Venus may be one of the brightest and most beautiful objects in our night sky, but don’t be fooled. Our neighboring planet is extremely unsuitable for life as we know it – a toxic, burning world that humans will never set foot on. Despite differences in habitability, Venus shares some striking similarities with Earth. Both planets have roughly the same size, mass, and density and have very similar compositions. This begs the question: Could Venus be habitable?

If a new study shows that Venus once had habitable conditions and liquid water on its surface, that was a long time ago and just a short time before the planet became the parched, arid world it is today. Planetary scientists Alexandra Warren and Edwin Kite of the University of Chicago modeled the history of the Venusian atmosphere to determine the rate and mechanisms of oxygen loss; More than 3 billion years ago.

This is what Venus is now. Extremely dry and extremely low in oxygen. Its atmosphere consists of 96 percent carbon dioxide and 3 percent nitrogen and other gases such as small amounts of sulfur dioxide. Its atmosphere is extremely thick, its pressure is 90 times that of Earth, and it is destroyed by strong winds and sulfuric acid rains. And because its atmosphere is so dense, heat cannot escape. Venus has the hottest surface temperature of any planet in the Solar System, with an average of 464 degrees Celsius (867 degrees Fahrenheit).

Earlier in the history of the solar system, when the Sun was less powerful, Venus could have been more temperate, with lakes and oceans of liquid water. Planetary scientists want to know how and why Venus got into its current state; Because Venus is so similar to Earth—some climate models suggest Venus may have had water less than a billion years ago—studying its history can help us understand how likely it is for our home planet to follow the same path.

The lack of oxygen in the atmosphere of Venus causes a certain mystery. If the planet had a liquid ocean, that water would evaporate into the atmosphere as Venus warms, decomposing into hydrogen and oxygen via photodecomposition, a chemical reaction triggered by sunlight.

The hydrogen would escape into space, but the oxygen would have to stay.

Warren and Kite wanted to know where the oxygen might have gone, so they built a model based on habitable Venus. They placed oceans of water on the surface of Venus, added mechanisms that could contribute to oxygen loss, and adjusted parameters such as the amount of water and the time period in which it could be present. They ran the model 94,080 times, and the levels of dioxide, water, and carbon monoxide at the end of the run were considered successful to be within the upper limits of gases in Venus’ atmosphere today.

In the end, only a small percentage of the model studies were successful and they showed some interesting trends. One possibility is that Venus’s oxygen combines with carbon released by volcanoes to form carbon dioxide, but this seemed unlikely. Rather, oxygen seemed to meet one of two fates: escape into space or be absorbed by oxidizing magma, such as basalt, on the planet’s surface. And the oceans must have dried up no later than 3 billion years ago.

But the magnitude of Venus’ past volcanic activity may have been limited by the amount of radioactive argon still present in the planet’s atmosphere. By determining how active Venus’ volcanism was in the past, the researchers were able to estimate how much water there might be on the planet. The answer is not much. Venus’ oceans cannot be more than 300 meters (984 feet) deep. That’s less than 10 percent of the average ocean depth on Earth of 3,688 meters.

The results thus reconcile the lack of oxygen in Venus’ current atmosphere with potential early conditions for life, but the researchers say the gap is narrow. This gap narrows even more if we take into account the argon registration. When considering all of Venus’ current atmosphere, less than 0.4 percent of runs were successful. Future missions could try to measure the composition of Venus’ surface to help determine if the planet did indeed fall into this very narrow gap.