Earliest atmospheres of terrestrial planets: Steam atmospheres, carbon atmospheres, and ocean planets
Linda T. Elkins-Tanton, Massachusetts Institute of Technology
Terrestrial planets may obtain atmospheres from three primary sources: Capture of nebular gases, degassing during accretion, and degassing from subsequent tectonic activity. Though the first atmospheres on the Earth and Mars have been changed and lost past recognition, they may have played important roles in determining the future of the planets’ surfaces and their habitability.
Compositions of primitive and differentiated meteorites provide a range of reasonable starting bulk materials for planetary formation and atmospheric degassing. These compositions may have been processed through a planetary melting event (a magma ocean), assumed to occur one or more times during accretion of planets in the first tens of millions of years of planetary formation through highenergy impacts of large bodies. These physical processes produce predictions for the mass and composition of degassed atmospheres as well as for the initial composition and density of the planetary silicate mantle, from which comes later degassing into the atmosphere.
A very small initial water content (less than a half mass percent) in the accreting Earth can produce a dense steam atmosphere, while a small change in chemistry can produce a carbon-based atmosphere, such as that on Venus. The presence of surface water may be critical in both preventing a Venusian runaway greenhouse and encouraging plate tectonics. Further, modest initial water contents (10
mass% of the planet and above) create planets with deep surface liquid water oceans soon after accretion is complete.