The uncertainty of science: A new study of Martian geology suggests that rivers ran on the surface are longer and later in the planet’s history than previously thought.

Seeking a better understanding of Martian precipitation, Kite and his colleagues analyzed photographs and elevation models for more than 200 ancient Martian riverbeds spanning over a billion years. These riverbeds are a rich source of clues about the water running through them and the climate that produced it. For example, the width and steepness of the riverbeds and the size of the gravel tell scientists about the force of the water flow, and the quantity of the gravel constrains the volume of water coming through.

Their analysis shows clear evidence for persistent, strong runoff that occurred well into the last stage of the wet climate, Kite said.

The results provide guidance for those trying to reconstruct the Martian climate, Kite said. For example, the size of the rivers implies the water was flowing continuously, not just at high noon, so climate modelers need to account for a strong greenhouse effect to keep the planet warm enough for average daytime temperatures above the freezing point of water.

The rivers also show strong flow up to the last geological minute before the wet climate dries up. “You would expect them to wane gradually over time, but that’s not what we see,” Kite said. The rivers get shorter—hundreds of kilometers rather than thousands—but discharge is still strong. “The wettest day of the year is still very wet.”

They also found that these rivers had been wider than those seen on Earth, which would make sense if there were few if any plant life to fix the banks in place, as on Earth. The lower Martian gravity probably plays an even larger role in this.

You can read the paper here. The study confirms many other previous studies of Martian surface features, which have repeatedly found evidence that liquid water once existed on Mars. That it found the water flowed later and more extensively only makes more difficult the deeper and probably biggest mystery of Martian geology, however, which is that scientists have not been able to come up with a historic atmospheric model that would allow that liquid water to exist. Mars today is too cold and its atmosphere is too thin for liquid water to flow, and the evidence from the past does not suggest an atmosphere different enough to change that.

It must have been different, but we don’t know how that was possible, based on the data we presently have. And this study makes solving that mystery even more difficult.