Our understanding of the role water played in the history of Mars has improved greatly in the last few years, with scientists uncovering evidence of vast oceans, buried ice fields and complex networks of rivers that once sprawled across its surface. A new study has examined the lattermost of these geographical features in close detail and found these Martian waterways were far thicker and dried out far later than previously thought, suggesting that for billions of years, Mars was home to gushing rivers even wider than those on Earth today.

Today, traces of water on Mars can be found in the form of vapor in the atmosphere and stowed away underground in ice sheets and lakes. But the plentiful dried-out riverbeds filled with smooth pebbles that snake their way around the Red Planet provide useful clues about its watery past. By studying these through imagery captured by orbiting spacecraft, scientists are slowly piecing together a very complex puzzle.

While the evidence is clear that the deep, ancient channels seen on Mars were carved out by water, the type of climate that facilitated these conditions remains very much a mystery. This is because of the planet's very thin atmosphere (and therefore weak greenhouse effect), and that in the planet's early history the Sun was far fainter and weaker, providing around 25 to 30 percent of the luminosity that it does today. And less heat, presumably, means less liquid water.

"Indeed, even on ancient Mars, when it was wet enough for rivers some of the time, the rest of the data looks like Mars was extremely cold and dry most of the time," says study author Edwin Kite, assistant professor of geophysical sciences at the University of Chicago.

Looking to fill in the blanks, Kite and his team studied photographs and elevation models of more than 200 ancient Mars riverbeds. By looking at characteristics such as their width, steepness and the size of gravel chunks within them, the scientists were able to draw some conclusions about the force of water that once flowed through them.

According to the team, this provided evidence that strong and persistent water flow existed possibly as recently as two billion years ago, far beyond when the planet's last wet climate is thought to have wrapped up around 3.5 billion years ago. What's more, analysis of the catchment areas indicates that these ancient rivers were wider than the rivers found on Earth today. This adds to our understanding of the ancient climate on Mars, but it throws up a few curveballs, too.

"Our work answers some existing questions but raises a new one," says Kite. "Which is wrong: the climate models, the atmosphere evolution models, or our basic understanding of inner solar system chronology?"

The research was published in the journal Science Advances.