Atmospheric Evolution and History of Volatiles

Mars's atmosphere may not always have been like it is today. Mars Odyssey observations indicate that the crust is enriched in water, yet the mantle is depleted and only trace of water is seen in the atmosphere. Furthermore, much of that water may have been liquid at one time. There are many features on Mars, gullies and canyon systems that appear to have a fluvial origin.

Three Views of Martian gullies. Image from NASA/JPL/Malin Space Science Systems

Today Mars is too cold for liquid water. Its average surface temperature is only 223K, a paltry 5K of which is due to its meager greenhouse effect. Another problem is that the surface pressure is too low. 7 mbar is just at the triple point of water. Occasionally, the temperature in the equatorial regions will rise above freezing, but then the ice sublimates rather then melts. To have liquid water on the surface of Mars requires both higher temperatures and pressures than present day values. Both of these requirements can be met by invoking a thicker atmosphere containing greenhouse gases

The Faint Young Sun Paradox

There is another complication, however. Models of stellar evolution indicate that the Sun's luminosity has increased by about 30% over the course of its life. Mars should have been even colder in the past, not warmer. A 5 bar atmosphere of greenhouse gases would not do the trick in this case.

Fortunately, the two dominant volatiles on Mars, CO₂ and water are excellent greenhouse gases. Models by Pollack et al. [1987] indicate that as little of 5 bars of a CO₂ and water atmosphere could warm the surface of early Mars to 273K. However, this greenhouse atmosphere on Mars leads us to another problem.

The Goldilocks Paradox

Venus, Earth, and Mars can be thought of as the three bowls of porridge in the Goldilocks and the three Bears story. Venus is too hot, Mars is too cold, but Earth is just right. We have invoked a thick greenhouse atmosphere for Early Mars to permit liquid water on the surface. However, if we assume all three planets had similar volatile compositions originally, Earth would have been too hot for liquid water. The water vapor would have increased the greenhouse heating on Earth. Earth would have suffered a runaway greenhouse and lost all its oceans in 100 million years. Clearly, this did not happen. Something else is going on here, and the problem is not really solved. Possibly, Mars had a higher amount of volatiles than Venus and Earth did originally. Mars formed farther out, in a region of the solar system where volatiles were more stable.

The Carbon Geochemical Cycle

Despite uncertainties in the original composition and greenhouse characteristics of the atmospheres of Earth and Mars, two things are known for certain: Mars is now too cold to have liquid water, and Earth is not. Both planets have lost much carbon dioxide from their atmospheres and probably in the same way. CO₂, in the presence of water, reacts with silica to form carbonate rocks. On earth, these rocks are eventually subducted and recycles. The gas is then released in volcanic eruptions or at spreading centers. Mars however, lost its heat more quickly, being a smaller planet. If plate tectonics ever occurred on Mars, they have long since shut down. Volcanism persisted for quite a while, but was never global, and that too shut down eventually. Thus there was a sink for CO₂ on Mars, but no source; the carbon trapped in the Martian rocks could never be recycled. However, this leads to yet another paradox; the Carbonate Paradox. If bars of CO₂ were pulled out of the martian atmosphere, we should see large amounts of carbonate rocks. Carbonates have been detected in small quantities in the SNC meteorites, but we would expect vast sedimentary formations on Mars like we see on Earth. These have never been observed.

The above is the model widely accepted by most planetary scientists, however it is not without its problems. The Sun was cooler in the past and Mars should have been too. If we invoke a greenhouse atmosphere to warm Mars enough to allow liquid water, it suggests that the Earth should be even warmer and have ended up like Venus. Also, the by-products of the removal of that CO₂, carbonate rocks are not observed on Mars in quantity. The White Mars section presents an alternative history which addresses these problems.

These pages designed by James Roberts
Last updated:  05 May 2002

http://anquetil.colorado.edu/~jhr/terraform/evolution-nf.html