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Mars today is a cold, dry planet. Its temperature averages 50 K below the freezing point of water. And its atmosphere is too thin for water to persist as a liquid. Geological evidence, however, shows that liquid water was abundant on the surface of ancient Mars (see my article with Michael Mellon, Physics Today, April 2004, page 71). The planet has features that imply the existence of rivers, streams, and shorelines early in Mars’s history (see figure 1).
The Sun was 30% dimmer then, so a thick greenhouse atmosphere must have been warming the planet. Carbon dioxide makes up 96% of today’s atmosphere and was likely the largest contributor to that greenhouse effect, though it may not have been the only greenhouse gas on Mars. Where did the CO2 from that earlier, thicker atmosphere go? Where did the water that carved the channels and eroded the surface go? Can water and CO2 be put back into the atmosphere and make Mars warm again?
It takes no more than a few bars of pressure from atmospheric CO2 to raise the temperature to the melting point of ice, and Mars initially may have had as much as 20 bars. By comparison, the total atmospheric pressure at Earth’s surface is about 1 bar. The abundance of water on Mars can be expressed as a global equivalent layer (GEL), which is the depth of the water if all of it existed at the surface as a liquid and was spread uniformly over the planet. Mars’s observed morphological features would have required a GEL of at least 50 m. Water may also reside as groundwater or as ice in the crust, in an amount that could raise the GEL by nearly an order of magnitude. Altogether, those reservoirs yield about 500 m GEL of water on the planet. By comparison, Earth’s oceans, if spread over the entire planet, would form a layer about 2 km thick.

Planets can lose water and CO2 above their surfaces in two ways. The Sun and its solar wind can strip water vapor and gaseous CO2 from the top of the atmosphere into space. The two compounds can also diffuse into the subsurface. There, CO2 and water can react with crustal materials to form CO2- and H2O-rich minerals.
The Mars Atmosphere and Volatile Evolution, or MAVEN, spacecraft has been tracking the stripping of the Martian atmosphere since 2014. Although the atmosphere is losing gas today at a rate of only about 2–3 kg/s, rates would have been much higher early in Mars’s history, when the Sun’s extreme UV rays and the solar wind were more intense. But by observing the processes today and knowing some history of the Sun, planetary scientists can extrapolate the loss rate into the past and estimate the total loss through time.

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