The Young Faint Sun Paradox
A Perfect Balance >
There are coincidences, and then there are amazing coincidences. If you've been reading this column for a while you know already that there are some pretty "fortunate circumstances" that have occurred over the last billions of years, circumstances without which you would not be alive to read this.
Our just-right distance from the sun, the preternatural creation of our Moon, at least two incredibly well-timed nearby supernovae before our sun was born, our extremely well-placed planetary neighbors, etc., etc. - the list is endless.
But when several sets of separate, seemingly unrelated events occur over a period of billions of years - a number of simultaneous and diverse phenomena which must work and change in flawless unison or it's curtains for life - then it's time now to drop the jaw in utter awe.
When life was first introduced here, nearly four billion years ago, the sun wasn't the bright and shiny star it is today. It was probably about 30% less luminous than now, meaning it poured out 30% less energy.
If we know that a drop in energy of only about a percent or two from what we have now would turn us into a giant snowball, then how could life survive back then at a time the sun was so much dimmer?
Well, back then, so the records show, there was a lot more carbon dioxide in the air. Carbon dioxide is a greenhouse gas, efficiently trapping energy like a huge blanket around our planet.
So although the sun poured out a lot less energy eons ago, the greenhouse effect was in full kick back then, offsetting the wimpy sun, and keeping the planet nice and warm.
The big questions now? Where did all the carbon dioxide go as the sun got brighter and brighter? Why didn't life on this planet bake away a long, long time ago under our heavy blanket? Put on the proverbial thinking cap and take a seat.
Carbon dioxide can be cleaned from the sky in several ways, but it can't happen too fast, nor too slowly – it has to happen at just the perfect rate to match the brightening sun. One way involves water.
Early oceans were important imbibers of CO2 from the air. Carbonated beverages are perfect examples of water filled with dissolved carbon dioxide. But even the oceans have a limit. Could land help out here perhaps?
Yes, land can absorb CO2, as well. There is a class of a very abundant rock called silicates that can undergo a chemical reaction with carbon dioxide. (Fear not! I won't give the equation!) When silicates become exposed to the air during erosion, they get together with CO2 to form carbonates and sand. Yes, sand.
But erosion rates and plate tectonics and the uplift of mountains to expose new rock, etc., all have to be perfect so that the decrease in CO2 matches the increased energy output of the sun.
Surprise! It was! And still is.
A long time ago, when there was much more intense lifting of the land and exposure of new silicates, more CO2 could be sucked up at just the right rate. But the plates and uplift have slowed considerably in the billions of years that have gone by. Why aren't we baking right now in all the carbon dioxide that couldn't be absorbed?
Amazingly, plant life showed up at just the right time to work as a team with the land. Remember from school that CO2 is something plants require. Well, early plants didn't require much CO2, later plants required more. These requirements helped offset the fact that the silicates were playing a smaller and smaller role in the removal of CO2.
And then shell critters showed up at just the right time to continue helping! These tiny beasts turn CO2 into special carbonates, the building blocks for their shells.
And when the plants and shell critters went and got themselves dead and buried they effectively took the CO2 with them.
So as the sun got brighter and brighter, and bathed us in more and more energy, our warm blanket got thinner and thinner.
It is even more amazing than I can describe in this short space. It is such an astonishing set of events it has its own name - the Faint Sun Paradox. But the Big Picture here, that several entirely disparate events took place at just the right time to maintain perfect temperatures for billions of years goes beyond cute coincidences and enters into the realm of the miraculous.
What a world!
Posted by Mark Ritter at 2006.05. 1 02:21 PM | Comments (0)
