system
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human
Did Venus once have an atmosphere like Earth, and are volcanoes the reason that changed?

I was watching a short documentary on Venus, and it mentioned a hypothesis that Venus was once much like Earth, but a major greenhouse effect drastically affected the climate there, giving us the hellscape we have in the night sky today. IIRC, one of the possible ways this may have happened was through excessive volcanic eruptions, with the after-effect being said drastic greenhouse effect. Is this a legitimate hypothesis for Venus' current conditions ?
gpt
There's evidence that Venus was basically covered in lava due to tremendous volcanic activity around 500M years ago. So much that it was basically resurfaced by the lava outflows and the solidified lava became the new surface of the planet. Needless to say, anything even approaching that level of volcanic activity would release enough carbon dioxide to send Venus into a runaway greenhouse climate if it wasn't already there in the first place. 

Scientists are divided on whether the resurfacing event 500M years ago was an isolated incident or the latest in a string of resurfacing events throughout the planet's history. There is no active plate tectonics on Venus today. The leading hypotheses on plate tectonics is that large amounts of liquid water are crucial in allowing continents to slide past one another (due to how water affects mineral formation, rock grain size and other factors that influence subduction and heat exchange from the mantle to the crust). Depending on when the Venusian oceans boiled away, plate tectonics could have stopped very early in the planet's history or in the lead up to the latest resurfacing event 500M years ago. When plate tectonics stops happening, heat cannot really be transfered out from the core & mantle nearly as effectively. That heat builds up until the crust fractures catastrophically and the built up heat and pressure escapes all at once as lava in a massive, planet-wide resurfacing event.

The loss of oceans on Venus also meant that geologic weathering stopped, preventing the main way to remove carbon dioxide from the planet's atmosphere. It could very well be that Venus might have been habitable and may have had primitive life in its oceans. Maybe the loss of its magnetic field caused the oceans to slowly be lost to space to kick the whole process off. Or maybe the primitive life on Venus never managed to get photosynthesis figured out and the planet slipped into an unstoppable runaway greenhouse as the Sun increased in brightness over billions of years. Venus was most likely made from the same kinds, amounts and ratios of materials the Earth was, so the planetary "ingredients" were very similar starting out.

The funny thing is, Earth experienced several major glaciations where thick ice caps basically expanded to the equator and the entire planet was covered in ice. The latest round of these severe ice ages happened right around the time Venus' oceans could have been boiling away or even within a few million years of Venus' last resurfacing event itself. How depressing or absolutely misleading it would have been if any interstellar explorers or probes visited the two planets during this time. One planet was a noxious hellhole with temperatures so high, METALLIC SNOW fell on its highest mountaintops. And the next planet out from the Sun was a frozen wasteland way colder than Antarctica is today, and could have been locked in a deep freeze for tens of millions of years at a time. Any notion of a "Goldilocks Zone" in our Solar System would be unthinkable given the contrast of these two planets even though they were not that much different initially.