Why Venus Turned to Hell

If you want to understand what a runaway greenhouse effect looks like, do not run a computer model. Look up. Venus is right there, 41 million kilometres away at its closest approach, gleaming with a reflected brilliance that made ancient astronomers call it the morning star. It is the brightest object in the night sky after the Moon. It is also, in every practical sense, hell.

The surface temperature is 465 degrees Celsius — hot enough to melt lead. The atmospheric pressure is 92 times that of Earth’s at sea level, the equivalent of being 900 metres underwater. Clouds of sulfuric acid at an altitude of 45 to 70 kilometres reflect 70 percent of incoming sunlight, yet still allow enough to reach the surface to drive a greenhouse effect so extreme that even the poles, shielded from direct solar input, are as hot as the equator.

It was not always this way. Probably.

The Early Venus Problem

Venus sits just inside the inner edge of what we loosely call the habitable zone — the region around a star where liquid water can exist on a planetary surface. In the early solar system, the Sun was perhaps 30 percent dimmer than it is today. Under those conditions, Venus might have had liquid water. Some climate models suggest it could have remained habitable for two billion years — longer than it took for multi-cellular life to evolve on Earth.

We do not know. We cannot know with current data. No mission has returned samples from Venus. Radar maps from orbit give us terrain but not geology. The Soviet Venera landers, the only spacecraft ever to touch down and survive on Venus’s surface, lasted between 23 minutes and two hours before the heat and pressure destroyed them. In those few minutes, they returned photographs of flat rocks and dust. Not enough.

The Runaway

At some point, Venus tipped. The precise mechanism is debated, but the broad strokes are understood. As the Sun brightened over billions of years, more energy reached Venus’s surface. Oceans — if they existed — began to evaporate faster. Water vapour is a potent greenhouse gas. More vapour meant more warming, which meant more evaporation, which meant more vapour. A feedback loop with no natural brake.

Eventually the oceans boiled away entirely. Ultraviolet radiation from the Sun broke water molecules in the upper atmosphere into hydrogen and oxygen. The hydrogen, light enough to escape Venus’s gravity, was lost to space. The oxygen combined with surface rocks. And Venus was left dry, hot, and suffocating beneath an atmosphere of carbon dioxide so thick that the sky at the surface is permanently dim, tinged orange by the haze of cloud above.

The last major volcanic resurfacing, based on crater count analysis, happened somewhere between 300 and 700 million years ago — a wide uncertainty that tells us how little we still understand. Whether Venus is geologically active today is an open question that two upcoming missions — DAVINCI and EnVision — may finally answer.

Key Takeaways

• Venus and Earth are near-twins in size and composition but diverged into radically different worlds.
• The runaway greenhouse effect is driven by a positive feedback between water vapour and surface warming; once started, it has no natural brake.
• Early Venus may have had liquid water for up to two billion years — longer than it took complex life to appear on Earth.
• The Venera probes, lasting 23 minutes to two hours on the surface, remain the only spacecraft ever to operate on Venus directly.
• DAVINCI and EnVision missions will attempt to determine whether Venus is still geologically active and reconstruct its atmospheric history.