Part of the What Do You Know About That series
VENUS — Depending on how much you like space and how big of a nerd you are, you may have heard that Mars is our own planet’s twin, or as close to it as we’ve got in our solar system. That’s because as far as we can figure, once upon a time Mars resembled us much more closely, a warm, wet world with a thick atmosphere. Of course, today the red planet looks nothing like the garden paradise we call home. But there is another planet nearby that could also put a claim on the title of twin: Venus.
For one, Venus is a lot closer in size to Earth than Mars is. While Mars is actually only about half the size of our planet, Venus is almost identical to us, at 7,521 miles across to our 7,926 miles. It’s also composed of the same stuff our planet is, and weighs roughly the same amount we do. It has a core, mantle, and crust. Of all the planets, it most closely matches Earth’s orbit around the sun. It was formed in the same inner part of the solar system Earth was.
Venus is also technically in what we deem the habitable zone, the distance from our sun in which it should be possible for life to form (Mars is as well). It used to have water on it, and may very well have been an ocean world like our own fledgling planet, with all the right ingredients to kickstart life. The trick is, we haven’t been able to test whether or not there could be fossils on the planet, partially because it is so incredibly deadly and awful to be there.
If you look up at Venus through a telescope, you’ll see a beautiful serene planet, with pale yellow swirling clouds covering the atmosphere. But those clouds are the dark reason Venus died.
It is important to understand a few other differences between our planet and Venus in order to understand how our toxic twin came to be the way it is today. And, as is so often the case when discussing how and why we are all alive on Earth today, the answer circles back to plate tectonics. My university geology professor would be so proud of me.
You see kids, the interior of our planet is very hot. The heat is a combination of radioactive elements decaying (and that’s a whole different chemistry lesson that requires more math than I’m capable of), heat left behind by massive impacts on our early proto-planet (like the one that likely formed our moon) and the leftover heat of the planet’s original formation.
The interior of our planet is formed of four primary layers: our solid inner core, liquid outer core, semi-solid mantle, and solid crust. The liquid outer core creates a buffer between the inner core and the mantle, which allows the inner core to spin differently than the rest of the planet. That spin is a big part of why we have a magnetosphere, which is a key part of keeping our atmosphere intact, protecting us from solar radiation that would otherwise cook us.
It is so hot that our mantle is a hot gloopy mess of molten rock that’s getting dragged around as we spin on our axis. About four billion years ago, as our crust was cooling, cooler chunks of rock got pulled down into the mantle (cold sinks, heat rises, remember?) which messed up the surrounding crust, making it weaker, which made those spots more likely to break later, which furthered the weaknesses until they cracked apart entirely, fracturing our crust into a series of plates. As the mantle moves around, so do the plates sitting on top of it, and various cracks and vents allow heat to escape on a semi-regular basis.
Venus on the other hand, despite having the same building blocks as us, has not developed a tectonic system. Its surface is smooth and uncracked, unmarked by separate plates and fault lines bleeding lava. Without those outlets, the heat inside the planet’s core just… keeps building. And building. When it finally reaches a breaking point, it explodes out like a massive oversize zit popping (you’re welcome for the visual), blooping truly enormous amounts of fresh lava across the whole surface. Venus actually has more volcanoes than any other planet in our solar system, with at least 80,000 that we’ve been able to detect via satellite. For a fun comparison, Earth currently has about 1,350 active volcanoes. About 90% of the surface of Venus is covered in basalt that flows out after the volcanoes pop, resealing the surface and filling in any cracks that may have formed as the pressure built up, a never-ending cycle.
Okay so there’s a lot of explode-y mountains up there, got it. There are plenty of things that live around volcanoes here on Earth, so why is that the relevant issue?
Well, remember how I mentioned our core rotates around and gives us a neato-magnetosphere that is really important? Well, Venus’s core doesn’t do that. While we’re not quite sure, as far as scientists have been able to determine, the core on Venus is entirely liquid, not solid, and it’s not got the ability to spin differently to create a protective shield. The planet has enough gravity naturally to hold onto heavier gasses without a magnetosphere, but light gases like hydrogen, helium, and water vapor get lost to space.
All these factors come together to turn Venus into a literally hot mess. Volcanoes are constantly belching greenhouse gases like carbon dioxide into the atmosphere, and the sun is constantly blasting it with radiation and heat. The carbon dioxide is heavy enough to not float away, so it stays and wraps the planet in a toastie blanket, keeping the heat of the planet, the volcanoes, and the sun all nice and cozy and at a balmy 864 degrees Fahrenheit. That’s hot enough to melt lead.
Those lovely swirling clouds? They’re not water vapor, as established, any water once on Venus has long since been lost to space as the planet heated up and the vapor was too light for the gravity to hold. Instead, the clouds on Venus are composed of sulfuric acid. It even rains acid up there, though the surface of the planet is so hot that the drops never reach it, re-evaporating into the atmosphere about 30km from the ground. So you know, you’d be safe from that at least on your visit.
Acid rain isn’t the only weird weather on Venus. The higher points on the planet are covered in something more reflective than the basalt plains in the lower areas. On our planet, you would assume that was snow. On Venus, the snow is made of metal, probably lead sulfide and bismuth sulfide.
If that weren’t enough for you, all that heavy carbon dioxide also puts a bunch of pressure on the surface, far more than we have to deal with here on Earth. At sea level here, the atmosphere is about 1 bar, or 14.7 pounds per square inch (psi). That doesn’t really have to mean anything to you for you to know that when the surface of Venus is at 92 bar, or 1,350 psi, that’s a pretty significant difference. It’s roughly equivalent to 2,920 feet down into the ocean. For reference, without anything to protect you, a human gets crushed at around 1,100 feet down.
But don’t let all that convince you to cancel your next Venus vacation! The planet also features a backward rotation, so the sun rises in the west and sets in the east. The planet does rotate way more slowly than Earth though, so one Venusian day is about 243 days on Earth. You may have to wait a while for that sunset.
Plus there’s the cultural component to it– while the planet may not have much in the way of civilized society itself, it has long been a fixture in the sky for humans here on Earth, who have admired it and assigned significance to it for millennia. It is the third brightest object in our sky only after the moon and sun. The ancient Greeks mistook it as two separate things, the morning and evening star, since it alternately appears in those times but doesn’t stay all night. They called it the light-bearer when it showed up in the mornings before the sun rose. They eventually did figure out it was just one thing, though the Romans who came after them continued to differentiate the two as different aspects of the same heavenly body. The Babylonians realized it was actually the same thing. They called it the Ninsi’anna, which translated to “divine lady, illumination of heaven.” The ancient Chinese viewed Venus as the deity Taibai Jinxing, which translates to ‘great white golden star’. The Aztecs also named Venus a god, Tlāhuizcalpantēcuhtli, or the morning star.
This week, Venus is a morning girly. Check out the astronomy column on page 5 for when it’s visible in our night sky.
