Mark Belan/Quanta Magazine
Venus is arguably the worst place in the solar system. A cloak of carbon dioxide suffocates the planet, subjecting its surface to skull-crushing pressure. Sulfuric acid rains down through the sickly yellow sky but never reaches the lava-licked ground. Venus is so hot — hot enough to melt lead — that the acid rain evaporates as it’s falling.
The planet’s extreme inhospitality is at the heart of one of the most beguiling mysteries in planetary science. Venus and Earth formed at the same time, from the same geologic building blocks, in pretty much the same part of the solar system. They’re even the same size. So why is Venus a hellscape, and Earth a garden?
A common refrain in the scientific community is that Venus is just several steps ahead — that it represents the end state of all large rocky planets, including Earth. The hypothesis is that these planets eventually lose the ability to sequester planet-warming greenhouse gases in their geologic underbelly. When those gases then accumulate in the atmosphere, the world enters a runaway greenhouse state — like the boiling hot Venusian climate. “Over the years, we’d always heard about Venus being a preview into Earth’s future,” said Stephen Kane, a planetary astrophysicist at the University of California, Riverside.
But is that long-held assumption true? In hundreds of millions or billions of years, will Earth’s climate go the way of Venus’, transitioning from a temperate world into a catastrophic hothouse? Kane and his colleagues have been trying to find out. Venus and Earth are often referred to as twins, with Venus being the evil one of the pair. In their Reuniting Twins project, the scientists have developed a digital model of Earth that combines solar physics, volcanology, plate tectonics and climate science. They’ve been pushing their model Earth to its extremes, trying every plausible way to break it and make it into Venus.
As well as exploring what went so wrong on the second rock from the sun, this work speaks to a query closer to home, said Paul Byrne, a planetary scientist at Washington University in St. Louis who was not directly involved with the project: “How long is Earth habitable for?”
Venus is 95% as broad as Earth, and 81.5% as massive.
NASA
A Planetary Whodunit
Forecasting Earth’s future requires an understanding of Venus’ past. Long ago, Venus might not have been too different from how Earth is now. Spacecraft surveys and telescope observations have revealed the existence of a rare, heavy form of water in the Venusian atmosphere — a telltale sign that regular water used to be abundant on the planet.
Researchers debate how that water resided there. One possibility is that the water on young Venus formed steam that wafted above the magma sea covering the newborn planet’s surface. That water vapor, a potent greenhouse gas, would have pushed the world into a scorching-hot greenhouse state not long after its birth. Alternatively, Venus’ initial, planetwide magma sea might have cooled and hardened into a crust quickly enough for liquid water — maybe even an ocean’s worth of water — to flow across it. If that is true, then what happened to all that water?
In 2020, Michael Way, a planetary scientist and climate modeler at NASA’s Goddard Institute for Space Studies in New York, looked at two possibilities for how a Venusian ocean could have boiled away.
The first hypothesis points to the sun, which, as it ages and burns through its hydrogen fuel, blasts out ever more sunlight, subjecting nearby planets — including Venus, which sits closer to the sun than we do — to an increasingly intense blaze. Planetary scientists estimate that by about a billion years after the solar system’s birth, the gradually brightening sun would have been able to efficiently vaporize any liquid water on Venus. Water vapor would then have flooded the Venusian atmosphere, potentially causing intensive global warming; this warming might have been exacerbated further by volcanoes off-gassing carbon dioxide. The combination might have pushed Venus into a runaway greenhouse state.
It’s a nice story. But according to Way’s models, the theory that the sun broke Venus by evaporating its oceans has problems. Venus spins very slowly on its axis, and one day there is equivalent to 116 Earth days, or nearly four months. If the planet’s dayside initially held liquid oceans, that water’s evaporation would have formed thick clouds. Those prodigious, persistent dayside clouds would have reflected sunlight, keeping Venus cooler than it otherwise might have been and actually preventing a runaway greenhouse effect and unrestrained global warming.
Way thinks that a more plausible suspect in Venus’ demise is a form of ruinous volcanism. In its own storied past, Earth experienced prolonged eruptions of lava in single regions, known as large igneous provinces (LIPs), that lasted for hundreds of thousands and perhaps millions of years. Each of these events injected copious carbon dioxide into the atmosphere and made the world, for a time, extremely hot. One LIP 252 million years ago triggered the worst recorded mass extinction in Earth’s history, almost sterilizing the planet.
The Magellan spacecraft imaged the desolate surface of Venus in the early 1990s.
NASA
Fortunately for life, after each of these LIP events, Earth gradually drew that excess carbon dioxide deep into its rocky bowels, and the planet cooled back down. It accomplished this through a process called subduction: When tectonic plates collide, one plate can descend below the other, drawing seawater rich in dissolved carbon dioxide into the abyssal depths. That carbon remains sequestered in the lower mantle for epochal lengths of time; some of it eventually erupts back into the atmosphere via volcanism. That, in a nutshell, is how Earth’s global thermostat is regulated.
It’s possible that back when there was water on Venus, it also had Earth-like plate tectonics with major subduction zones. But the system wasn’t widespread or large-scale enough to save it from a planetary immolation — especially if several LIP events occurred at roughly the same time. Way’s models show that several concurrent LIPs could have thrown vast quantities of carbon dioxide into Venus’ atmosphere, warming the world so severely that much of its liquid water boiled away into the sky, accelerating the warming further. With no oceans to speak of, all that carbon dioxide could not be reabsorbed. Moreover, water is an enabler of subduction: It lowers the melting point of rocks, allowing tectonic plates to more easily bend and break. And so, with no water left at the surface, major subduction zones would grind to a halt, preventing the entombment of carbon dioxide.
Other scientists tend to agree with Way’s assessment: The sun alone cannot be responsible for making Venus the awful place it is today. “I really do think you need multiple extreme volcanic episodes … to get there,” said Anna Gülcher, a planetary scientist at the University of Bern in Switzerland.
How to Break the World
While studying Venus and its runaway greenhouse wasteland, Kane became morbidly curious: Could the same fate befall Earth? “What if we shut down the carbon cycle on Earth?” he said. “Could you produce a Venus?”
To find out, he and his team built a virtual world-destroying machine. “Everybody loves a post-apocalyptic or doomsday scenario, provided it’s 5 billion years in our future,” he said.
The planetary astrophysicist Stephen Kane has investigated whether Earth’s future will resemble Venus’ present.
Courtesy of Stephen Kane
The first step was to fast-forward their model of Earth about 3.5 billion years, to when the sun and planets will be 8 billion years old. At that point, the sun will shine brighter than it does today, and Earth’s atmosphere will receive the same level of roasting-hot starlight that Venus did when it was just 1 billion years old. Back then, it’s thought, Venus would have been at a tipping point: either temperate and waterlogged, or burnt to a crisp. The 8 billion-year-old sun will push Earth to a similar, climatic knife-edge.
The team’s model suggests that in 3.5 billion years, Earth’s oceans could begin evaporating and become heat-trapping water vapor in the atmosphere. That could be sufficient to kill off Earth’s major subduction zones, since without water, there might be no subduction (plus, there would be less water for carbon dioxide to dissolve into in the first place). “We’re losing the ability to draw back that CO2 into the Earth’s mantle. So it just builds up,” said Michelle Hill of Stanford University, a member of the Reuniting Twins project.
The shutdown of major subduction zones would mean that the Earth’s tectonic plates stop clashing and jostling about. Instead, they would form a near-united rocky shell around the hot mantle. For a time, the mantle would get hotter, since the shell around it would trap heat generated by radioactively decaying compounds inside it. As heat accumulates in the interior, the Earth in Kane’s simulations would experience an uptick in volcanism lasting for about 15 million years.
This period, known as a “stagnant lid regime,” adds even more carbon dioxide to the sky. But the eruptive spike is short-lived. The mantle cools down and the crust thickens until it becomes nearly impermeable to any major carbon-erupting volcanism.
So 15 million years after its stagnant lid forms, Earth will have reached a new equilibrium. Occasional spurts of volcanism would still happen. (Likewise, scientists have found compelling evidence that Venus is volcanically active today.) But Kane explained that these sporadic eruptions aren’t expected to add much carbon to the atmosphere.
At that point, the team’s simulations stop. How Venus-like is their model of the future Earth?
Basalt lava flowed from the Kīlauea volcano on Hawai‘i in 2012. Occasionally throughout Earth’s history, continent-size basalt floods have triggered global warming and mass extinctions. Some scientists suspect that similarly enormous outpourings of lava helped scorch Venus.
USGS
There’s No Place Like Venus
If or when Earth’s large-scale subduction shuts off in about 3.5 billion years, kneecapping the planet’s ability to bury carbon, Kane and his team’s simulations indicate that the carbon dioxide level in the atmosphere will rise anywhere from 0.1 bars to 0.8 bars. (For reference, the total atmospheric pressure at sea level today is 1 bar, and roughly 0.04% of that, or 0.042 bars, comes from carbon dioxide.) Even in their best-case scenario of a 0.1-bar rise in carbon dioxide, Earth’s surface temperature surpasses 100 degrees Celsius (212 degrees Fahrenheit). The same happens at 0.8 bars, but far more quickly.
Either way, the surface of the world becomes literally boiling hot. Earth will “turn into a post-runaway greenhouse state,” Kane said. “The surface temperature will be too hot for any water. It’ll all boil away.” Nothing sitting on the world’s skin would survive.
Still, Earth won’t get close to the state that Venus is in today. “It’d be Venus lite,” Kane said.
Venus has a 93-bar atmosphere consisting of 96.5% carbon dioxide. Kane and his colleagues’ doomsday machine, no matter how hard they push it, cannot take Earth to those levels. “I was surprised by that,” he said. Because the mantle is sealed off by a stagnant lid, volcanism drops, protecting Earth from a Venus-style roasting better than he thought.
Independent scientists have praised the Reuniting Twins project for challenging prior assumptions and adding significantly to the discussion about the terminal state of rocky planets.
“I like their idea,” Way said, adding that the team’s version of a future Earth “doesn’t sound unreasonable.”
“You end up with a world that’s stinkingly hot,” said Byrne, the Washington University planetary scientist. But, he said, the possibility that it may not be Venusian levels of hot is intriguing.
Kane’s team acknowledged that their model hasn’t considered LIP-style mega-eruptions, and that these events could feasibly add a bounty of trapped carbon to the atmosphere at any point in the future. Maybe Earth gets unlucky and experiences multiple, simultaneous LIP events (though this grows less likely over time, Kane said, as the mantle cools and its churning slows). If so, that scenario could push Earth to be more like Venus than the team’s model suggests.
Uncertainties abound. But if Kane’s team is even broadly correct, it suggests that Venus has a uniquely grim history. Something — perhaps an inundation of lava — burned that planet to the bone. Earth, meanwhile, has so far been unable to bring about its own destruction. Let’s hope that remains the case long into the future.
