Air may be thin, but not insubstantial |

Air may be thin, but not insubstantial

Floating over rock and metal and water, the atmosphere extends dozens of miles above our heads. The weight of dozens of miles of air presses down on us with a force, at sea level, of 14.7 pounds on every square inch.

The surface of the Earth covers 184 million square miles, or three-quarters of a billion billion square inches. At 14.7 pounds per square inch, the entire atmosphere weighs some five and a half million billion tons.

With so much weight above it, the lower atmosphere is seriously squashed.

Over billions of years, photosynthesis has filled the atmosphere with a million billion tons of oxygen – a fifth of all air. Folks at sea level breathe the densest air, and get the most oxygen with each breath; those at higher elevations breathe air that’s less dense, and get less oxygen.

On Banner Mountain, only 90 per cent of the atmosphere presses down on you … the other 10 per cent lies between your feet and sea level. Few tourists seem to notice the missing ten per cent. At Donner Summit, however, around 7000 feet, a good twenty per cent of the atmosphere lies beneath you. The thinner air doesn’t add to the challenge of, say, watching television, but it makes exercise noticeably harder. Aboard a jetliner cruising cross-country, the experience is similar; an altimeter indicates a constant altitude of seven or eight thousand feet – the level at which the craft is pressurized.

Just north of Tioga Pass into Yosemite, Gaylor Peak, at 11,000 feet, rises above a third of the atmosphere.

Leaving the Sierra for Alaska or South America, one can reach 18,000 feet (and above), putting one above half the Earth’s air. This elevation is beyond human endurance for permanent habitation; workers in the highest mines in the Andes live in camps thousands of feet below.

Nearly all climbers on Everest carry their oxygen with them, to supplement the meager third of the atmosphere available at 29,000 feet.

On the far side of the sun, Mars is now out of sight. In a year, however, the red planet will approach Earth almost as close as it ever gets. A flotilla of spacecraft will take advantage of that proximity to visit Mars next year. The landers among them will use the Martian atmosphere to slow themselves down before making planetfall. They won’t have much air to play with, however – the pressure at the Martian surface is about that at 100,000 feet above the surface of Earth.

Carrying a Geiger counter aboard a jetliner, we find that cosmic radiation (high-energy particles from exploded stars and such) increases dramatically with altitude. With only one-fourth of the atmosphere above you, you lose three-fourths of the air’s radiation shielding. Cosmic radiation will be a serious threat to astronauts living under the thin air of Mars.

I recently participated in a field trip to the Mojave, an attempt to view the desert as an analog for Mars. While the Mojave, in the words of one scientist, was “too damn wet,” it did share some features with Mars.

Both Martian and Californian deserts see high winds and wind-blown dust. On the rocks of the Mojave, wind-blown sand has carved facets and flutes, turning the rocks into “ventifacts” – “objects fashioned by wind.” Such ventifacts have also been found in images returned from the surface of Mars.

Alan Stahler trained as a biologist and is an amateur astronomer.

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