On being rarefied

Natural History, Feb, 1998 by Neil de Grasse Tyson

Everybody knows that air is thin. How else could we be impressed when a magician pops a rabbit in and out of an otherwise empty hat? Most of the time we are not even aware the air is there. But when the rabbit disappears, nobody tells you that a cubic centimeter of pure, invisible, sea-level air contains more molecules (ten quintillion) than an average beach has grains of sand. Is this a lot? Or is this a little? Missing rabbits may be interested to know that the average density of matter in the entire universe is much, much less than the air in a magician's hat. The adage "nature abhors a vacuum" applies famously to life under our blanket of Earth's atmosphere, but it fundamentally misrepresents the rest of the cosmos.

Before laboratory vacuums, air was the closest thing to nothing that anyone could imagine. Along with earth, fire, and water, air was one of Empedocles' original four elements that composed the known world. Aristotle then postulated a fifth element known as the quintessence. Otherworldly, yet lighter than air and more ethereal than fire, the rarefied quintessence was presumed to compose the heavens.

We needn't look as far as the heavens to find rarefied environments. Our upper atmosphere will suffice. Beginning at sea level, air pressure is about fifteen pounds per square inch, which means that if you cookie-cut a square inch of atmosphere from thousands of miles up all the way down to sea level and put it on a scale, it would weigh fifteen pounds. In comparison, a square-inch column of water would need to be a mere thirty-three feet tall to weigh fifteen pounds. On mountaintops and high up in airplanes, the cookie-cut column of air above you is shorter and therefore weighs less. At the 14,000-foot summit of Mauna Kea, Hawaii, home to some of the world's most powerful telescopes, the atmospheric pressure drops to about nine pounds per square inch. While observing on site, astronomers must occasionally breathe from oxygen tanks to retain intellectual acuity. Earth's atmosphere extends thousands of miles above its surface, but the compressibility of air enables the atmosphere to pack over 99 percent of its molecules below twenty miles. Above fifty miles, where there are no known astronomers, the air is so rarefied that your average gas molecule moves for a relatively long time - an interminable 1/100,000 of a second - before colliding with another.

If, in between collisions, the molecules are slammed by an incoming subatomic particle, they become temporarily excited and then emit a unique spectrum of colors before their next collision. When the incoming particles are the constituents of the solar wind, such as protons and electrons, the emissions are seen as curtains of undulating light that we commonly call an aurora. When the spectrum of auroral light was first measured, it had no counterpart in the laboratory. The identity of the glowing molecules remained unknown until we learned that excited, but otherwise ordinary, molecules of nitrogen and oxygen were to blame. At sea level, their rapid collisions with each other - clocked at seven billion per second - absorb this excess energy long before they have had a chance to emit their own light.

Earth's upper atmosphere is not alone in producing mysterious lights. Spectral features in the Sun's corona long puzzled astrophysicists. An extremely rarefied place, the corona is that beautiful, fiery-looking outer region of the Sun that becomes visible during a total solar eclipse. The unexplained features were attributed to "coronium," an element that was later recognized to be iron. We then learned that the solar corona is heated to millions of degrees, which puts normally familiar iron into an unfamiliar state in which most of its outer electrons are stripped away and float free in the gas.

The term "rarefied" is normally reserved for gases, but I will take the liberty, to apply it to the solar system's famed asteroid belt. From movies and other descriptions, you would think it was a hazardous place, fraught with the constant threat of head-on collisions with Winnebago-size boulders. The actual recipe for the asteroid belt? Take a mere 2 1/2 percent of the Moon's mass (itself just 1/81 the mass of Earth) and crush it into thousands of assorted pieces, but make sure that over 90 percent of the mass is contained in just four asteroids. Then spread all the pieces across a 100-million-mile-wide belt that traces a 1.5-billion-mile path around the Sun. Yes, traversing the asteroid belt is more dangerous than not traversing the asteroid belt. But it is not as dangerous as you might think.

Leaving aside pesky chunks of rocks and boulders, interplanetary space has an average density of about ten atoms and molecules per cubic centimeter, which is about the same as the best laboratory vacuums on Earth. Comet tails, as tenuous and rarefied as they are, represent an increase in density by at least a factor of 100 over the ambient conditions of interplanetary space. By reflecting sunlight and reemitting energy absorbed from the Sun, a comet tail possesses remarkable visibility, given its nothingness. Fred Whipple, of the Harvard-Smithsonian Center for Astrophysics, is generally considered a parent of our modern understanding of comets. He has succinctly described a comet's tail as the most that has ever been made of the least. Indeed, if the entire volume of a 50-million-mile-long comet tail were compressed to the density of ordinary air, all the tail's gas would fill a ten-mile cube.