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cs like the Earth's but it turns off and on。 Right now； he proposes； the plate tectonics are off； 〃continents〃 do not move along the surface； do not crash into one another； do not thereby raise mountain ranges； and are not later subducted into the deep interior。 After hundreds of millions of years of quiescence； though； plate tectonics always breaks out and surface features are flooded by lava； destroyed by mountain building； subducted； and otherwise obliterated。 The last such breakout ended about 500 million years ago； Turcotte suggests； and everything has been quiet since。 However； the presence of coronae may signify—on timescales that are geologically in the near future—that massive changes on the surface of Venus are about to break out again。
EVEN MORE UNEXPECTED than the great Martian volcanos or the magma…flooded surface of Venus is what awaited us when the Voyager 1 spacecraft encountered lo； the innermost of the four large Galilean moons of Jupiter； in March 1979。 There we found a strange； small； multihued world positively awash in volcanos。 As we watched in astonishment； eight active plumes poured gas and fine particles up into the sky。 The largest； now called Pele—after the Hawaiian volcano goddess—projected a fountain of material 250 kilometers into space； higher above the surface of Io than some astronauts have ventured above the Earth。 By the time Voyager 2 arrived at Io； four months later； Pele had turned itself off； although six of the other plumes were still active； at least one new plume had been discovered； and another caldera； named Surt； had changed its color dramatically。
The colors of Io； even though exaggerated in NASA's color…enhanced images； are like none elsewhere in the Solar System。 The currently favored explanation is that the Ionian volcanos are driven not by upwelling molten rock； as on the Earth； the Moon； Venus； and Mars； but by upwelling sulfur dioxide and molten sulfur。 The surface is covered with volcanic mountains； volcanic calderas； vents； and lakes of molten sulfur。 Various forms and pounds of sulfur have been detected on the surface of Io and in nearby space—the volcanos blow some of the sulfur off Io altogether。* These findings have suggested to some an underground sea of liquid sulfur that issues to the surface at points of weakness； generates a shallow volcanic mound； trickles downhill； and freezes； its final color determined by its temperature on eruption。
* Io's volcanos are also the copious source of electrically charged atoms such as oxygen and sulfur that populate a ghostly； doughnut…shaped tube of matter that surrounds Jupiter。
On the Moon or Mars； you can find many places that have changed little in a billion years。 On Io； in a century； much of the surface should be reflooded； filled in or washed away by new volcanic flows。 Maps of Io will then quickly bee obsolete； and cartography of Io will have bee a growth industry。
All this seems to follow readily enough from the Voyager observations。 The rate at which the surface is covered over by current volcanic flows implies massive changes in 50 or 100 years； a prediction that luckily can be tested。 The Voyager images of to can be pared with much poorer images taken by ground…based telescopes 50 years earlier； and by the Hubble Space Telescope 13 years later。 The surprising conclusion seems to be that the big surface markings on Io have hardly changed at all。 Clearly； we're missing something。
A VOLCANO in one sense represents the insides of a planet gushing out； a wound that eventually heals itself by cooling； only to be replaced by new stigmata。 Different worlds have different insides。 The discovery of liquid…sulfur vulcanism on Io was a little like finding that an old acquaintance； when cut； bleeds green。 You had no idea such differences were possible。 He seemed so ordinary。
We are naturally eager to find additional signs of vulcanism on other worlds。 On Europa； the second of the Galilean moons of Jupiter and Io's neighbor； there are no volcanic mountains at all； but molten ice—liquid water—seems to have gushed to the surface through an enormous number of crisscrossing dark markings before freezing。 And further out； among the moons of Saturn； there are signs that liquid water has gushed up from the interior and wiped away impact craters。 Still； we have never seen anything that might plausibly be an ice volcano in either the Jupiter or Saturn systems。 On Triton； we may have observed nitrogen or methane vulcanism。
The volcanos of other worlds provide a stirring spectacle。 They enhance our sense of wonder； our joy in the beauty and diversity of the Cosmos。 But these exotic volcanos perform another service as well： They help us to know the volcanos of our own world—and perhaps will help one day even to predict their eruptions。 If we cannot understand what's happening in other circumstances； where the physical parameters are different； how deep can our understanding be of the circumstance of most concern to us？ A general theory of vulcanism must cover all cases。 When we stumble upon vast volcanic eminences on a geologically quiet Mars； when we discover the surface of Venus wiped clean only yesterday by floods of magma； when we find a world melted not by the heat of radioactive decay； as on Earth； but by gravitational tides exerted by nearby worlds； when we observe sulfur rather than silicate vulcanism； and when we begin to wonder； in the moons of the outer planets； whether we might be viewing water； ammonia； nitrogen； or methane vulcanism—then we are learning what else is possible。
CHAPTER 13 THE GIFT OF APOLLO
The gates of Heaven are open wide；
Off I ride 。 。 。
—CH'U TZ'U /ATTRIBUTED TO CH'U YUAN）； 〃THE NINE SONGS；〃 SONG V； 〃THE GREAT LORD OF LIVES〃 （CHINA； CA。 THIRD CENTURY B。C。）
It's a sultry night in July。 You've fallen asleep in the armchair。 Abruptly； you startle awake； disoriented。 The television set is on； but not the sound。 You strain to understand what you're seeing。 Two ghostly white figures in coveralls and helmets are softly dancing under a pitch…black sky。 They make strange little skipping motions； which propel them upward amid barely perceptible clouds of dust。 But something is wrong。 They take too long to e down。 Encumbered as they are； they seem to be flying a little。 You rub your eyes； but the dreamlike tableau persists。
Of all the events surrounding Apollo 11's landing on the Moon on July 20； 1969； my most vivid recollection is its unreal quality。 Neil Armstrong and Buzz Aldrin shuffled along the gray； dusty lunar surface； the Earth looming large in their sky； while Michael Collins； now the Moon's own moon； orbited above them in lonely vigil。 Yes； it was an astonishing technological achievement and a triumph for the United States。 Yes； the astronauts displayed death…defying courage。 Yes； as Armstrong said as he first alighted； this was a historic step for the human species。 But if you turned off the by…play between Mission Control and the Sea of Tranquility； with its deliberately mundane and routine chatter； and stared into that black…and…white television monitor； you could glimpse that we humans had entered the realm of myth and l