What if Venus had Earth's air?

Venus and Earth are about the same size. But Venus is much, much hotter.

What if Venus had air like Earth's? Would it be cooler? Let's find out.

Venus and Earth are close in size and built in a similar way, with an iron core, a hot rock mantle and a thin rocky crust. Yet NASA says Venus is the hottest planet in our solar system, hot enough to melt lead.

The big difference is the air. So what if Venus had Earth's air instead of its own? Would it cool down? Would it rain? This is a thought experiment. Choose a path to explore why Venus is so hot today and what a NASA computer model suggests about a cooler Venus long ago.

Venus and Earth share similar size and interior structure, and NASA notes both appear to have harbored oceans when young. Their atmospheres now differ enormously: Venus's is mostly carbon dioxide at about 93 times Earth's surface pressure, Earth's mostly nitrogen and oxygen.

This thought experiment swaps the atmospheres. Evidence comes from spacecraft measurements of Venus today and from a NASA Goddard Institute for Space Studies (GISS) climate model of ancient Venus. That model is about the past, under a dimmer young Sun; what Earth's air would do on Venus today is an idea, not a result.

Venus is covered in thick clouds. The air at the ground is very heavy and very hot.

It is hot enough to melt some metals!

Venus's air is about 96.5 percent carbon dioxide, with clouds made of sulfuric acid. NASA says the surface is about 467 °C, hot enough to melt lead, and the air presses down about 93 times harder than Earth's at sea level. That's like being more than 900 meters under the ocean.

Venus spins very slowly, and backward. Its day lasts 243 Earth days, longer than its year of 225 days. Probes that landed there long ago lasted only a short time before the heat stopped them.

NASA's fact sheet gives Venus's atmosphere as 96.5% carbon dioxide and 3.5% nitrogen. NASA Science gives the surface pressure as about 93 times sea level on Earth, like being about 930 m underwater, with a surface near 467 °C and clouds of sulfuric acid.

Venus rotates retrograde, once every 243 Earth days, longer than its 225-day year, and its 3-degree tilt is too small for noticeable seasons. The Soviet Venera program landed ten probes between 1961 and 1984; the longest-lived lasted two hours. About 50 km up, though, temperatures of 30 to 70 °C and Earth-like pressures prevail.

Earth's air is mostly nitrogen and oxygen. It has only a tiny bit of carbon dioxide.

If Venus had air like that, it might not trap so much heat.

Earth's air is about 78 percent nitrogen and 21 percent oxygen, with only a tiny amount of carbon dioxide. Venus's is almost all carbon dioxide, the same kind of gas that traps heat on Earth.

Swap in Earth's air and Venus might hold in far less heat. But Venus is closer to the Sun than Earth and gets more sunlight, so it might still be hotter than Earth. No source here gives a temperature for today's Venus with Earth's air, so these are ideas, not predictions.

NASA's Earth fact sheet lists dry air as 78.08% nitrogen and 20.95% oxygen, with carbon dioxide at about 420 parts per million, roughly 0.04 percent; Venus's air is 96.5 percent carbon dioxide at far higher pressure.

Replacing it with Earth's air would remove most of the greenhouse gas, so the surface might cool substantially. But Venus is closer to the Sun, and GISS notes even ancient Venus, under a Sun up to 30 percent dimmer, received about 40 percent more sunlight than Earth does now. How warm a present-day Venus with Earth's air would be, and whether water could stay, is not modeled here; treat any answer as an idea.

Venus's thick air is full of carbon dioxide. This gas traps heat, like a blanket.

That is why Venus is the hottest planet of all.

Carbon dioxide is a greenhouse gas: it traps heat, a bit like a blanket. NASA says the same gas drives the greenhouse effect on Earth too. Venus has so much carbon dioxide that its greenhouse effect ran away.

NASA scientists explain how that may have happened. Venus likely had an ocean long ago, but it gets more sunlight than Earth. The ocean evaporated, sunlight broke the water vapor apart, and the hydrogen escaped to space. With no water left, carbon dioxide built up in the air, trapping more and more heat.

Carbon dioxide is the gas that drives the greenhouse effect on both Venus and Earth, NASA notes; on Venus it produces a runaway greenhouse that makes the surface hotter even than Mercury's.

GISS summarizes the leading account. Pioneer Venus measurements in the 1980s first suggested an early ocean. Because Venus receives far more sunlight than Earth, that ocean evaporated; ultraviolet light split the water-vapor molecules and the hydrogen escaped to space. With no water left on the surface, carbon dioxide accumulated in the atmosphere, producing the runaway greenhouse that set present conditions.

Scientists used a computer to imagine Venus long ago. In their model, Venus had air like Earth's and a shallow ocean.

It rained, and thick clouds kept the ground cool!

In 2016, scientists at NASA's Goddard Institute for Space Studies reported results from a computer model of ancient Venus. It was similar to the models used to predict climate change on Earth. They gave Venus an atmosphere like Earth's, a shallow ocean and a fainter young Sun.

In the model, Venus's slow spin kept its day side in sunlight for almost two months at a time. That made rain and a thick layer of clouds that shaded the ground like an umbrella. The model's average temperature came out a few degrees cooler than Earth's today. It suggests Venus might have been habitable for up to 2 billion years.

Michael Way and colleagues at GISS, reporting in Geophysical Research Letters, simulated an early Venus with an Earth-like atmosphere, today's 117-day solar day, a shallow ocean filling the lowlands mapped by Magellan's radar, and a Sun up to 30 percent dimmer. Co-author Anthony Del Genio explained that the slow rotation exposed the dayside for almost two months, driving rain and thick clouds that shielded the surface; mean temperatures were a few degrees cooler than Earth's today.

The model suggests a shallow ocean and habitable temperatures for up to 2 billion years. It is a model result about the past, not an observation, and NASA's Venus page notes no compelling evidence of life there.