Silicate weathering and atmospheric CO2

Carbon dioxide is one among many greenhouse gases that can be present in a planetary atmosphere, but it plays a distinguished role in the evolution of atmospheres of rocky planets like Earth, Mars and Venus because of its participation in chemical reactions that allow it to be exchanged between the atmosphere and minerals in the crust and planetary interior. What's more, acting over sufficiently long time scales, the dependence of the interchange on temperature can potentially act as a thermostat and help to keep the planet in the habitable range despite considerable changes in solar luminosity and other factors. Understanding the precision with which this feedback mechanism controls planetary temperature, the limits within which it can operate, and the circumstances under which it can break down, is one of the most central of the Big Questions.

It is likely that there are other gases important to climate which undergo similar exchanges, but CO2 exchanges have been far more extensively studied than any other case, and so this problem serves as a template for thinking about more exotic possibilities, some of which will me mentioned at the end of this section.

Carbon dioxide exchanges with the solid crust and interior of a rocky planet through reaction with silicate minerals to form carbonates. The class of reactions involved in this exchange is typified by the idealized Ebelmen-Urey reactions

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