pale blue dot -carl sagan-第17章
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ans and extensive continents at the same time。 It will be interesting to see what the answer turns out to be。
What I've just told you is a kind of scientific progress report。 Tomorrow there might be a new finding that clears up these mysteries and contradictions。 Maybe there's something wrong with Muhleman's radar results； although it's hard to see what it might be： His system tells him he's seeing Titan when it's nearest； when he ought to be seeing Titan。 Maybe there's something wrong with Dermott's and my calculation about the tidal evolution of the orbit of Titan； but no one has been able to find any errors so far。 And it's Bard to see how ethane can avoid condensing out at the surface of Titan。 Maybe； despite the low temperatures； over billions of years there's been a change in the chemistry； maybe some bination of ets impacting from the sky and volcanoes and other tectonic events； helped along by cosmic rays； can congeal liquid hydrocarbons； turning them into some plex organic solid that reflects radio waves back to space。 Or maybe something reflective to radio waves is floating on the ocean surface。 But liquid hydrocarbons are very underdense： Every known organic solid； unless extremely frothy； would sink like a stone in the seas of Titan。
Dermott and I now wonder whether； when we imagined continents and oceans on Titan； we were too transfixed by our experience on our own world； too Earth…chauvinist in our thinking。 Battered； cratered terrain and abundant impact basins cover other moons in the Saturn system。 If we pictured liquid hydrocarbons slowly accumulating on one of those worlds； we would wind up not with global oceans； but with isolated large craters filled； although not to the brim； with liquid hydrocarbons。 Many circular seas of petroleum； some over a hundred miles across； would be splattered across the surface—but no perceptible waves would be stimulated by distant Saturn and； it is conventional to think； no ships； no swimmers； no surfers； and no fishing。 Tidal friction should； we calculate； be negligible in such a case； and Titan's stretched…out； elliptical orbit would not have bee so circular。 We can't know for sure until we start getting radar or near…infrared images of the surface。 But perhaps this is the resolution of our dilemma： Titan as a world of large circular hydrocarbon lakes； more of them in some longitudes than in others。
Should we expect an icy surface covered with deep tholin sediments； a hydrocarbon ocean with at most a few organic encrusted islands poking up here and there； a world of crater lakes； or something more subtle that we haven't yet figured out？ This isn't just an academic question； because there's a real spacecraft being designed to go to Titan。 In a joint NASA/ESA program； a spacecraft called Cassini will be launched in October 1997—if all goes well。 With two flybys of Venus； one of Earth； and one of Jupiter for gravitational assists； the ship will； after a seven…year voyage； be injected into orbit around Saturn。 Each time the spacecraft es close to Titan； the moon will be examined by an array of instruments； including radar。 Because Cassini will be so much closer to Titan； it will be able to resolve many details on Titan's surface indetectable to Muhleman's pioneering Earth…based system。 It's also likely that the surface can be viewed in the near infrared。 Maps of the hidden surface of Titan may be in our hands sometime in the summer of 2004。
Cassini is also carrying an entry probe； fittingly called Huygens； which will detach itself from the main spacecraft and plummet into Titan's atmosphere。 A great parachute will be deployed。 The instrument package will slowly settle through the organic haze down into the lower atmosphere； through the methane clouds。 It will examine organic chemistry as it descends； and—if it survives the landing—on the surface of this world as well。
Nothing is guaranteed。 But the mission is technically feasible； hardware is being built； an impressive coterie of specialists； including many young European scientists； are hard at work on it； and all the nations responsible seem mitted to the project。 Perhaps it will actually e about。 Perhaps winging across the billion miles of intervening interplanetary space will be； in the not too distant future。 news about how far along the path to life Titan has e。
CHAPTER 8 THE FIRST NEW PLANET
1 implore you； you do not hope to be able to give the reasons for the number of planets； do you？ This worry has been resolved 。 。 。
— JOHANNES KEPLER； EPITOME OF COPERNICAN ASTRONOMY； BOOK 4 / 1621
Before we invented civilization； our ancestors lived mainly in the open； out under the sky。 Before we devised artificial lights and atmospheric pollution and modern forms of nocturnal entertainment； we watched the stars。 There were practical calendrical reasons； of course； but there was more to it than that。 Even today； the most jaded city dweller can be unexpectedly moved upon encountering a clear night sky studded with thousands of twinkling stars。 When it happens to me after all these years； it still takes my breath away。
In every culture； the sky and the religious impulse are intertwined。 I lie back in an open field and the sky surround me。 I'm overpowered by its scale。 It's so vast and so far away that my own insignificance bees palpable。 But I don't feel rejected by the sky。 I'm a part of it； tiny； to be sure； but everything is tiny pared to that overwhelming immensity； And when I concentrate on the stars； the planets； and their motions； I have an irresistible sense of machinery； clockwork； elegant precision working on a scale that； however lofty our aspirations； dwarfs and humbles us。
Most of the great inventions in human history—from stone tools and the domestication of fire to written language—were made by unknown benefactors。 Our institutional memory of long…gone events is feeble。 We do not know the name of that ancestor who first noted that planets were different from stars。 She or he must have lived tens； perhaps even hundreds of thousands of years ago。 But eventually people all over the world understood that five； no more； of the bright points of light that grace the night sky break lockstep with the others over a period of months； moving strangely…almost as if they had minds of their own。
Sharing the odd apparent motion of these planets were the Sun and Moon； making seven wandering bodies in all。 These seven were important to the ancients； and they named them after gods not any old gods； but the main gods； the chief gods； the ones who tell other gods （and mortals） what to do。 One of the planets； bright and slow…moving； was named by the Babylonians after Marduk； by the Norse after Odin； by the Greeks after Zeus； and by the Romans after Jupiter； in each case the king of the gods。 The faint； fast…moving one that was never far from the Sun the Romans named Mercury； after the messenger of the gods； the most brilliant of them was named Venus； after the goddess of love and beauty； the blood red one Mars； after the god of war； and the most sluggish of the bunch Saturn； after the god of time。 These metaphors and allusions were the best our ancestors could do： They possessed no scientific instruments beyond the naked eye； they were confined to the Earth； and they had no idea that it； too； is a planet。*
* There was one moment in the last 4；000 years when all seven of these celestial bodies were clustered tightly together。 Just before dawn on March 4； 1953 B。C。； the crescent Moon was at the horizon。 Venus； Mercury； Mars； Saturn； and Jupiter were strung out like jewels on a necklace near the great square in the constellation Pegasus—near the spot from which in our time the Perseid meteor shower emanates。 Even casual watchers of the sky must have been transfixed by the event。 What was it—a munion of the gods？ According to the astronomer David Pankenier of Lehigh University and later Kevin Pang of JPL； this event was the starting point for the planetary cycles of the ancient Chinese astronomers。 There is no other time in the last 4；000 years （or in the next） when the dance of the planets around the Sun brings them so close together from the vantage point of Earth。 But on May 5； 2000； all seven will be visible in the same part of the sky—although some at dawn and some at dusk and about ten times more spread out than on that late winter morning in 1953 B。C。 Still； it's Probably a good night for a party。
When it got to be time to design the week—a period of time； unlike the day； month； and year； with no intrinsic astronomical significance—it was assigned seven days； each named after one of the seven anomalous lights in the night sky。 We can readily make out the remnants of this convention。 In English； Saturday is Saturn's day。 Sunday and Mo＇o＇nday are clear enough。 Tuesday through Friday are named after the gods of the Saxon and kindred Teutonic invaders of Celtic/Roman Britain： Wednesday； for example； is Odin's （or Wodin's） day； which would be more apparent if we pronounced it as it's spelled； 〃Wedn's Day〃； Thursday is Thor's day； Friday is the day of Freya； goddess of love。 The last day of the week stayed Roman； the rest of it became German。
In all Romance languages； such as French； Spanish； and Italian； the connection is still more obvious； because they 4 derive from ancient Latin； in which the days of the week were named （in order； beginning with Sunday） after the Sun； the Moon； Mars； Mercury； Jupiter； Venus； and Saturn。 （The Sun's day became the Lord's day。） They could have named the days in order of the brightness of the corresponding astronomical bodies—the Sun； the Moon； Venus； Jupiter； Mars； Saturn； Mercury （and thus Sunday； Monday； Friday； Thursday； Tuesday； Saturday； Wednesday）—but they did not。 If the days of the week in Romance languages had been ordered by distance from the Sun； the sequence would be Sunday； Wednesday； Friday； Monday； Tuesday； Thursday； Saturday。 No one knew the order of the planets； though； back when we were naming planets； gods； and days of the week。 The ordering of the days of the week seems arbitrary； although perhaps it does acknowledge the primacy of the Sun。
This collection of seven gods； seven days； and seven worlds the Sun； the Moon； and the five wandering planets entered the perceptions of people everywhere。 The number seven began to acquire supernatural connotations。 There were seven 〃heavens；〃 the transparent spherical shells； centered on the Earth； that were imagined to make these worlds move。 The outermost—the seventh heaven—is where the 〃fixed〃 stars were imagined to reside。 There are Seven Days of Creation （if we include God's day of rest）； seven orifices to the head； seven virtues； seven deadly sins； seven evil demons in Sumerian myth。 seven vowels in the Greek alphabet （each affiliated with a planetary god）； Seven Governors of Destiny according to the Hermetists； Seven Great Books of Manichaeism； Seven Sacraments； Seven Sages of Ancient Greece； and seven alchemical 〃bodies〃 （gold； silver； iron； mercury； lead； tin； and copper—gold still associated with the Sun； silver with the Moon； iron with Mars； etc。）。 The seventh son of a seventh son is endowed with supernatural powers。 Seven is a 〃lucky〃 number。 In the New Testament's Book of Reve
