As best we know, Mars today is a lifeless, dried-up red abyss but it wasn’t always that way. Most who study the Red Planet agree that it was populated with flowing rivers and lakes billions of years ago. The only problem, as The Verge highlights, is that scientists aren’t really sure how the planet was warm enough to support liquid water.

Since arriving on Mars roughly four and a half years ago, NASA’s car-sized Curiosity rover has found evidence suggesting that water once pooled in the Gala Crater (the area of Mars the rover landed in) around 3.5 billion years ago.

At that time, however, it is believed that the Sun was only generating around 70 percent of the energy it churns out today. If that was indeed the case, it means that some other factor would have had to contribute to Mars being warm enough for water to exist in liquid form.

Some believe that the ancient Mars atmosphere had an abundance of carbon dioxide, a greenhouse gas that traps heat. If true, this could explain how the Red Planet was able to retain enough heat for liquid water. Again, if true, scientists believe that the gas would have dissolved into the Martian water and created something called carbonic acid.

Said acid is known to weather rocks beneath water and eventually produce what’s known as carbonate minerals. The problem is that NASA’s Curiosity rover hasn’t found any traces of carbonate minerals in the rocks it has studied to date.

That’s not to say the mineral isn’t located elsewhere on Mars; it just hasn’t been spotted in the Gala Crater using the rover’s CheMin instrument. What’s more, it’s possible that the carbonates are present but are in quantities too small to be detected by the instrument.

As of now, there appear to be more questions than answers.