Mars today is a cold planet with an arid, hyperoxidising surface and a thin, dry atmosphere, quite incapable of supporting liquid water. However exploration of the surface of the planet has shown the presence of numerous sedimentary structures, including dendritic valley networks, fluvuvial conglomerates, open-basin lakes, and fluvolacustrine deposits, all of which strongly imply the presence of abundant liquid water on the surface of Mars billions of years in the past. This is somewhat puzzling, as the Sun has not always shone as brightly as it does today, and, at the time when these deposits were laid down, is predicted to have produced between 75 and 80% of the heat and light it does today so that even had Mars had an atmosphere thick enough to allow the presence liquid water, the temperatures would have been even cooler than today (the current average temperature on the surface of Mars is -60 °C), again ruling out the presence of liquid water. A number of possible methods of heating have been proposed for early Mars, including a Greenhouse Effect caused by a thick Carbon Dioxide atmosphere, and heating from large asteroid impacts or volcanic eruptions, but none of these has been able to produce a model that satisfactorily explains the presence of the sedimentary structures seen on Mars.
Reull Vallis, a riverbed-like structure about 7 km across in Mars' southern hemisphere. Mars Express/European Space Agency/Deutsches Zentrum für Luft- und Raumfahrt/Freie Universität Berlin.
In a paper published in the journal Geophysical Research Letters on 21 January 2017, Robin Wordsworth of the School of Engineering and Applied Sciences, and Department of Earth and Planetary Sciences at Harvard University, Yulia Kalugina of the Department of Optics and Spectroscopy at Tomsk State University, Sergei Lokshtanov of the Chemistry Department at the Lomonosov Moscow State University, and the Obukhov Institute of Atmospheric Physics of the Russian Academy of Sciences, Andrei Vigasin, also of the Obukhov Institute of Atmospheric Physics, Bethany Ehlmann of the Division of Geological and Planetary Sciences, and Jet Propulsion Laboratory at the California Institute of Technology, James Head of the Department of Earth, Environmental and Planetary Sciences at Brown University, Cecilia Sanders, also of the Department of Earth and Planetary Sciences at Harvard University, and the Division of Geological and Planetary Sciences at the California Institute of Technology, present the results of a new model of the early climate of Mars, presuming an atmosphere made up of carbon dioxide, methane and hydrogen.
Models of the ancient atmosphere of Mars have been attempted before, but a high Methane content has not been considered as, in the absence of a protective ozone layer, methane molecules are prone to photodissociation into hydrogen and carbon dioxide, with the hydrogen rapidly escaping into space. However Wordsworth et al. calculate that the lower luminosity of the Sun 3.8 billion years ago (when the sedimentary structures on Mars where being laid down) would have led to methane being lost at a much lower rate, estimating that an atmosphere as dense as that of the modern Earth, with a 5% methane content, would take around 250 000 years to lose its methane content. Since methane is a major component of volcanic emissions, and Mars was formerly a volcanically active planet, Worsdworth et al. estimate only one major volcanic eruption every 100 000 years would be enough to keep methane levels high enough to fuel a greenhouse effect.
Wordsworth et al. used a range of carbon dioxide/hydrogen/methane combinations in their model, and found that a predominantly carbon dioxide atmosphere, with a surface pressure of 1.5 bars would only need about 3.5% each of hydrogen and methane to support liquid water on the surface of Mars, a level low enough to be maintained by infrequent volcanic eruptions.
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Models of the ancient atmosphere of Mars have been attempted before, but a high Methane content has not been considered as, in the absence of a protective ozone layer, methane molecules are prone to photodissociation into hydrogen and carbon dioxide, with the hydrogen rapidly escaping into space. However Wordsworth et al. calculate that the lower luminosity of the Sun 3.8 billion years ago (when the sedimentary structures on Mars where being laid down) would have led to methane being lost at a much lower rate, estimating that an atmosphere as dense as that of the modern Earth, with a 5% methane content, would take around 250 000 years to lose its methane content. Since methane is a major component of volcanic emissions, and Mars was formerly a volcanically active planet, Worsdworth et al. estimate only one major volcanic eruption every 100 000 years would be enough to keep methane levels high enough to fuel a greenhouse effect.
Olympus Mons on Mars, the largest volcano in the Solar System. Weizmann Institute of Science.
Wordsworth et al. used a range of carbon dioxide/hydrogen/methane combinations in their model, and found that a predominantly carbon dioxide atmosphere, with a surface pressure of 1.5 bars would only need about 3.5% each of hydrogen and methane to support liquid water on the surface of Mars, a level low enough to be maintained by infrequent volcanic eruptions.
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