National Geographic : 1977 Jan
OZONE 15 MILES 1 \ CLOUDS o MILE P C E PAINTINGSBY PIERREMION Earth Six times the volume of Mars, our planet has a dense atmosphere rich in water vapor that protects and nourishes life. A band of ozone, thickest about 15 miles above the surface, shields against the sun's deadly ultraviolet rays. An earthling must climb 21 miles to experience the atmospher ic pressure on the surface of Mars. In such thin air, blood would boil. Mars The red planet probably has a thick, rocky crust, because, being smaller than earth, it cooled faster. The Martian ozone region begins at ground level, but there isn't enough ozone to block out ultra violet rays. Many earthly micro organisms would die in seconds. Viking gave scientists the first proof of nitrogen in the Martian atmosphere, and detected signifi cant leakage of the gas into space. They deduced that Martian air was once much denser, perhaps dense enough to permit rain to fall. Scientists also found fast-moving atmospheric "tides," able to pro pel a gas molecule in a few days from the surface to a level known as the turbopause, where gases cease their turbulent mixing. On earth that takes about a century. From orbit a camera shows how morning sun warms the ground (below, left), vaporizing frozen water that recondenses in low spots as patches of fog (arrows, below).