National Geographic : 1980 Jan
bow shock. It is indeed an area of great shock, for it is there that the forces of the sun clash with those of Jupiter. The sun's outer surface is continuously blowing away, sending out streams of hot ionized gases called the solar wind, which is traveling at a million and a half kilometers an hour when it collides with the bow shock. Then its speed suddenly slows to only 400,000 kilometers per hour-and most of its particles are deflected. Prior to Voyager, it was thought that some particles would leak through the tail end of this shield and be carried right into Jupiter's polar regions. There they would strike atoms in the atmosphere, which in turn would glow, creating auroral displays like those on earth. Voyager did indeed find great auroras, but their cause was a sur prise. Those auroras, at least, are apparent ly triggered not by solar-wind particles, but by electrons streaming in from Io's torus. What Voyager Saw: Jupiter'sDazzling Realm ULLEDAND TUGGED by the gravity ofJupiter,Europa,and Ganymede, the crust oflo seethes with tidalforces that heat its upper layer, inducing volcanism. Eruptions spew a hundred kilometers or more above the surface (left), unchecked by atmosphereor strong gravity. Voyager 1 also recorded splotched ter rain(top), marked by black spots and cres cents. Six hours later, bluish clouds, probably of sulfur dioxide (above), had appearedover the crescent at right. Were Io an earth satellite, it would shine six times brighter than our moon.