BY JOSE MA. EDUARDO P. DACUDAO
GLOBAL warming has been a hot issue for decades already. The mass media has seen it in a negative light, and thus has been advocating the reduction of greenhouse gases. We will tackle the gas that has been hypothesized to be the main cause of global warming.
In our Solar System and also in the Universe, atoms are proportioned into almost 98% hydrogen and helium, 1% oxygen, 0.5% carbon, and another 0.5% into the rest of the elements. You’d expect any planet to have huge amounts of oxygen and carbon. Oxygen reacts strongly with silicon, magnesium, aluminum, and calcium, forming solid silicate minerals, and so its atoms typically make up half of a rocky planet’s crust and mantle. On the other hand, the said rocky planets (like Earth) severely lack carbon. There are two hypotheses for this.
The first atmosphere of Earth 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 Earth 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 Tera, 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 a second reason for the depletion of Carbon in the Earth’s early atmosphere, crust and mantle. It’s known that liquid hot pressurized iron under the conditions in Earth’s outer core has a high carbon to oxygen partition coefficient. So 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 Earth’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 Earth mostly got locked into calcium and magnesium carbonate minerals in the crust. So only a relatively small amount from Earth’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 nitrogen, and water. 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. (To be continued)/PN
