It’s hard to imagine a less-hospitable location in the solar system than the surface of Venus. Humans can’t survive without spacesuits and life support systems anywhere besides Earth, but Mars, the Moon, and Europa present challenges we could probably meet with current technology. Venus’ atmosphere is 92 times thicker than our own — step outside the comfort of a hypothetical space craft, and you’d be crushed like the organic equivalent of a beer can. The question of how Venus, the planet most like Earth in size, gravity, and composition, ended up a toxic hellstew of sulfur dioxide with a runaway greenhouse effect has fascinated scientists for decades. Now, new research suggests that Venus might have been the first habitable place in our solar system — and it might have remained so for billions of years.
Our current models suggest that Venus and Earth formed from similar materials, which would strongly imply that the planet initially had substantial water reserves. The scientists in this report used computer modeling to simulate how Venus might have evolved if it began as an Earth-like planet with shallow oceans and an Earth-like atmosphere. Keep in mind that “Earth-like” refers to the conditions of the ancient Earth, not the markedly different ones we find ourselves inhabiting today.
The researchers found that Venus’ simulated rotation speed had a profound impact on how the climate of paleo-Venus evolved over time. Currently, Venus spins extremely slowly, with a year that’s actually significantly shorter than its day. When the climate models kept this slow spin, the temperatures on ancient Venus remained within habitable ranges for a substantial amount of time — up to 2 billion years.
Speed up the rotation, however, and the situation goes south in a hurry. If the Venusian day is “just” 16x slower than our own, surface temperatures skyrocket in a hurry. One of the noteworthy characteristics of Venus is that its high-altitude wind speeds dwarf anything on Earth, with wind speeds up to 60x faster than the planet rotates. In hypothetical early Venus, with a slow rotation speed, the climate model predicts significant layers of cloud cover that would’ve shielded the young planet from the increased level of solar radiation it received relative to Earth. Speed up Venus’ rotation, and the weather patterns that dominate its atmospheric behavior change. As a result, surface temperatures rise markedly.
While our ability to estimate ancient Venusian climate is limited by our understanding of the planet and its evolution, results like this are interesting when considered through the lens of a Fermi Paradox solution we discussed earlier this year. One argument for why we’ve found no evidence of other life to date is that while the conditions for life to arise may be initially abundant, only a handful of planets manage to sustain life long enough for that life to begin reshaping its own biosphere on a global level. On Earth, events like the Great Oxygenation Event reshaped our entire atmosphere and, by extension, our entire biome. On Venus or Mars, even if life initially arose, it was unable to overcome other forces that were heating the planet and driving a runaway greenhouse effect (Venus), or cooling it, leading to the evaporation and sublimation of available water (Mars). Venus’ lacks plate tectonics but has been extensively reshaped by volcanism; these eruptions are thought to be partially responsible for the current climate and toxic hell-stew atmosphere.
Humans will likely never live on the surface of Venus; the environment is hilariously noxious to our own existence. The challenges Venusian terraformers would face make Mars look like a walk in the park, though there’s actually been some interesting proposals to create floating colonies in the upper layers of the Venusian atmosphere. Still, understanding how Venus’ atmosphere and characteristics evolved over time could help us focus our efforts to find stars with planets within their own habitable zones.
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