Planetary Sciences News

A Flip-Flopping Climate Could Explain Mars's Watery Past

A new hypothesis might reconcile two opposing theories that have tried to explain Mars's mysterious history for more than 40 years.


In the 1970s, images of Mars taken by the Mariner and Viking spacecraft revealed enormous channels and valley networks—both of which are reminiscent of catastrophic floods and river drainage systems on Earth. The fluvial features were the first sign that 3.8 billion years ago, the planet was once a lush oasis, awash with oceans, lakes, and rivers.

Apparent delta in Jezero Crater on Mars.
Scientists have long debated how enough water could have existed on Mars—with its thin atmosphere and great distance from the Sun—to create such features as this apparent delta in Jezero Crater. The feature may have formed when rivers shepherded claylike minerals (green) into a crater lake early in Mars’s history. Credit: NASA/JPL/JHUAPL/MSSS/Brown University

But how was early Mars so wet? That question sparked a 40-year-long debate that has divided planetary astronomers into two camps: those that think Mars must have once contained a thicker and warmer atmosphere—which made the Red Planet hospitable to liquid water and potentially the evolution of life—and those that think Mars was mostly cold save for short bursts of warmth.

A new hypothesis might reconcile both camps, some scientists say. In a study accepted for publication in Earth and Planetary Science Letters, Natasha Batalha, a graduate student from Pennsylvania State University in University Park, and her colleagues suggest that Mars flip-flopped between a deep-freeze climate and a habitable one.

Batalha’s adviser at Penn State, coauthor James Kasting, said that he previously supported the hypothesis that early Mars was long enshrouded in a thick, warm atmosphere but no longer does. “We now agree with the geologists who say that Mars had to be cold most of the time,” he said, speaking of himself, Batalha, and their coauthors. However, Mars somehow got “warm enough for long enough to form the valleys” and other fluvial features we see today.

A Climate That Oscillates

Although multiple theories have tried to explain how these fluvial features formed on early Mars, Batalha and her colleagues assert that those explanations all overlooked a phenomenon that is crucial for Earth: the carbonate-silicate cycle.

Volcanoes on Earth spew carbon dioxide into the air, creating an atmosphere that’s thick with greenhouse gases. This increases the temperature of the planet, which increases rainfall. The precipitation then scrubs the carbon dioxide from the air by forming carbonic acid. When the acid splashes the surface, it dissolves silicate rocks and creates carbonate, which is then carried into the oceans, where it is eventually subducted into the Earth’s mantle. The cycle completes itself—a half a million years later or so—when volcanoes spew carbon dioxide back into the air.

Could a similar cycle have existed on Mars? Batalha thought it was worth investigating. But when she and her colleagues added the cycle to an atmospheric model for early Mars, they found a stark difference from the Earth.

Because the Red Planet is so far from the Sun, the cycle begins on a world coated with glaciers. Volcanoes spew greenhouse gases into the atmosphere, increasing the temperature until it’s finally warm enough to rain. Downpours then scrub the potent gases from the atmosphere rapidly, shutting down the greenhouse and plunging the planet back into its glaciated state. Whereas on Earth the cycle acts as a built-in temperature control system, keeping our planet habitable on million-year-long timescales, the team’s simulations showed that on Mars it forces the planet to oscillate between a glaciated world and a habitable one.

“We were really surprised to see that when you add this to these early Martian atmospheric models you get these dramatic climate cycles that give you about 10 million years of warmth in between these 120-million-year frozen states,” Batalha said. “And 10 million years is exactly the amount of time that you need to form all these fluvial features that we see on Mars.”

“The idea is a happy medium,” said Ramses Ramirez, an astronomer at Cornell University in Ithaca, N.Y., who was not involved in the study. “Maybe Mars was mostly cold, but then you had very transient warm periods, marrying the two hypotheses.”

Where Are All the Carbonates?

But simulating such climate cycles on early Mars doesn’t mean they actually happened. To model them, Batalha and her colleagues had to endow the Red Planet with an early history of plate tectonics. Plate tectonics on Mars is still a speculation, although there is some evidence for linear volcanic zones and steep cliffs that could have been generated by fault lines. The researchers also had to add a lot of carbon dioxide and hydrogen to the Martian atmosphere that scientists aren’t certain were present in the planet’s early days.

“There’s no way to go back 3.8 billion years ago and study what the atmosphere was made of then,” said Batalha. “In fact, we have a hard time nailing down what’s going on in the Martian atmosphere today,” she continued, referencing the mysterious methane signatures that scientists have seen belching from the Red Planet but can’t yet explain.

Still, Batalha and her colleagues are pretty sure the young planet once outgassed plenty of hydrogen. Previous studies have found that meteorites from Mars are rich in hydrogen compared to rocks on Earth, supporting the notion that abundant outgassing of the element may have occurred there.

But whether the planet also outgassed a lot of carbon dioxide is a harder question to answer. Itay Halevy, an astronomer at the Weizmann Institute of Science in Israel, who was not involved in the study, and Ramirez both told Eos that if that outgassing occurred, Mars should be overrun with carbonate rocks—and it isn’t.

But Batalha and her colleagues have a reply to that too: An atmosphere laced with carbon dioxide would also form highly acidic rain, they say, which would remove any carbonates from the surface and place them soundly underground. And Ramirez agrees that this is plausible. “There are some craters where you can see that there could be some carbon dioxide [locked in the rocks in the form of carbonates] in there,” he said. “It’s hard to tell how much there is.”

“You just have to dig deeper”—metaphorically and physically, Batalha said.

—Shannon Hall, Freelance Writer; email: [email protected]

Citation: Hall, S. (2016), A flip-flopping climate could explain Mars’s watery past, Eos, 97, doi:10.1029/2016EO059281. Published on 13 September 2016.
© 2016. The authors. CC BY-NC-ND 3.0
  • Colin

    Gosh, are they trying to suggest that climate on a planet can change all by itself with no human influence at all? Utterly absurd, without man-made CO2 in the atmosphere a climate cannot possibly change. Everyone knows that. Deniers!

    • Euro_Hero

      FYI in case you missed it 2016 is on course to be the warmest year on record, surpassing 2015 which itself surpassed 2014 as the warmest year. And some say global warming has stopped. Hmmm. As for CO2 you have to go a million years back to find levels comparable to today’s – and obviously it was warm then. However the rates at which this is happening are unprecedented. If you think that the fact that CO2 levels having risen from 200ppm to 400ppm thanks to man-made production as well as temperature having increased almost 1C in the last 150 years, all coinciding with the rise of industry, is all coincidental and should not even be looked at, and rather use some far more far-fetched explanations, then I rest my case

      • Colin

        That kool-aid is tasty, huh? You’ve got all the catch-phrases and buzzwords present and correct. Well done!

        The temperature has increased a whopping 1C in 150 years? Gasp!That’s what all the fuss is about? Newsflash – the climate is capable of making that kind of change all by itself, with no human input at all, and has demonstrably done so many times in the recent past. That means that our CO2 emissions – which by the way are just a fraction of the total 400ppm – are making no measurable difference to the natural variations in Earth’s temperature.

        Your side of this argument has been crying wolf and predicting doom for so long and been so wrong every time that the general public is tuning you out. Take the predictions of ‘imminent’ and ‘soon’ sea level rise due to the polar ice caps melting. If anyone believed that, the value of coastal properties around the world would be plunging, and people would be moving inland. But of course, what is happening is the complete opposite; prices are rising, and folks want to live by the ocean.

        All you have is models and projections; you cannot demonstrate man-made climate change in the real world to anyone where they live, can you?

        • Alex Slonimer

          It’s a complex system Colin. The oceans have been buffering the rise in atmospheric carbon (which has lead to the ongoing acidification of the ocean) as well as helping to regulate the temperature by absorbing massive amount of energy. However, the warming of the ocean will lower the solubility of carbon, which will in the very near future, lead to sky-rocketing atmospheric carbon and intensified warming.

          Also, the anthropogenic emissions of carbon, both through industry, deforestation, and other land use changes, really does attribute for the rising atmospheric carbon. Admittedly, other sources of atmospheric carbon exist, but these have always been around, and have typically been in balance with the carbon cycle.

    • drklassen

      Yes; all climate scientists know that climate can change in response to non-human forcing factors. It *also* changes in response to human induced forcing factors. Your straw man is complete poppycock.

  • catsend

    Too bad they can’t do ice core analysis at the poles as we do on Earth. Ice cores here on earth contain the atmospheres from ancient times contained in ice formed at that time. Problem is, the polar ice on Mars seems to totally melt in summers.

  • davidlaing

    There’s a lot of arm-waving in this article, and no wonder. The authors take a number of assumptions about Mars, take them for granted, and weave them into a complicated model that leaves a lot of holes and requires a lot of explaining. They also ignore one very important fact: Earth has a massive satellite, Moon, and Mars doesn’t. I suppose they can’t be blamed for this glaring omission because virtually everyone has made it before, but the fact is that Moon has 1/81 the mass of Earth, and it exerts a significant gravitational pull on Earth’s equatorial bulge. This, in turn, stabilizes the nutation of Earth’s rotational axis, keeping it within a degree or so of its present 23.4 degrees. This means that Earth’s polar regions have never pointed directly, or even substantially, toward Sun during the solstices, which would have subjected a significant portion of Earth’s oceans to 24/7 irradiation. This would have caused overheating and evaporation and subsequent photodissociation of water (an ozone shield didn’t develop on Earth until the early Proterozoic). That this didn’t happen on Earth, but did on Mars and Venus (which both lack large natural satellites) is reflected in the very large duterium to hydrogen ratios on Mars and Venus compared to the same ratio on Earth (on dissociation, the lighter hydrogen would have escaped to space). This rather obvious explanation neatly explains all the observed phenomena on Mars and Venus and leaves no holes and requires no arm-waving. This highlights the pitfalls of Cartesian thinking, i.e., focusing so closely on the object under study that you miss important factors nearby that may influence that which is under study.