A new study from the University of Texas at Austin is helping scientists piece together the ancient climate of Mars by showing how much precipitation and melting snow filled lake bottoms and river valleys 3.5 to 4 billion years ago. The study, published in the journal Geology, represents the first time that scientists have quantified the amount of precipitation that should have been present on the entire planet. It looks like the Mars 2020 Perseverance rover is heading to the Red Planet to land on the bottom of the former lake and this is critical to this new exploration.

The ancient climate of Mars is a mystery to scientists. For geologists, the existence of riverbeds and paleolakes – basins of secular lakes – paint a picture of a planet with significant precipitation or snowmelt. But scientists specializing in computer models of the planet’s climate have failed to reproduce an ancient climate with large amounts of liquid water present long enough to account for the observed geology.

During the study, scientists found that rainfall had to be between 4 and 159 m per episode in order to fill the lakes and in some cases provide enough water to overflow and break through the lake basins.

Scientists investigated 96 indoor and outdoor lakes and their watersheds, which are believed to have formed between 3.5 and 4 billion years ago. Open lakes are lakes that have burst through the flood; closed, on the contrary, are intact. Using satellite imagery and topography, they measured lake and catchment areas as well as lake volumes and took into account potential evaporation to figure out how much water is needed to fill the lakes.

By looking at the ancient closed and open lakes and river valleys that fed them, the team was able to determine the minimum and maximum rainfall. Closed lakes provide an indication of the maximum amount of water that could fall in one go without disturbing the side of the lake basin. Open lakes show the minimum amount of water needed to cover the lake basin, causing water to burst overboard and out.

In 13 cases, the researchers found paired basins containing one indoor and one outdoor basins that were fed by the same river valleys, which provided key evidence for both maximum and minimum rainfall in a single event.

Another big mystery is how long one episode of rain or melting snow should last: days, years, or thousands of years. According to scientists, this is the next stage of the study.

At the time of this study’s publication, NASA had recently launched the Mars 2020 Perseverance Rover to visit Jezero Crater, which contains one of the open lakes used in the study. The data collected from the crater could be important in determining how much water was on Mars and if there are signs of past life.