Venus's topography

The Venusian Geology Night Thread

Merve mentioned this as an example of a boring topic for a night thread, so tonight it’s my job to convince you to care about the geology of Venus.

Venus is a terrible planet. The woooorst. It’s close to Earth in size, but it’s a “runaway greenhouse” planet with a carbon dioxide atmosphere 90 times as heavy as Earth’s, and a surface temperature hot enough to melt lead. Any water it might have had has long since been lost. Why? Is it just because Venus is closer to the sun, or is it connected to Venus’s weird geology?

Because Venus’s geology is very weird. We can’t get good photos of the surface because it’s constantly covered by sulfuric acid clouds (!!), but NASA’s Magellan spacecraft and the Arecibo observatory have made good maps of the surface using radar. In the image below, color indicates height: lowlands are blue and mountains are red and white. The mountains are up to 14 km tall, similar to the height of Earth’s tallest mountains above the seafloor.

Radar topography of Venus, collected by NASA’s Magellan spacecraft and the Arecibo observatory.

Venus is also like Earth in that it has very few big meteorite craters, unlike the Moon, Mercury, or Mars. That means its surface is young — in fact it’s probably still geologically active, just like Earth.

But otherwise, Venus’s geography looks nothing like the Earth. The Earth has big flat plates of high and low elevation (the continents and ocean floors). Venus doesn’t seem to have plates: it’s just wrinkly all over. There are a few high regions, called “tesserae”, where the wrinkles seem to be pushed together to form high mountain ranges (on the left edge and right center of the image), and lots of areas, called “coronae” and “arachnoids”, where a circular high region is surrounded by a deep ring-shaped valley.

A chain of coronae, possibly caused by blobs of hot mantle trying to push through the crust to the surface.

Earth has plate tectonics: the ocean plates slide underneath the continental plates into the mantle, making way for new crust to be formed. Venus doesn’t do that. We think rising plumes of hot mantle rock are pushing up against the crust from below to create the round coronae, pushing other landscapes aside, then sinking back down to create the ring-shaped valleys. In other places, sinking blobs of mantle pull the crust together to form the tesserae. It’s a bit like a stew that’s been boiling so long that a a thick skin has formed on top, and the bubbles can’t break through.

Some scientists have proposed that Venus’s geology isn’t a gradual process: that once every few hundred million years, the crust of the whole planet overturns and is replaced with molten rock, in a global volcanic catastrophe! The jury’s still out on this hypothesis.

Why does Earth have plate tectonics and Venus doesn’t? Possibly because Earth’s water changes the chemistry of its rocks, making it easier for continental plates to float, and oceanic plates to sink under them. But since plate tectonics may be necessary for maintaining Earth’s oceans and atmosphere in a state friendly to life, by understanding Venus better, we may unlock clues about what makes planets like Earth habitable.

One of only six pictures we have from the surface of Venus.

But we may have reached the limit of what we can figure out by radar: the next step is to go to the surface and explore. The problem is, as I said, Venus is terrible. The Soviets sent landers to the surface in the 1980s: they each lasted an hour before being destroyed by the incredible heat. In 2021, NASA approved two small new missions to Venus, including Davinci+, which will parachute to the surface and take pictures on the way down. But plans for a long-term lander are still in the early stages: we may have to invent a whole new type of computer chip to tolerate the heat.

Venus is terrible. But also fascinating!