Today, Venus is a planet reminiscent of Dante Alighieri’s seventh ring of Hell. The second planet from the Sun, Venus is a little smaller than Earth and covered with mountains and large lava plains that see scorching temperatures and a weighty atmosphere mainly made of carbon dioxide with thick clouds of sulfuric acid. But new research suggests that Earth’s “twisted sister” planet may have once been characterized by temperate conditions and liquid water capable of supporting life.

That is, until a dramatic transformation more than 700 million years ago restructured the planet’s atmosphere.

“Our hypothesis is that Venus may have had a stable climate for billions of years. It is possible that the near-global resurfacing event is responsible for its transformation from an Earth-like climate to the hellish hot-house we see today,” said Michael Way, from The Goddard Institute for Space Studies, in a statement.

The new study presented last week at the EPSC-DPS Joint Meeting builds on previous research from NASA’s Pioneer Venus that suggested the planet once had shallow oceans of water. This time around, scientists ran five simulations that assumed different levels of water coverage across the planet in order to see if a stable climate would have been able to support it. Three of the five scenarios assumed Venus had a topography similar to what we see today but with an ocean averaging 310 meters (1,017 feet), a shallow layer of water averaging 10 meters (33 feet), and a small amount of water that was locked in soil. They then compared those against a scenario accounting for a planet with Earth’s topography and a world with oceans reaching 158 meters (518 feet) deep.

Researchers then simulated environmental conditions at 4.2 billion years ago, 715 million years ago, and today by adapting a 3D general circulation model to account for an increase in solar radiation seen in our Sun over time and changing atmospheric compositions over Venus.

According to the simulations, Venus would have been able to maintain stable temperatures between 20 and 50°C (68 and 122°F) for about 3 billion years. Around 4 billion years ago, Venus would have rapidly cooled after its formation, resulting in a carbon dioxide-dominated atmosphere. Over the next 3 billion years, the planet may have evolved like Earth and locked CO2 into silicate rocks in the planet’s surface. Sometime around 715 million years ago, researchers believe the atmosphere may have been dominated by nitrogen with trace amounts of CO2 and methane much as Earth is today.

“Venus currently has almost twice the solar radiation that we have at Earth. However, in all the scenarios we have modeled, we have found that Venus could still support surface temperatures amenable for liquid water,” said Way.

These conditions may have been maintained up until today if a mysterious cataclysmic event hadn’t occurred. Between 700 and 750 million years ago, a series of events released – or “outgassed” – carbon dioxide stored in the planet’s rocks. Though the exact cause is unknown, scientists believe that it was likely linked to volcanic activity such as what occurs when magma bubbles and releases CO2 from molten rocks into the atmosphere. If these rocks solidified before reaching the surface, a barrier would have been created that prevented the gas from being reabsorbed into the rocks, leading to a greenhouse effect.

“On Earth, we have some examples of large-scale outgassing, for instance the creation of the Siberian Traps 500 million years ago which is linked to mass extinction, but nothing on this scale. It completely transformed Venus,” said Way.

How this process occurred depends on a deeper understanding of how quickly Venus had initially cooled following its formation and whether it was able to condense liquid water on its surface in the first place. It’s unknown whether the outgassing event occurred in one event or over the course of several. In all, researchers say a deeper understanding of the rocky planet is needed in order to answer these questions.