Space News Deep Dive: Plate Tectonics on Venus

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I rather imagine that it comes as no surprise to anyone who reads this blog that I am a space nerd. But within my general and wide-ranging space nerdery, I have certain niche topics that I particularly love to dwell on. A news story from last week happened to nestle perfectly into one of those niches, so naturally I got all excited and naturally I’m going to blog about it.

Let’s get all up in Venusian geological history and talk about plate tectonics. It’s actually really cool, I swear. Or well, hot. It is Venus we’re talking about.

Welcome to Hell World

Let’s start with a little modern day Venus, which is basically Hell. Venus is actually very close in size to the Earth, having a radius about 95% of Earth’s and a mass around 82% of Earth’s. But that’s about where the similarities stop these days.

Venus is the hottest world in the solar system, thanks to its absurdly thick atmosphere. Atmospheres are blankets for their planets. Mercury, which is closer to the Sun than Venus, doesn’t have one. That means while the daytime side of Mercury is very hot, the nighttime side is actually “kill you dead real fast” cold. It doesn’t retain any of that daytime heat. 

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Venus’s atmosphere is stupidly thick for a rocky world and that atmosphere is 96% carbon dioxide which is, in case you haven’t been paying attention to the last few decades on Earth, a terrific gas for trapping solar heat. This means Venus’s surface temperature is well up over 800 F all over the planet. So Venus will definitely try to cook you.

This thick atmosphere also weighs a lot. We don’t often think of atmosphere has having weight, but that’s essentially what air pressure is, the weight of the atmosphere above you. The weight of Venus’s ridiculous atmosphere means that the pressure at the surface is 92 times that of sea level on Earth. To find an equivalent pressure on Earth you’d have to go half a mile deep into the ocean. So Venus will not only try to cook you, it will also try to crush you.

For extra funsies, the atmosphere also contains clouds of sulfuric acid. Because in case Venus can’t cook you or crush you it will splash some acid on you as a backup. It only makes sense that the Hell Planet would have a healthy dose of sulfur in it. What’s Hell without a little brimstone?

A Pleasant Past

So modern day Venus is no place to build a vacation home, but we suspect it was not always such an unpleasant locale. There are many models that suggest that Venus, at some point in its past, looked like Earth, complete with oceans of liquid water. 

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We don’t have the kind of direct evidence for water on Venus in the past the way we do on Mars because, as with most things Venus-related, the atmosphere is a problem. We can’t see the surface easily with all the clouds, so observing from orbit is hard. And spacecraft that have gone to the surface have historically died pretty fast, with the current record belonging to Venera 13, which lasted for 2 hours and 7 minutes back in 1981. But for something the size of Venus to form where it did, the models suggest an Earthlike origin and some period of very pleasant environments.

How long this state lasted depends on which model you look at. Some models suggest that Venus turned into the level of “Dante’s Inferno” it is today fairly early and that the oceans were only around for a few million years. In this scenario solar radiation was the culprit, causing increased evaporation of the Venusian oceans and then causing the water molecules in the atmosphere to split, freeing the oxygen to combine with carbon and form the carbon dioxide atmosphere we see today.

Other models suggest that the oceans stuck around for a good long time, perhaps as recently as 700 million years ago (very recent for a planet). In this scenario, Venus would have been Earthlike for a vast majority of the solar system’s history, a phase that ended in some extreme volcanic event that outgassed huge amounts of carbon dioxide from the crust, where planets like to store carbon dioxide—Earth also has a bunch in its crust.

We know that Venus is still volcanically active today 

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 (and you should thank your lucky stars you weren’t in the Planetarium office when that news broke. I was literally singing about it all day while I ricocheted off the walls in sheer excitement. I’m amazed my coworkers still talk to me), so the idea that it suffered some insanely powerful eruption event a few million years ago isn’t out of the question.

So we have many reasons to suspect that past Venus would have looked quite familiar to an Earthling for some amount of time or other. This, of course, always brings up the question of whether life could have ever formed on Venus before it all went to pot. And this is where the new news comes in, because it turns out past Venus was even more like Earth than we thought.

Let’s Get Tectonic

Earth is the only planet in our solar system whose crust is broken up into large sections called tectonic plates, which move around on top of a semi-liquid mantle and like to smack into each other or push under each other or spread away from each other. At least, it’s the only planet with plate tectonics now. This new study suggests that Venus also used to be part of that club.

And that wasn’t even the point of the study! The study actually started out as a way to determine how to use the atmospheres of distant exoplanets to figure out their geologic histories. IThe scientists involved used Venus as a local model, and quickly ran into a problem. 

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In their models, Venus’s current atmosphere was…not happening. At least, it wasn’t when they were assuming that Venus has always had the solid crust it has today. Once they added early plate tectonics into the mix, though, current surface pressures and nitrogen content appeared as expected. This heavily suggests at least a brief tectonic history for our evil twin, from roughly 4.5-3.5 billion years ago. And that, in turn, has implications.

For one thing, it seems to prove that plate tectonics can be a thing and then, at some point, not be a thing anymore. Which is new and wild. Up until this point we had pretty much assumed that either a planet had tectonics or it didn’t. It raises a bunch of interesting questions. Why did Venus stop having plate tectonics? Why did Earth not stop? Will it stop one day? Is the Earth model where tectonics keep on going for billions of years the normal model, or will most tectonically-active worlds will go quiet eventually like Venus, making Earth the universe’s geological weirdo? These are some pretty awesome questions to ponder.

Then there’s the other big thing: life.

Venusian Life?

We think plate tectonics may have played a big role in getting life started on Earth. It speeds up the cycling of elements, minerals, and gasses through the planetary system. It’s a key part of the planet’s carbon cycle. It allows heat from the mantle to move into the crust. It powers the volcanism that can play a huge role in affecting the composition and structure of the atmosphere (and, as we’ve seen, the atmosphere plays an enormous role in whether or not a planet is livable). We don’t think tectonics are necessary for life to develop, but it would seem to help.

And now we have a model of past Venus with active tectonics, making it even more like past Earth than we thought. Essentially you had two planets of almost equal size fairly close to each other (Venus is our closest planetary neighbor), both with active surfaces powering the kinds of processes we think help make life.

The possibility of past Venusian life was already out there. Heck, the possibility of present Venusian life is out there, but that’s probably a blog post for a different day because it’s complicated and there’s a lot to say about it. What we have now is an even greater chance that Venus had, back in the day, very similar conditions to those that gave rise to life on Earth. And boy isn’t that intriguing!

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Frankly, Venus keeps dangling these exquisitely tantalizing hints about its geologic history in our faces. It’s really making the case for why missions like DAVINCI and VERITAS need to happen (especially since there’s a question over whether VERITAS actually will). Mars gets a lot of the attention, but maybe it’s time for our evil twin to get the spotlight a little. She’s got a heckuva story to tell. 

And look, I’m not saying that we’ll ever find evidence that there was ever any sort of life on Venus. All I’m saying is that if after all of the attention we’ve put into finding evidence of life on places like Mars, Europa, Enceladus, Titan, etc., the place we first find evidence of extraterrestrial life is Venus, you’re going to hear me cackling with glee clear out to Cape Cod.