We are a curious species–seduced by the unknown, and fascinated by strange journeys of exploration into the most secretive and shadowy corners of the Universe. Mysteries sing the song of the sirens to us, seducing us and enchanting us with whatever wonders there may be lurking just beyond the horizon of our knowledge. The possibility of life on worlds beyond our Earth has always haunted the daydreams of those who dare to dream, and the little red world Mars has held a special fascination for us as the most likely “nearby” abode of life that is unearthly. Mars is a chilly world with a sky that is the color of butterscotch–a planet that has shown plenty of geographical evidence that rivers of life-loving liquid water have periodically flowed across its reddish surface. In January 2017, Harvard University scientists announced that this world–cold by Earthlings’ standards–may have once been warm enough to allow these flowing rivers of liquid water to exist as a result of an intermittent and powerful greenhouse effect caused by bursts of methane.
The apparent existence of liquid water on Mars is puzzling. During the time period that the rivers were supposed to wander over the surface of the Red Planet–about three to four billion years ago–Mars should have been much too chilly to support water in its liquid phase.
How did Mars manage to stay so delightfully warm?
In a paper published in Geophysical Research Letters, scientists from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) in Cambridge, Massachusetts, have now proposed that an ancient dance between methane, carbon dioxide and hydrogen in the early Martian atmosphere may have been responsible for these mysteriously balmy periods. This interaction may have created warm episodes that enabled Mars to support liquid water on its surface.
“Early Mars is unique in the sense that it’s the one planetary environment, outside Earth, where we can say with confidence that there were at least episodic periods where life could have flourished. If we understand how early Mars operated, it could tell us something about the potential for finding life on other planets outside the Solar System,” Dr. Robin Wordsworth explained in a January 24, 2017 SEAS Press Release. Dr. Wordsworth is an assistant professor of environmental science and engineering at SEAS, and the first author of the research paper.
A Weird Warm World
Mars orbits our Sun in the relatively warm and well-lit inner region of our Solar System, where our Star is bright enough to blast its alien sky with its fabulous light and delightful distant fires. Mars is the fourth planet from our Sun, and the second-smallest planet in our Sun’s familiar family of eight major planets. The smallest major planet in our Solar System is Sun-bedeviled Mercury, the innermost of our Solar System’s worlds.
Mars was named after the Roman god of War because of its rust-colored surface–which is why it is frequently referred to as the “Red Planet.” This rusty-red color is caused by iron oxide which is very abundant on the Martian landscape. A terrestrial planet with a thin atmosphere, the features that characterize the surface of Mars display haunting similarities to the polar ice caps, valleys, and deserts of our Earth–as well as the scattered scars created by impact craters that pock-mark our own planet’s large Moon. The Martian seasons, as well as its period of rotation, are also similar to those of Earth.
In contrast to Earth’s relatively large Moon, Mars is orbited by a pitiful–albeit interesting–duo of tiny irregular moons. This misshapen duo, dubbed Phobos and Deimos, are generally thought to have been born as asteroids dwelling in the Main Asteroid Belt, that is located between Mars and our Solar System’s behemoth–the gas-giant planet, Jupiter. According to this scenario, the two little deformed asteroids wandered away from their birthplace into the space between planets–only to be snared and ultimately captured by the powerful gravity of the Red Planet that they now orbit.
Mars is a mysterious, fascinating world, that has been made the target of numerous current scientific investigations, seeking clues about whether ancient life could possibly have once existed there–as well as the intriguing chance that Mars may still host some lingering tidbits of life. Because of this bewitching possibility, Mars has been made the target of a number of planned astrobiology missions–including the Mars 2020 and ExoMars rovers.
The existence of liquid water is necessary for life as we know it to evolve and flourish on worlds other than our Earth. Because of the low atmospheric pressure on Mars, the presence of liquid water on its surface is scanty–limited to areas of lowest elevation, and here only for very short periods of time. The two Martian polar ice caps are apparently composed mostly of water. The volume of water ice in the south polar Martian ice cap, if it were to warm up and melt, would be sufficient to cover the entire Martian surface to a depth of 36 feet. On November 22, 2016, NASA announced the detection of significant amounts of subsurface ice in the Utopia Planitia region of the Red Planet. The amount of water discovered has been estimated to be about equal to the volume of water in Lake Superior.
As far back as the 1970s, planetary scientists were discovering channels and valleys on Mars which they thought may have been carved out and eroded by rain and surface runoff–just like on our own planet. In August 2016, widespread systems of fossilized riverbeds were discovered on an ancient region of the Martian surface, thus adding credibility to the theory that the now frigid and arid world once had a welcoming warm, damp climate approximately four billion years ago. This observation was made by a team of planetary scientists, led by researchers on the faculty of University College London (UCL) in England.
The paper describing this research, published in the journal Geology and funded by the Science & Technology Facilities Council and the UK Space Agency, identified more than 17,000 kilometers of ancient former river channels on a northern plain dubbed Arabia Terra. This discovery strengthened the theory that liquid water once flowed on the surface of Mars.
“Climate models of early Mars predict rain in Arabia Terra and until now there was little geological evidence on the surface to support this theory. This led some to believe that Mars was never warm and wet but was a largely frozen planet, covered in ice-sheets and glaciers. We’ve now found evidence of extensive river systems in the area which supports the idea that Mars was warm and wet, providing a more favorable environment for life than a cold, dry planet,” explained study lead author, Dr. Joel Davis, in an August 24, 2016 UCL Press Release. Dr. Davis is of the Department of Earth Sciences at UCL.
The study examined images covering a region roughly the size of Brazil at a considerably higher resolution than was possible in earlier observations. While a few valleys were discovered, the team revealed the existence of numerous systems of fossilized riverbeds. These very ancient riverbeds can be seen as inverted channels that are spread across the Arabia Terra plain.
The inverted channels are similar to those observed elsewhere on Mars and Earth. They are composed of gravel and sand that had been deposited by a flowing river. When the river dried out, the channels remained to tell the ancient story. The channels now appear as upstanding features as the surrounding material erodes. On our own planet, inverted channels often form in dry, desert regions like Oman, Egypt, or Utah, where erosion rates are sluggish. In most other environments, the channels are worn away long before they can become inverted.
“The networks of inverted channels in Arabia Terra are about 10m high and up to 1-2 kilometers wide, so we think they are probably the remains of giant rivers that flowed billions of years ago. Arabia Terra was essentially one massive flood plain bordering the highlands and lowlands of Mars. We think the rivers were active 3.9 -3.7 billion years ago, but gradually dried up before being rapidly buried and protected for billions of years, potentially preserving any ancient biological material that might have been present,” Dr. Davis noted in the August 24, 2016 UCL Press Release.
“These ancient Martian flood plains would be great places to explore to search for evidence of past life. In fact, one of these inverted channels called Aram Dorsum is a candidate landing site for the European Space Agency’s ExoMars Rover mission, which will launch in 2020, commented Dr. Matthew Balme in the same UCL Press Release. Dr. Balme is Senior Lecturer at The Open University in the UK, and is a co-author of the study.
Ancient Mars Warmed By Bursts Of Methane
Four billion years ago, our Star was approximately 30 percent fainter than it is now. Also, significantly less solar radiation (heat) was able to make its way to the surface of ancient Mars. The small amount of radiation that did manage to make its weary way to the Red Planet was captured by the Martian atmosphere. This resulted in stretches of time that were both warm and wet. For decades, scientists have tried to devise a model that precisely explains how Mars was insulated.
The apparent cause is carbon dioxide. Carbon dioxide accounts for approximately 95 percent of today’s Martian atmosphere, and it is both the best known and most abundant greenhouse gas on Earth.
But carbon dioxide alone cannot explain Mars’ early warm temperatures.
“You can do climate calculations where you add carbon dioxide and build up to hundreds of times the present day atmospheric pressure on Mars and you still never get temperatures that are even close to the melting point,” Dr. Wordsworth explained in the January 24, 2017 SEAS Press Release.
Because carbon dioxide alone cannot acount for the ancient Red Planet’s balmy temperatures, there has to be some missing ingredient present in the Martian atmosphere that also contributed to the greenhouse effect.
Rocky (terrestrial) planets, such as our Earth and Mars, cannot keep a good gravitational grip on their lighter gases, such as hydrogen. Eventually, these light gases are lost to interplanetary space. Indeed, the oxidation that accounts for the rusty red hue of the Martian surface is the direct result of the loss of hydrogen.
Dr. Wordsworth and his team studied these long-lost light gases (reducing gases) in order to shed new light on a possible explanation for Mars’ primeval climate. The team of scientists were especially interested in methane, which is currently not abundant in the thin Martian atmosphere. However, billions of years ago, certain geological processes could have been sending significantly more methane into the Martian atmosphere. This methane may have gradually experienced a sea-change into hydrogen and other gases, by way of a process that is similar to what is currently occurring on Saturn’s smoggy, tormented, hydrocarbon-slashed moon, Titan.
In order to understand how this ancient Martian atmosphere may have behaved, the scientists needed to gain an understanding of the fundamental properties of these molecules.
“When you’re looking at exotic atmospheres, you can’t compare them to Earth’s atmosphere. You have to start from first principles. So we looked at what happens when methane, hydrogen and carbon dioxide collide and how they interact with photons. We found that this combination results in very strong absorption of radiation,” Dr. Wordsworth explained in the January 24, 2017 SEAS Press Release.
Back in 1977, the late Dr. Carl Sagan, an astronomer at Cornell University in Ithaca, New York, was the first to speculate that hydrogen warming could have been an important factor affecting ancient Mars. However, the SEAS study marks the first time that scientists have been able to calculate its greenhouse effect precisely. It is also the first time that methane has been shown to be an effective greenhouse gas on ancient Mars.
“This research shows that the warming effects of both methane and hydrogen have been underestimated by a significant amount. We discovered that methane and hydrogen, and their interaction with carbon dioxide, were much better at warming early Mars than had previously been believed,” Dr. Wordsworth continued to explain in the SEAS Press Release.
The scientists hope that future missions to Mars will reveal the nature of the geological processes that produced methane billions of years ago.
Dr. Wordsworth added that “One of the reasons early Mars is so fascinating is that life needs complex chemistry to emerge. These episodes of reducing gas emission followed by planetary oxidation could have created favorable conditions for life on Mars.”