BY JOSE PALU-AY DACUDAO
THE FIRST atmosphere of Tera was mostly hydrogen from the solar nebula. Any element that combines readily with hydrogen to form a gas would get severely depleted as the heat from the planet’s continuing accretion of other solar system bodies would drive it off into space. One of these elements is carbon, which combines readily with hydrogen to form gaseous methane (CH4). Carbon also combines readily with abundant oxygen to form gaseous carbon monoxide and dioxide. Imagine young Terra as a lightweight proto-planet that lacks sufficient gravity to hold on to gases permanently; with an atmosphere of hydrogen, helium, water, ammonia, CH4, CO and CO2; and a continuous rain of impactors up to the size of small planets. This first atmosphere would get blown off into space quite easily. Later the mass of the solids in the impactors would add to that of early Terra, and increase its gravity so as to be able to hold on to a more permanent second atmosphere, but by this time, almost all of the carbon in the planet’s outer layers would have been ejected into space.
There might be another reason. It’s known that liquid hot pressurized iron under the conditions in Tera’s outer core has a high carbon to oxygen partition coefficient. Thus, when minerals sink into the outer core, oxygen gets squeezed out back into the mantle. But carbon compounds decompose, and the carbon dissolves into the outer core’s liquid iron. As the outer core transforms into the solid inner core at deeper levels, the carbon is retained as solid iron carbides. And Tera has a larger iron core than any other rocky planet, certainly able to take in huge amounts of carbon. There are speculations that most of Tera’s carbon that survived getting ejected into space are now trapped in the planet’s core.
The remaining carbon in the surface layers of early Terra mostly got locked into calcium and magnesium carbonate minerals in the crust. Terra has enormous amounts of calcium and magnesium. So only a relatively small amount from Terra’s original carbon budget was left that could combine with oxygen to form carbon dioxide in our planet’s second atmosphere.
Then photosynthetic organisms evolved. They extracted CO2 and released molecular oxygen. The free oxygen oxidized any remaining methane (CH4) and ammonia (NH3) in the second atmosphere into CO2, free diatomic nitrogen (N2), and water (H2O). Thus, our planet’s third atmosphere came into existence – nitrogen, oxygen, argon (from the decay of radioactive Potassium-40 isotope over billions of years), and a bit of carbon dioxide.
Photosynthesis in earnest may have begun about 3 billion years ago. Around 2.5 billion years ago, after most of the reduced compounds of the air, sea, and land got oxidized, molecular oxygen began to accumulate in the atmosphere. The CO2-depleting effect caused by steady photosynthetic activity added up over the eons.
Then there is what could be described as the acute geophysical reason for CO2 sequestration. (To be continued)/PN
