The surface of modern Mars is a parched memory of water. What little remains of the life-giving liquid trickles from salty seasonal seeps, languishes in pockets as underground lakes, or sits frozen in sheets of ice.
Yet the planet's rusty rocks record a past flush with water; deep valleys carve through a landscape speckled with dry lake beds, alluvial fans, and smooth river pebbles. While scientists have long thought the planet's warm and wet period was relatively brief, a study published today in Science Advances hints that these rivers may have stuck around for much longer than previously thought.
According to the new analysis, these ancient channels are wider than similar channels on Earth today. What's more, water rushed through these hefty features at places around the globe between 3.4 and 2 billion years ago. That's late in Mars's wet history, a period when many scientists believe the red planet was already desiccating.
“The traditional story of Mars’s climate history is that it used to be warm and wet, and now it’s cold and dry. But the evidence suggests that Mars’s climate evolution is more complicated than that,” Kathryn Steakley of NASA's Mars Climate Modeling Center, who was not involved in this work, says via email.
The mention of water on Mars inevitably draws excitement, since where there was water, there might also have been life as we know it. But don't start dreaming up names for fossil Martians quite yet. Many questions remain about what was going on during this extended period of Mars's past and how rivers could fill under the changing conditions.
“It actually makes the problem of what’s allowed early Mars to be warm and wet—which is already a difficult problem—more difficult,” says study author Edwin Kite, a planetary scientist at the University of Chicago.
Gushing rivers, languishing lakes
While Mars's current atmosphere is too meager to trap much heat from the sun, many scientists agree that a thicker version likely once blanketed the red planet and fostered a wetter world. Even then, Mars was no tropical escape. The ancient sun was 25 to 30 percent weaker than it is today, which means that much less solar radiation warmed Mars's rocky terrain.
“Things were always kind of right at the edge of being able to have water flowing across the surface,” says Alan Howard of the Planetary Science Institute in Tucson, Arizona, who was not involved in the work.
A few factors could've helped ease this liquid conundrum. On Earth, our churning outer core whips up a protective magnetic field that keeps our relatively thick atmosphere from being stripped away by solar wind. The same was likely true for early Mars. And perhaps the mix of gasses differed from that in Mars's current atmosphere. For instance, some experts suggest that erupting volcanoes were once pumping greenhouse gasses into the Martian sky.
However it happened, this warm and wet period didn't last. The ancient atmosphere seems to have eroded away, and with it went many of the Martian lakes and rivers. Kite and his colleagues initially thought that after this time, as rivers lingered mostly in lower altitudes, the rushing waters also slowed to a trickle.
“That was the hypothesis,” Kite says. “And we were wrong.”
Go with the flow
Bolstered by the stunning resolution of Mars-orbiting instruments like the High Resolution Imaging Science Experiment (HiRISE), the researchers analyzed the dimensions of more than 200 ancient riverbeds. Based on the channels' sizes, meandering size, and the relative ages of surrounding terrain, the team found what seems to be a curiously persistent and late period of liquid runoff.
What was driving this flow at such an unexpected time remains unclear. Some researchers, including Kite and his team, are investigating whether water-ice clouds could flourish in low atmospheric pressures. Such clouds still linger above Mars today, and if they were thicker, they might trap enough heat to melt snow and ice. Or perhaps the dates of river formation are wrong, which would mean the channels formed during a more ancient time when a thicker atmosphere heated the Martian snowpack.
Kite acknowledges that without better estimates for channel depth or the size of channel sediments, it's tough to precisely say how much water once gushed through them. Width can only tell you so much, Howard agrees, noting that this measure may slightly inflate estimates, since the flow of any given river may not span the full channel.
Still, based on the information at hand, “the basic premise and conclusions that they are making—that there are fairly hefty discharges—I think are realistic,” Howard says.
Scientists may soon get even more clues: The Mars 2020 rover is slated to land in Jezero crater, which contains one of these late-stage river deltas, Kite notes. That rover can take pictures of the sediments, which would help scientists determine how much water poured into the crater. But the only definitive, albeit unrealistic, solution is to send an orbiter back billions of years to check out the red planet's churning surface.
As Howard says with a laugh, “that would destroy all the controversy and interest in trying to piece it together from the sparse evidence we have at the present time. So the science would be less interesting.”