Image in header: On Earth, the atmosphere, hydrosphere, biosphere and lithosphere are in equilibrium with each other. That balance is always in long-term evolution, though. Source : https://qsstudy.com/evolution-atmosphere-hydrosphere/
We rather easily forget that the atmosphere is constantly changing over the long term. The world around us, with modern animals (including humans) and plants are completely adapted to our current atmosphere. For instance, the two dominant forms of metabolism – photosynthesis and respiration – are fine-tuned to the prevailing concentrations of acidoph gas (O2) and carbon dioxide (CO2). And if we look at the full spectrum of electromagnetic radiation emitted by our sun, and what the Earth’s atmosphere lets through of it, we see the following:
Electromagnetic radiation with higher energy levels than UV (i.e. X-rays and Gamma-rays) interact strongly with the various molecules in the air, especially O2 and N2 so. Their photons contain enough energy to ‘knock’ electrons out of orbit. Our relatively thick atmosphere contains so many molecules that the X and gamma rays actually lose all their energy through interactions before they reach the lower layers. This is why we are protected from these radiations on the ground.
The higher-energy UV rays (UVb and UVc), in turn, lose their energy in the stratospheric ozone layer (most fanned out at 20-30 km altitude). This is because the ozone molecule disintegrates when a UV photon strikes it. This UV photon has exactly the amount of energy the orbitals of ozone need to disintegrate. The ozone disintegrates into atomic oxygen and oxygen gas. These products very quickly start reacting with each other again to form new ozone molecules with release of heat energy, but in the meantime the UV radiation has been absorbed. So on the ground, we are also protected from this radiation. Terrestrial terrestrial organisms have cells and tissues that are rapidly damaged by UVb, UVc, X-rays and gamma rays. This, of course, is no accident.
Looking further at the electromagnetic spectrum above, we see that there is also shielding for certain parts of the infrared bands. Reality shows more variation than in the simplified drawing above, but as a general trend it is fair to say that infrared rays get through our atmosphere less and less as they have longer wavelengths. This is mainly because greenhouse gases absorb this radiation.
So in the spectrum that reaches the ground, we see – besides the longest wavelengths such as radio waves – that mainly visible light and then a little UV and a little IR on the Earth’s surface. These are precisely also the wavelengths that our star emits the most. Animal eyes, of course, are not accidentally sensitive to exactly these wavelengths (which is why we call this band ‘visible light’). And photosynthetic cells have learned just fine to use visible light as an energy source to make living cells. Everything seems to be ideally aligned. But on the young Earth, it wasn’t always like this. In early versions of Earth’s atmosphere, the plants and animals we know would immediately die. We give a brief overview of those early atmospheric evolutions in the following paragraphs.
