©Mar 2014 Dale Alan Bryant
Keyboard Bill Cosby once posed a question in the title of one of his early, classic albums of stand-up comedy – ‘Why is there air?’ I used to listen to Cosby as a kid and I once told myself that if I ever found the answer to his question, I'd let him know - well, I hope you're reading this, Bill!...
To answer Bill's question, let's start with another fundamental question: “What is air?” In the Earth's case, air is its atmosphere - a blanket of various gases approximately 250 miles deep surrounding its outer crust. Air can be any gas, or combination thereof. Earth's atmosphere is mostly nitrogen, carbon dioxide and argon - but also contains a highly questionable component - oxygen, at 21%. Most planetary atmospheres contain very little oxygen if any, and indeed, the early Earth's atmosphere contained none. Earth's oxygen (O2) is mainly a by-product of living organisms, specifically vegetation. Vegetation converts the ambient CO2 (carbon dioxide) in the atmosphere into oxygen, which higher forms of life have been adapted to utilize.
The ‘Great Oxygenation period, as it is known, occurred sometime during the Cambrian period of the Paleozoic era some 542 million years ago, just preceding the ‘Cambrian Explosion’ of life. During this period, vegetation began to spread worldwide, which in turn began producing an abundance of oxygen. Only simple, unicellular life forms existed before this time which had no need of the nasty stuff! Nevertheless, as more-complex multicellular forms arose, they began to adapt to the increased oxygen levels generated by the abundance of early plant life. The present level of oxygen in our atmosphere is, as mentioned, 21%, but it actually got as high as 61% during the reign of the great reptiles, the dinosaurs. By this time, living organisms had begun to utilize oxygen so efficiently, and it was so abundant, that a general trend toward growth and increased size of multicellular forms of life took place - fossilized spiders have been found measuring up to 3 ft. across!
(Egads, I'm feeling a little faint...)
Now, earlier, I inferred that oxygen was a bit on the nasty side. Well, fond of the stuff as you may be, this is true. Oxygen is not an inert gas; it is highly reactive, corrosive and poisonous and is almost entirely responsible for all of the corrosion, rusting and decay that takes place on our Earth. As a rule, an atmosphere - and especially one containing oxygen, is not a good thing for anything that comes into contact with it - nor is it even the norm in the universe (the near-vacuum of space is the norm; an average cubic-inch of space contains only around 10 hydrogen atoms and a stray photon or two.)
So where did all this air come from? Almost all of our solar system’s planets and moons have some type of atmosphere with varying compositions and thicknesses. As the Earth was beginning to cool, shortly after its formation, gases trapped within its rocky mantle and outer crust began to escape through cracks and vents (this is called ‘outgassing’) to uniformly surround the planet. Moreover, because of our atmosphere, conditions here on Earth are just right for the continual re-shaping of its surface.
Air holds both moisture and heat. Moisture, combined with oxygen, causes things to corrode and anything containing iron to rust. Air is also the source of erosion, brought on by rain and wind continually reshaping the planet's surface. Moreover, moisture and heat combined support microorganisms that cause disease and decay. Only in space can the destructive forces of an atmosphere be avoided entirely. Space is cold, dry and still. Nothing can ever rot, burn or erode in space; even food will remain eternally "fresh" there. Moreover, our metallic robot explorers, like Pioneers 10 & 11 and the Voyager spacecraft, launched in the 1970's, are in pristine condition - as new and shiny as the day they were built. (Too bad they didn’t realize this back in 1957 - they could've put a ‘57 Chevy right off the assembly line into orbit and today it’d be in factory-new condition!)
Now, Earth’s moon has no atmosphere, making conditions on its surface the same as they are in space. Instruments left on the surface, by the astronauts of the Apollo program are in new condition and some are still in working order. Because there is no wind or rain, the boot prints of the astronauts left in the lunar dust are as fresh and crisp as the day they were imprinted there - and will remain so for millions of years to come. Only the extremely slow accumulation of meteoritic dust on the ground, coming from space, will dull their outlines slightly after many millenniums.
On Earth - and perhaps only on Earth - we have atmospheric conditions that are both conducive and detrimental to life. Oxygen reacts with nearly everything that it touches and is highly flammable. Oxygen fuels our campfires and our spacecraft; liquid oxygen (LOX) has been used in our rockets, from the German V-2, the U.S. Mercury and Gemini program's Redstone, Delta, Agena and Atlas boosters, the Apollo program’s Saturn-V booster, to the Space Shuttle’s triple-engine booster.
Currently, other astrobiologists, using data from the Kepler Space Telescope, are focusing on searching for exoplanets possessing atmospheres with at least some oxygen content that might support some form of life and that lie within a certain distance from their host suns, termed the habitable, or ‘Goldilocks Zone’. This is a reasonable approach, since most of the only forms of life that we know of - Earth life - utilize both of those conditions. The breadth of the ‘Goldilocks Zone’ in our own solar system runs from Venus out to about Jupiter; Earth is about in the middle.
Nevertheless, with life being as adaptable as we have seen it to be (filling niches from the deepest super-hot oceanic vents to as deep as a mile into the Antarctic ice sheet), we may need to modify our current models of ‘acceptable’ atmospheric content and habitable zone widths.
Oxygen - and 'Goldilocks' - aren’t necessarily the only shows in town...