Planetary Profile: The Vagaries of Venus

This week’s post is inspired by two things. First, as mentioned in this week’s newsletter, China has announced its intention to build a Venus atmosphere sample return mission. On the face of it this seems like a strange choice, until you dig into why we’re so interested in Venus’s clouds.

That, in turn, leads to a lot of other things that are weird and interesting about Venus, which reminded me of the post I did a few weeks ago outlining some of Mercury’s oddities. Since every planet has its own little quirks, doesn’t Venus deserve the same attention? And so here we are.

What are the traits that make Venus a unique world, and why is the idea of an atmosphere retrieval mission so potentially exciting? Let’s dig into the weirdnesses of Earth’s twin. 

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Twinsies?

We have to start out that nickname: Earth’s twin. At first glance Venus looks nothing like Earth. At second, third, and fourth glance, it still doesn’t. At twenty-seventh glance you’d still be scratching your head. It all comes down to size. Venus and Earth are almost exactly the same size, with Venus’s radius only about 200 miles smaller than Earth’s.

That’s mostly where the resemblances end these days. From the top of the atmosphere to internal behavior Venus and Earth are definitely not twinsies. That said, there was a time, we think, when you might have mistaken them for each other if you were looking from space.

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While today Venus is a perfect hell world, we do think it’s possible, maybe even likely, that it once looked like Earth, complete with oceans of liquid water. Two planets of roughly the same size forming in more or less the same place probably had similar formations. The question is how long this state on Venus would have lasted.

In one scenario solar radiation caused Venus’s oceans to evaporate early, after only a few million years. In another, Venus may have retained its oceans for most of its history, only losing them after a massive volcanic event a few hundred million years ago gave it its modern atmosphere, which serves as a suffocating carbon dioxide blanket making the surface the kind of place no water molecule in its right mind would hang out.

All of this is based on models since Venus’s surface is much harder to observe than, say, Mars’s, and things like volcanism and that intense atmosphere (neither of which are modern Martian concerns) would be likely to erase the signs of past liquid water that our redder neighbor shows off so readily. But keep this past, pleasant Venus in mind. We’re gonna come back to it.

The Weird Stuff

I promised to talk about how Venus is weird. Let’s start with the fact that it spins backwards. On every other planet in the solar system, if you could stand on it (I know, gas giants, but work with me here) you’d see the Sun rise in the east and set in the west. If you stood on Venus (and, you know, could see the Sun through the clouds), you’d see it rise in the west and set in the east.

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It also spins slower than it orbits, with its day being longer than its year. It takes Venus 225 days to orbit the Sun, but its day is 243 Earth days. At least…it’s more or less 243 days. Because here’s another weird thing about Venus: its days are not a consistent length.

The Magellan and Venus Express spacecraft each made measurements of Venus’s rotation 16 years apart. Each measurement was very precise. And they were off from each other by over 6 minutes. This can be attributed to that ridiculous atmosphere and being so close to the Sun. The heavy atmosphere getting heated by the Sun differently on different parts of the planet can cause the air to have a kind of tidal effect on the planet, similar to but less precise than the way the oceans do on Earth. It’s a drag effect that pulls on the planet and messes with its rotation. All in all, it means keeping time on Venus would be a frustrating endeavor.

When you get under the surface, things don’t get less weird. You might assume that Venus would be similar on its insides to Earth, given their sizes and the fact that they probably had similar formations. But part of what goes on inside Earth is due to the way it rotates and, as we’ve just seen, Venus is marching to a different drummer when it comes to rotation.

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Venus does have a mantle and core like Earth does (we’re pretty sure, anyway), and its core is probably still at least partially liquid. Theoretically, then, Venus has the stuff needed to generate a global magnetic field like Earth does. Only (stop me if you’ve guessed it) it doesn’t.

While we’re not fully certain why Venus doesn’t have an Earth-like magnetic field, it probably has to do with the way the planet moves. Getting the liquid iron in the core moving is what generates the field. Earth’s rotation and internal convection cause its field. Venus probably just isn’t spinning fast enough, and its insides are probably not roiling with the convection that Earth’s insides have. No roiling, no magnetic field. That said, there is a very weak field that gets generated by the interaction between the upper atmosphere and the solar wind, but that’s doing exactly jack to protect anything from solar radiation. 

Combine all this with a surface pressure equivalent to that half a mile deep in Earth’s oceans, a surface baking at a nice toasty 900F or so, apparent rampant volcanism, and acid drops suffused through the atmosphere, nowhere else in the solar system is quite like Venus.

Okay, But About That Atmosphere…

So we’ve established that Venus’s surface is a blasted hellscape. It’s hot, it’s volcanic, and the only thing between you and pure solar radiation is 60 miles of acidic clouds that are also trying to squish you. Even if the planet was once a delightful place, it certainly isn’t any longer.

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But we do think it was once the kind of place that life could have thrived, and if there’s one thing we know about life it’s that once it’s around, it’s very hard to destroy it completely if you give it half an inch.

Venus’s surface is awful, but there’s a layer of atmosphere roughly 25-40 miles (40-60 km) up where the pressures are far more pleasant and the temperatures don’t mean automatic death. Enough radiation gets filtered out by the cloud layers above that by the time you get to this region you’re not getting cooked anymore than you would on Earth’s surface. There’s acid drops, yes, but that’s not actually a dealbreaker for certain kinds of simple life. We sometimes call this Venus’s “habitable zone”, which definitely stretches the definition of the term, but you get it.

Let’s propose a gigantic “if”: IF life formed on Venus during its friendlier era, it’s entirely possible that some microbial form of it could still survive in the cloud layers, even as anything closer to the ground or higher up got eradicated. And after all, Earth has an aerial biosphere of its own—maybe Venus can support one too. 

Such Tantalizing Hints

I remember the day, back in 2017, when I was doing my usual perusal of the day’s space headlines (what, you don’t think I only started doing that when the newsletter became a thing, do you?) and I read about the mystery of UV light absorption in Venus’s clouds. I got more excited than one probably normally should about light absorption, because there are potential implications.

Something is absorbing large amounts of UV light in Venus’s atmosphere. The authors of the 2017 paper dutifully listed several possible explanations. Being thorough, they even listed the one that seemed mostly unlikely: microbes. Certain kinds of microbes absorb UV light at the exact wavelengths being absorbed in the Venusian clouds. Iiiiinteresting.

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Then there’s the phosphine debate. In 2020 a paper came out claiming the detection of large amounts of phosphine in Venus’s atmosphere. This could be a potential biomarker—or it could be not that at all. And then the arguments started about whether it was actually detected at all. The phosphine isn’t really there. No wait, it is. But no, not really. Unless it actually is and this time it’s accompanied by ammonia, another potential biomarker.

At the moment the momentum is in the “yes, there’s phosphine” camp, though even if it is it doesn’t necessarily mean it’s being produced biologically. But it could be, is the point. 

There and Back Again

This is why the idea of a Venus atmosphere sample return mission is so blasted exciting. It’s hard to observe details about the Venusian habitable zone all the way from Earth—but if we can bring a sample of it back to Earth and study it here, we can really see what’s going on.

Of course the sampling vehicle will have to be able to survive intense solar radiation, sulfuric acid droplets, and the incredibly strong winds of Venus to do it, not to mention getting there and back again. None of that is easy. And when we get the sample home, it will undoubtedly help answer a lot of questions we have, but it might not be filled with anything revolutionary.

Or we might crack that sample container and find a whole crowd of Venusian microbes indignant that they’ve been so rudely snatched out of their lovely, acidic clouds and brought to this frigid place. The first place we find life away from Earth might just be the last place we once thought to look for it. That’s still possible. And it’s the possibilities that help keep us going.