Why did Venus turn inside-out?

The second planet from the Sun is hot, bright, inhospitable, and brimming with puzzles to solve.
By | Published: December 22, 2023

Some planetary scientists refer to our cosmic neighbor Venus as “the planet that threw up on itself.” Let me explain. The brightest planet in our skies, an object of eternal mystery, Venus is shrouded in thick clouds that have defied historical attempts to understand our “sister world.” Early attempts at planetary exploration in the 1960s and 70s were haltingly successful, marred by some failures and finally short shelf lives once landers arrived due to the incredibly hostile conditions. The first measurement of the Venusian temperature from a lander registered at 800° F before the instrument failed.

Other spacecraft from both the United States and the Soviet Union pegged even higher temperatures and crushing pressures that made their return stream of data short-lived. Throughout the 70s, Soviet and U.S. missions both landed and orbited and began to assemble a clear understanding of the planet’s harsh conditions. But the real breakthroughs began with the American Magellan spacecraft, which constituted a radar mapping mission that commenced in 1990 and operated for four years. Magellan produced the landmark imagery that allowed us to really begin to understand what Venus is all about.

Where are all the craters?

Magellan produced a substantial number of surprises tucked away in its data, but the most significant one came when planetary scientists analyzed the surface imagery. The most striking thing that jumped out when examining radar imagery, that sliced through the thick blanket of clouds, was that very few craters exist on the Venusian surface. This was a very strange thing for a terrestrial planet in the inner solar system. Planetary scientists know that in the solar system’s early history the inner solar system was pelted with a huge number of impactors in a period called the Late-Heavy Bombardment, some 4 billion years ago, as planetesimals and small bodies flew around the region, before being cleared out by numerous collisions. You can see a well-preserved record of this on Mercury and the Moon. On Earth, many resurfacing processes are at work, covering up the scars of our past. But what the heck happened to take away this evidence of a harsh environment on Venus?

Venus is a strange place in many respects. Its oven-like temperatures, carbon dioxide-rich atmosphere, and incredibly high pressures form a hellish characteristic. Unlike Earth, on Venus, no plate tectonics exist which could help to hide old craters. If all the water in Venus’s atmosphere were condensed into a layer on the surface, it would be only 10 cm deep. And the ratio of deuterium to hydrogen on Venus suggests that a huge amount of water that existed early in the planet’s history escaped into space long ago.

And yet Magellan’s breakthrough discovery, that the surface of Venus is comparatively very young, is the key to understanding the planet’s history. Magellan captured synthetic radar images of approximately 98 percent of the planet’s surface. The craft also captured data on the planet’s gravity field. These are other data helped to form a physical model of the Venus system for the first time.

Something big had to have happened

The Magellan data also revealed that the planet is rich in shield volcanoes and lava plains. Many of the planet’s highlands are formed around areas similar to upwelling volcanic areas on Earth such as Hawaii and the Canary Islands. Strange tectonic features also exist, some of which are forms that are unique to Venus, such as tesserae, which are multiply fractured zones of crumpled terrain, and coronae, which are circular features believed to have formed from upwelling plumes.

The completed first rounds of examining Magellan data left planetary scientists almost speechless. It became clear that sometime in the past, huge volcanic flows covered up what previously had been surface terrain on Venus. What could have caused such a catastrophic, global resurfacing? We just don’t yet know. But rather than having a surface that is 3 or 4 billion years old, as we would have expected, it’s clear that a big event inside Venus, perhaps three-quarters of a billion years ago, turned the planet inside-out and covered most of the planet with a new, lava-rich surface.

Whatever triggered this big event, instabilities deep within Venus rewrote the history of our planetary neighbor. We know that dramatic and catastrophic things can happen on terrestrial planets. Scientists are still searching for more and more lessons we might apply to our own planet, a fragile ecosystem that is changing thanks to our own effects.

David J. Eicher is Editor of Astronomy, author of 26 books on science and history, and a board member of the Starmus Festival and of Lowell Observatory.