Colliding Molecules in Mars’ Atmosphere May Solve an Ancient Climate Mystery

By Jeffrey Marlow | January 26, 2017 11:12 am
Features on Mars like this one, a likely river delta deposit, point to a warmer ancient past. (Image: NASA/JPL)

Features on Mars like this one, a likely river delta deposit, point to a warmer ancient past. (Image: NASA/JPL)

Climate change on Earth is a well-established phenomenon, but scientists have long struggled to explain an even more dramatic change of conditions, long ago in a far-off land.

Mars is a dry, frigid planet today, with an average ground temperature of about -60 °C. Liquid water seems to be possible only under a narrow range of circumstances, but for the most part, water sublimates directly from solid ice to gaseous water vapor. And yet, features on the surface of Mars tell a very different story. Dramatic river channels and canyons indicate that vast quantities of water once flowed from the southern highlands to northern plains.

Clearly, Mars must have been warmer to allow all of this liquid water to persist on its surface. But how were such distinct conditions created? What was different about the planet billions of years ago?

Several options have been proposed over the years. Perhaps heavy clouds of carbon dioxide or sulfur dioxide spewed from volcanoes trapped enough heat in the atmosphere? Maybe changing orbital cycles melted ice and caused massive, landscape-gouging floods? Could hydrogen or ammonia in the atmosphere have done the trick? No, no, and no, according to a number of scientific studies over the years: all of these scenarios have disqualifying flaws.

But a new proposal may have cracked the case, and it relies on a physics parlor trick called collision induced absorption. Atmospheric gases trap heat by absorbing incoming (from the Sun) and outgoing (from the planet’s surface) radiation at certain wavelengths; the activated molecules then vibrate faster and re-radiate some of the energy, transferring heat throughout the atmosphere. While individual gas molecules account for much of this “greenhouse effect” – key players like carbon dioxide and methane are well known – interacting gas molecules contribute an additional dimension of absorption that is frequently overlooked.

The recent study, led by Robin Wordsworth, an assistant professor of environmental science and engineering at Harvard University, focuses on the heat absorption that occurs when two pairs of molecules collide: CO2 & H2, and CO2 & CH4. He and his colleagues found that CO2 is the key player – its oxygen atoms pull electrons away from carbon, and this distributed field of electron density creates a wider array of absorptive states, enhancing the heat-trapping properties of colliding molecules.

CO2, which is abundant in the martian atmosphere, absorbs energy most effectively at wavenumbers around 700 cm-1; the CO2–H2, and CO2–CH4 pairs fill a conspicuous gap, absorbing relatively well between 250-500 cm-1.

With the fundamental physics in hand, Wordsworth was able to tweak modeled compositions of the ancient martian atmosphere to see what was needed to get the temperature above water’s freezing point and enable flowing rivers. Assuming a thick, 1.5-bar atmosphere of carbon dioxide, just 3.5% each of CH4 and H2 would do the trick.

This doesn’t seem like a tall order, but today’s CH4 and H2 concentrations on Mars are miniscule, so where could these key ingredients have come from? The researchers propose volcanic emissions or input from meteorites, as well as reactions between water and volcanic rock that are known, on Earth, to generate both molecules.

Viewing atmospheric molecules not as independent actors, but as a dynamic milieu whose interactions could dramatically change a planet’s thermal history, is an important conceptual shift that could substantially expand the search for life beyond Earth.

  • Uncle Al

    Assuming a thick, 1.5-bar atmosphere of carbon dioxide
    “אויב מיין באָבע האט באַללס זי וואָלט זיין מיין זיידע”

    • OWilson

      Assuming an earlier Martian civilization that had no Paris Accord!

  • Jim Reekes

    Before we go any further with yet another climate change theory, someone has to explain why Mars doesn’t have a magnetosphere, or if it did when did it end and why. Without a magnetosphere Mars can’t have an atmosphere for this “collision induced absorption” idea.

    • Logan Edwards

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  • Robert T Cunniff

    I don’t have access to the full study on Geophysical Research Letters, but from what I can read both the Discover article author and the original study authors, are making a rather silly and obvious attempt to support anthropogenic causes for climate change on Earth. But without invoking Martians as the cause for climate change on Mars, it’s a juvenile argument, because if true (which in this case I doubt) it really just supports non-anthropogenic causes.

  • Woodrow Wilson

    Mars was wet once, but wet with what? Mars was a planet with lakes and rivers. Satellites map washes and basins left behind when the planet dried up. Robots find rocks rounded by eons of tumbling down rivers and streams. Early Mars was a wet and wild place.

    What liquid flowed across the Martian plains? Mars has never been warm enough for liquid water. It could never have been an Earth-like ocean planet. Conditions on the cold moons of Jupiter and Saturn are more representative of what early Mars might have been like.

    Saturn’s giant moon Titan has an ecosphere based on methane rather than on water. Its rivers and lakes are filled with liquefied natural gas. It rains methane on Titan. Mars was probably cold enough for methane oceans once. The early sun was not as hot as it is today. Venus may have had liquid water then, and the Earth might have been an ice planet. The colder Mars could have been hospitable to liquid methane. As the sun warmed, the Goldilocks zone—where temperatures are right for liquid water—expanded. Venus became Hell, and Earth grew into the Garden of Eden. The Goldilocks zone for methane expanded as well. Mars became too warm for liquid methane and its seas evaporated.

    Alternatively, Mars might have hosted liquid water under a thick layer of ice. The moons Enceladus and Europa store vast quantities of liquid water under miles of ice. This configuration is surprisingly common throughout the solar system. Data from the New Horizons probe suggest it may occur even on the distant dwarf planet Pluto. Closer to home, much of Earth’s fresh water lies in Antarctica under miles of ice. It’s there in rivers and lakes moving as the flows on Mars might have at one time. Could Mars have hosted liquid water under a mantle of ice? Where did all that water go?

    We know Mars had rivers and lakes. We don’t know what flowed in them yet. What life might have evolved in them? What surprises are in store for us there? Mars has a lot to teach us.


The Extremo Files

The Extremo Files traces the science that is pushing the boundaries of biology, from the deep sea to outer space to the brave new world of synthetic biology.

About Jeffrey Marlow

Jeffrey Marlow is a geobiologist exploring the limits of life, from the role of microbes in global elemental cycles to the possibility of life beyond Earth and the brave new world of synthetic biology. He received his PhD from the California Institute of Technology and is currently a Postdoctoral Scholar at Harvard University, where he studies the inner workings of methane-metabolizing organisms.


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