12 March 2008

Martian Greenhouse: Volcanic CO2 Doesn't Cut It

Posted by Ryan Anderson

With all the evidence for water on the surface of Mars in the distant past, we always return to the same question: how was it possible for water to be stable back then? These days any liquid water on the surface would boil due to the low pressure or freeze due to the low temperature (or maybe do both at the same time!).

To explain liquid water on the past, there has been a lot of work done to see whether a thicker atmosphere of CO2 could provide high enough pressure and temperature to allow stable water. Today Marc Hirschmann gave an interesting talk about whether volcanoes could give off enough gas to do the job.

It turns out that since the sun was dimmer in the past, and since Mars is farther from the sun than the Earth, the early CO2 atmosphere of mars would have to be several times as thick as Earth’s current atmosphere to keep liquid water stable at the surface. Hirscmann estimated how much volcanism there was on Mars based on the heat flow from the planet, and then calculated the amount of CO2 released based on the amount of carbon in the mantle. Hirschmann showed that on Mars, with the amount of oxygen available to react with rocks (the so-called “oxygen fugacity”), the carbon in many lavas would likely be in the form of graphite, or even diamonds deep in the mantle where the pressure is higher.

With reasonable estimates of the amount of volcanism and the amount of carbon in the lavas, Hirschmann estimated that even in the “best case” scenario, volcanism can only provide an atmosphere with 0.1 bars of CO2: only a tenth of Earth’s atmospheric pressure and not nearly enough to make Mars warm. This led Hirschmann to look at the possibility of CO2 from the formation and stabilization of the crust. Depending on the assumptions you make, this process can give either 10s of bars of CO2, or a negligible amount.

Hirschmann concluded that if crust formation was the source of a thick atmosphere, it would require the most extreme limits of likely conditions. Alternatively, a CO2 atmosphere could have somehow persisted from the very earliest period of martian evolution, when the planet had a “magma ocean”. Alternatively, there may have been a greenhouse effect caused by something other than CO2, such as SO2 or methane. This “alternative greenhouse gas” idea is pretty new, but it shows a lot of potential for explaining a warm early climate. I don’t know enough about it yet to post intelligently, but if I learn more, I’ll share it here.

Ok, back to the talks!