Venus, the planet that went wrong

Venus is about 95 percent of Earth’s diameter and has about 80 percent of its mass. It formed from similar material, in a similar part of the solar system, at the same time. And yet the surface temperature is about 465 degrees Celsius, and the clouds are made of sulfuric acid. The air pressure, more than 90 times Earth’s, matches what a submarine would feel about 900 meters down in the ocean.

A paradox of sunlight

The obvious explanation is that Venus is closer to the Sun, and it is wrong, or at least badly incomplete. Sunlight at Venus is about 1.9 times as intense as at Earth. But the planet’s bright clouds reflect around three-quarters of it back to space. Run the numbers and Venus absorbs about 156 watts per square meter, averaged over its surface, while Earth absorbs about 238.

So Venus takes in roughly a third less solar energy than Earth does, and its surface is hotter than Mercury’s. Whatever is happening there is not about how much sunlight arrives. It is about how hard it is for heat to get out.

A blanket of carbon

Venus’s atmosphere is about 96 percent carbon dioxide, and it is enormously thick. The heat radiated by the surface cannot escape through that dense layer except very slowly, so the lower atmosphere settles at a temperature high enough to force the energy out. The surface there stays almost as hot at night and at the poles as anywhere else.

Here is the comparison that makes Venus a little unsettling. Earth is not short of carbon. Most of it is locked away in limestone and other rocks, formed over geological time as carbon dioxide dissolved in rain and oceans and reacted with minerals. Add it up and Earth’s buried carbon is about the same order as all the carbon dioxide in Venus’s atmosphere. The two planets may have comparable inventories. Earth has simply stored its supply in stone, and it was able to do so because it had liquid water.

Earth has simply stored its supply in stone.

Did Venus ever have oceans?

One leading idea is that early Venus had liquid water, perhaps even oceans. As the young Sun brightened, the water began to evaporate. Water vapor is itself a greenhouse gas, so evaporation raised temperatures, which drove more evaporation. Eventually the water rose high into the atmosphere, where sunlight split the molecules apart and the hydrogen escaped to space. Without liquid water, carbon dioxide had no way back into rock.

The evidence for that history is in the atmosphere. Venus’s remaining water is strongly enriched in deuterium, the heavy form of hydrogen, by a factor of more than a hundred compared with Earth. Lighter hydrogen escapes more easily, so a large enrichment suggests that Venus once had far more water than it does now. Climate models published in 2016 found that the planet could have stayed temperate for billions of years, given slow rotation and the right cloud behavior.

Others are unconvinced, and the argument has sharpened. Some models find that a young, steamy Venus may never have cooled enough for rain to reach the ground at all. In 2024, a study of the chemistry of volcanic gases concluded that the Venusian interior is dry, which is difficult to square with a long-lived ocean era. Deuterium shows that water was lost. It does not say whether that water ever formed seas.

A world that may still be active

Venus is often described as dead, and that description is under pressure too. In 2023, researchers re-examining radar images from NASA’s Magellan mission, taken around 1991, found a volcanic vent near Maat Mons that changed shape and grew over an eight-month span, the first direct evidence of a volcanic change on the surface. Data more than thirty years old, read with patience, turned out to contain something new, a pattern familiar from the Voyager flyby of Uranus.

Why it matters

For astronomers studying rocky planets around other stars, Venus is a warning about labels. A planet of the right size and in roughly the right place may be habitable or may be an oven, and from light-years away the two could look alike. Terms like “Earth-sized” and “in the habitable zone”, which appear in many exoplanet announcements, describe an opportunity and promise nothing. Understanding where the line falls between an Earth and a Venus is essential for interpreting what telescopes see elsewhere.

Going back

The Soviet Union landed a series of Venera probes on the surface between 1970 and 1982; Venera 13 survived for 127 minutes in 1982 and sent back color images of a flat, rocky landscape under an orange sky. The last landers arrived in 1985, carried by the Soviet Vega missions on their way to Halley’s Comet. None has gone since.

Three missions are planned for the 2030s, though their dates have been moving. NASA’s DAVINCI would drop a probe through the atmosphere, measuring its composition all the way down, and is targeting a launch around the end of 2030. NASA’s VERITAS and the European Space Agency’s EnVision would map the surface with radar in far greater detail than before. Congress restored funding for both NASA missions after proposed cancellations, but VERITAS is ramping up slowly, and reported launch dates for it and for EnVision differ. If they fly, the 2030s will bring the most thorough study of Venus in half a century.

Sources

• Way et al., “Was Venus the first habitable world of our solar system?”, Geophysical Research Letters, 2016
• Constantinou et al., “A dry Venusian interior constrained by atmospheric chemistry”, Nature Astronomy, 2024
• Herrick and Hensley, “Surface changes observed on a Venusian volcano during the Magellan mission”, Science, 2023
• Space.com: Venus missions planned for the next decade