The Diverse Planetary Ingassing/Outgassing Paths Produced over Billions of Years of Magmatic Activity

Abstract

The C-H-O-N-S elements that constitute the outgassed atmosphere and exosphere have likely been delivered by chondritic materials to the Earth during planetary accretion and subsequently processed over billions of years of planetary differentiation. Although these elements are generally considered to be volatile, a large part of the accreted C-H-O-N-S on Earth must have been sequestered in the core and mantle, with the remaining part concentrated at the Earth’s surface (exosphere: $\text{atmosphere} + \text{ocean} + \text{crust}$). The likely reason for this is that, depending on the prevailing pressure (P), temperature (T) and oxidation state (oxygen fugacity, fO₂) in the planet’s interior, the C-H-O-N-S elements can behave as siderophile, lithophile, refractory, magmatophile, or atmophile. It is not clear if these elements might be sequestered in the interiors of planets elsewhere, since the governing parameters of P-T-fO₂ during the diverse magmatic processes controlling magmatic differentiation vary greatly over time and from planet to planet. The magma ocean outgassed the first atmosphere, which was probably also the largest in terms of mass, but its nature and composition remain poorly known. Meanwhile, a significant, but unknown, part of the accreted C-H-O-N-S elements was sequestered in the core. These will probably never be liberated into the atmosphere. A secondary atmosphere was then fuelled by volcanism, driven by mantle convection and most likely enhanced by plate tectonics. The Earth still has active volcanism, and the volume and volatile contents of its magma are closely linked to geodynamics. Earth’s volcanoes have long emitted relatively oxidized gases, in contrast to Mars and Mercury. Mantle oxidation state seems to increase with planetary size, although the role of plate tectonics in changing the Earth’s mantle oxidation state remains poorly understood. Water contents of magma from elsewhere in the solar system are not so different from those produced by the Earth’s depleted mantle. Other elements (e.g. N, S, C) are unevenly distributed. A great diversity of speciation and quantity of magmatic gas emitted is found in planetary systems, with the key inputs being: 1 – degassing of the magma ocean, 2 – mantle oxidation state (and its evolution), and 3 – plate tectonics (vs. other styles of mantle convection). Many other parameters can affect these three inputs, of which planetary size is probably one of the most important.