Mars is a frigid desert today, but the planet was rich in water 3.5 to 4 billion years ago. Some of this water was free-flowing on the surface and some was carving the terrain under giant ice sheets. 

A simple but complex question is how much precipitation, whether rain or snow, fell during those wetter days. A new study published in the journal Geology has tried to quantify just how much water likely fell in a single event. The answer is anything between 4 and 159 meters (13 and 520 feet). The range shows just how uncertain we are about the climate, but it does suggest that if this work is correct, Mars experienced some portentous floods.

"This is extremely important because 3.5 to 4 billion years ago Mars was covered with water. It had lots of rain or snowmelt to fill those channels and lakes. Now it's completely dry. We're trying to understand how much water was there and where did it all go," lead author Gaia Stucky de Quay, from The University of Texas at Austin, said in a statement.

"It's a huge cognitive dissonance. Climate models have trouble accounting for that amount of liquid water at that time. It's like, liquid water is not possible, but it happened. This is the knowledge gap that our work is trying to fill in."

The estimates are based on the 96 open-basin and closed-basin lakes and their watersheds that are believed to have formed between 3.5 and 4 billion years ago. The idea behind the research is quite simple: If we know the area and depth of these basins, we can figure out how much water would be needed to fill them. And if these lakes overflowed into a river, how much extra water would be needed to cause that.

There are still many uncertainties, such as the rate of evaporation, how long these rainfalls lasted, and how often they happened.