •The surface baked, the carbonates that might once have formed broke down and returned their gas, and the atmosphere settled at its present mass
A dead planet, or a young one
Radar mapping from the Magellan spacecraft in the early 1990s showed a surface with volcanoes, lava plains and few craters, and the craters that exist are spread evenly, as if the whole surface were the same age, about 300 to 700 million years. One reading is that Venus resurfaced itself in a single global outpouring of lava around then, perhaps because it has no plate tectonics to release heat gradually, and stores it until the crust fails all at once. In 2023 a comparison of Magellan images taken eight months apart found a volcanic vent that had changed shape between them, the first evidence that the planet's volcanoes are active now, and traces of phosphine in the clouds reported in 2020 set off an argument about microbial life in the one layer of the atmosphere where the temperature is mild; most researchers now think the signal was something else. Three missions, two American and one European, are due to arrive in the 2030s to settle both questions.
The lesson for Earth
The Sun is brightening slowly, as stars of its kind do, and in something like a billion years Earth will receive the sunlight Venus received when it lost its oceans; the runaway greenhouse will then happen here, whatever anyone does. On the timescale that matters, Venus is not a forecast, since Earth's oceans and rock cycle would take far more carbon dioxide than there is fossil fuel to burn before the feedback ran away. What it is, is the proof. The same gas that keeps Earth 33 degrees warmer than it would otherwise be keeps Venus 450 degrees warmer, and the difference is only how much of it is in the air, which is the one number about the atmosphere that humans have been changing.
The takeaway
Venus is hotter than Mercury because its atmosphere is almost pure carbon dioxide ninety times as massive as Earth's, and a greenhouse effect of that strength holds the surface at 465 degrees despite clouds that reflect most of the sunlight. It got that way because extra sunlight evaporated its oceans, ultraviolet light split the water and the hydrogen escaped, and without water there was no way to turn volcanic carbon dioxide into rock, so it all stayed in the air; Earth's carbon is in limestone, and the difference between the twin planets is the difference between wet and dry.