As recently as a billion years ago, scientists say, rivers on Mars were substantially larger than rivers on Earth, possibly filling deep aquifers with liquid that today might each year still erupt to the surface to wash gullies in the steep walls of Martian craters.

The finding comes from two independent lines of study, published separately last month in the journals Science Advances and Nature Geoscience.

In the first, a team led by Edwin Kite, a planetary scientist at the University of Chicago, Illinois, looked at high-resolution topographic maps of hundreds of Martian river channels, comparing them to the size of the drainage basins that fed them.

His team found that although these rivers were intermittent, they were about twice as wide as similar ones on Earth today, and therefore carried comparably more water.

Kite’s team also found and that these intense flows occurred as recently as a billion years ago, and that they corresponded to average runoff rates on the order of 0.3 to 2.0 millimetres per day over the regions feeding them.

By comparison, Kite’s own city, Chicago, averages 2.56 millimetres of precipitation per day, and Adelaide, Australia, averages 1.5. During its wet periods, therefore, ancient Mars might have been nearly as wet as large parts of modern-day Earth.

The source of all this water, Kite adds, remains a mystery. “We don’t know if the water source was snowmelt or rainfall,” he says.

But it does prove that the planet remained warm enough, long enough, for that much surface water to have existed at a time when scientists thought its thinning atmosphere had already sent it into the deep freeze.

“The problem is to warm the planet, not, ‘Where did the water come from?’,” Kite says.

One possibility, he adds, is that warming cycles were driven by variations in evaporation of carbon dioxide from the Martian ice caps or by cyclical increases in other climate-warming gases, such as methane or water vapour.{%recommended 1673%}

Ultimately, he says, much of that liquid water would have wound up lost to space, or trapped at the Martian poles.

But some might have gone underground, where it may well still remain in deep aquifers, such as those known to underlie Earth’s Sahara Desert.

From there, says Essam Heggy, author of the second study and a planetary scientist at the University of Southern California, Los Angeles, it may still be emerging each summer to create short-lived stream channels that each Martian year decorate the walls of some Martian craters with dark streaks, unpoetically called recurring slope lineae.

There is some dispute over whether these streaks are indeed created by water (an alternative theory says they are created by rockfall or related avalanche-like processes), but Heggy thinks they could be caused by the movement of groundwater from hundreds of metres deep in the Martian subsurface, up through fractures created by ancient impacts.

Like artesian springs, bubbling to the surface of the Earth under the pressure of overlying strata, these aquifers, he posits, burst forth at weak spots in crater walls and flow downslope, eroding gullies and darkening the surrounding rocks with what appears to be the glint of moisture.

“These features are very mysterious on Mars,” he says, “because they are among the very few dynamic features we see, which means that they appear in the spring and disappear in the fall.”

The streaks have been observed for years, but prior suggestions for their origins (other than the ones that don’t require water) have suggested that they are produced by some kind of ice-melting process occurring near the surface.

“We are saying this is not from the surface,” Heggy says. “This is from leakage coming up from the subsurface.”

In part, the find comes from modelling the behaviour of water, or brine, under the conditions expected to be found 750 or so metres beneath the Martian surface. But it’s also the case that locations prone to recurring lineae appear to be correlated to known impact-related fractures that could provide underground water an escape route to the surface.

The results, Heggy says, are important for understanding not only Mars, but also Earth.

“It could be very similar to places we see in the Sahara,” he says, noting that the Sahara, too, has regions where flow features once associated with surface water might actually have been caused by leakage from deep aquifers via fractures similar to the ones his team is now studying on Mars.

On Mars, he says, discovering leakage from such aquifers is not only proof that subsurface water still exists, but that surface water must once have existed to fill them.

It’s also a demonstration, he notes, of how much we still have to learn about the behaviour of fossil water in our own deserts, whether they are the Sahara, Arabia, or wide swaths of Australia.

“We need to understand the groundwater dynamic in desert environments to help us understand Mars and other planets,” he says.

And for those interested in the long-term future of the Earth, he adds, “understanding how groundwater has formed on Mars, where it is today, and how it is moving helps us understand the similarities to our own planet, and if we are going through the same climate evolution and the same path that Mars is going [through].

“Understanding Mars’s evolution is crucial for understanding our own Earth’s long-term evolution, and groundwater is a key element in this process.”