National Geographic : 1970 Aug
MERCURY NEW MEXICOSTATEUNIVERSITYOBSERVATORY Best likeness of Mercury, taken through a 24-inch telescope, captures only faint, undecipherable shadings. GEOGRAPHIC ARTDIVISION(q N.G.S. Mercury's new twist: Radar proved in 1965 that the planet rotates on its axis every 59 earth days, as indicated here by an imaginary spot on the sur face. Earlier dogma held the rotation to be synchronous with the orbit once every 88 earth days. Actually, in those 88 days the spot would move precisely from noon to midnight. Thus a Mercury year lasts only half a Mercury day. And there is apparently no atmosphere such as our own planet enjoys for a shield. At any rate, the way Mercury reflects and po larizes light is similar to that of the airless moon. With low gravity (only a third that of Earth) and high temperatures, atoms and molecules of most gases would move so rap idly that, over the eons, they would escape to interplanetary space. The pictures telemetered back to Earth in the Mercury mission will arouse unprece dented interest among scientist and layman alike. Astronomers have never seen Mercury really well, even though it is reasonably close to Earth as distances go in the solar system. The planet stays so close to the sun in its rela tively tiny orbit that, to the naked eye, it is 162 Desolate cinder scorched by the sun's fiery breath, Mercury unfolds a moonscape of meteorite pits and ridgelike crater rims. Apparently devoid of protective atmosphere, the copper-hued orb cooks at 650° F. on the daylight side, while nights plunge to a brittle -300° F. Orbiting close to the blinding sun, Mercury confounds earthbound ob servers. But radar, uncannily reading echoes as weak as if bounced from a dime 10,000 miles away, detects huge areas of rough terrain.