National Geographic : 1967 Dec
Pieced together from centuries of meticulous observations, these maps locate the three basic Mar tian features: dark areas, bright areas, and the frostlike cap that covers each of the polar regions in winter. Like earth, Mars experiences seasons. Spring causes polar caps to recede, and a wave of dark ening sweeps from them toward the equator. Once scientists con sidered dark regions bodies of water, describing them in Latin as mare-sea, lacus-lake, and sinus-gulf. But the author be lieves dark areas generally cor respond to highlands; brightones, he thinks, are dusty basins analo gous to earth's ocean basins. Mars measures some 4,200 miles in diameter. It has only a tenth of earth's mass, but exerts a gravitational pull about 40 percent that of earth. Infrared readings indicate surface tem peratures at the Martian equator vary daily from 700 to minus 150° F. A Martian day lasts 24 hours, 37 minutes, 22.7 seconds. Mariner IV-beeping signals through Mars' atmosphere helped scientists calculate the surface air density to be about 1 percent of earth's. They think this tenuous atmosphere is com posed largely of carbon dioxide, with only a trace of water vapor. © N.G.S . In stately orbits, planets revolve about the sun. Farther out and slower than earth, Mars takes 687 days to make one circuit. Unlike maps, photographs are usually reproduced with Mars' south pole at the top, because telescopes invert images. Hazy bluish cloud over the north pole on pages 820-21, called the polar hood, appears in late fall.