- The moon photographed on July 31, 2011 out the window of the space station. Seen near setting, the moon appears to hover at edge of outer space. The lowest layer of air, tinted orange by twilight, is the troposphere. A brownish layer called the tropopause separates it from the higher stratosphere. Credit: NASA
By convention, outer space starts at 100 km or 62.1 miles above the Earth’s surface, but to get there, a plane or ship must first pass through five layers of atmosphere. The recently released photo was taken from about 230 miles up by an astronaut on board the International Space Station. It’s not only beautiful to look at but clearly shows several layers of our planet’s atmosphere. The lowest region, which contains 80 % of the atmosphere’s mass and 99% of its water vapor, is called the troposphere after the Greek tropos meaning ‘turning’ or ‘mixing’.
Mixing is the word in the lower atmosphere thanks to high and low pressure regions, winds and turbulence. Home to clouds, aerosols and weather, the troposphere is by far Earth’s most colorful air layer. Think of all the pinks, oranges, red, blues and even the occasional green we pass through and breathe in during a year.
- Different phenonomena occur in different layers of Earth’s atmosphere. The troposphere is characterized by clouds and weather; the mesosphere by the aurora borealis. Satellites and the space station orbit at the extreme edge called the exosphere. Credit: NASA with my own additions
Up above about 10 miles we enter the stratosphere, a calm region of thin, cold, clear air. If you take a cross-country plane trip, you’ll be cruising through its lower reaches. Unlike the troposphere, temperature increases as you ascend the approximately 30 miles from the bottom to the top due to the absorption of UV light from the sun by the ozone layer. This crucial region was created by life through photosynthesis and in turn protects life by absorbing much of the dangerous, short-wave UV radiation.
To get beyond the stratosphere into the gaspingly-thin air of the mesosphere, you’ll need to be an astronaut. Despite its tenuousness, the mesosphere’s one of the most action-packed places around, hosting everything from flaming meteors to the aurora borealis to fondue parties. Just wanted to see if you were paying attention. Things get chilly again as we ascend this layer. The bottom can be as toasty as the low 20s while the top dips to around 200 below, making it the coldest region of the atmosphere.
Higher up, the air continues to thin out but the temperature rises once again through the thermosphere reaching over 4,500 degrees F during the daytime. Since heat is a measure of the motion of atoms or molecules, the few remaining oxygen atoms at these altitudes dart about rapidly as they absorb energetic radiation from the sun. Yet the air is so rarefied, we’d never feel the heat; there are just too few atoms to bounce off of us to create a sensation of warmth. The sun’s energy also electrifies atoms at these heights making them capable of reflecting radio waves beamed from the Earth long distances from one station to another.
Once we’ve climbed the 250 miles into the exosphere, Earth falls away as we penetrate deeper into outer space. While 250 miles sounds like a lot of air, and illustrations make the atmosphere appear like a deep pool, the reality is much more stark. If our planet were shrunk to the size of an onion, the Earth’s atmosphere would only be as thick as the onion’s outermost skin. The second Earth-moon photo, also taken by astronauts, hints at how fragile that fuzzy layer really is. Breath deeply and enjoy.