Of all Earth’s wonders, perhaps the most wonderful is something that often is invisible and is almost always taken for granted: the atmosphere, the cocoon of gases that envelops the planet and makes life possible. All of human history (until very recently, with the advent of space travel) and all phenomena associated with the globe’s climate and weather have taken place in this relatively thin layer of gases, which can be seen as a ribbon of blue swaddling the planet in this NASA photograph. The atmosphere serves our world as a kind of celestial moat, a protective barrier that shields Earth from the damaging ultraviolet rays of the sun and also from the potentially harmful impacts of the some 1.4 million small meteoroids that burn up and vaporize as they come in contact with it each day.
The crater-scarred surface of the moon, whose atmosphere is very thin, is a chilling reminder of what our planet might look like were it not for the atmosphere. And the extinction of the dinosaurs, now believed by many scientists to have been caused by the impact of an enormous object that barreled into the planet some 65 million years ago, is a reminder that some asteroids are much too large to be consumed by the planet’s airy buffer zone.
Earth’s vast ocean of air extends, if thinly, to some 430 miles above the planet’s surface. It is composed of gases such as nitrogen, oxygen, argon, carbon dioxide water vapor, as well as the host of small, wind-borne liquid and solid particles that scientists call aerosols: dust and soil, pollen, ash from volcanoes, sea salts churned up and made airborne by breaking waves. It is these particulates that turn the sunset red and orange, when the rays of the sun strike earth through a thick swath of the atmosphere as the star declines to the horizon.
Nitrogen and oxygen make up 99% of the atmosphere; yet the carbon dioxide that composes only some 0.03% of air is the focus of grave scientific concern. Carbon dioxide and water vapor are greenhouse gases; like the panes of glass on a greenhouse, these substances allow solar heat to pass through the atmosphere but not back out, trapping it inside. As the amount of CO2 in the air grows, largely owing to mankind’s burning of fossil fuels, the resulting gradual heating of the planet is driving unpredictable climate change.
Scientists divide the planet’s atmosphere into four distinct strata:
- Troposphere: The lowest layer of the planet’s blue cocoon is our layer: the ocean of air in which humans and animals live and weather events occur. It extends upward from the surface of the planet for 12 miles (19.3 km); temperatures drop as the distance from Earth increases and the air thins. The troposphere’s thickness changes with the seasonal position of the planet relative to the sun.
- Stratosphere: The next layer of the atmosphere extends to 31 miles (50 km) above the planet. Oxygen here is far too scarce to support life – but temperatures actually increase in the higher regions of the stratosphere, for this is where the ozone layer lies, and ozone heats up as it absorbs the sun’s harmful ultraviolet rays.
- Mesosphere: the third layer of the atmosphere onion extends 53 miles (85 km) above the planet. Beyond the reach of research balloon, it is a relatively unexplored region of the atmosphere.
- Thermosphere: The atmosphere’s highest layer – made visible by auroras – extends to the verge of outer space, some 430 miles (692 km) above Earth. Its thin air makes up a tiny fraction of the atmosphere’s total mass.
About the top picture: The atmosphere, which is most often seen by humans in the form of a red sunset or smog, is clearly seen as a ribbon of blue surrounding the planet in this 1999 photo.