pale blue dot -carl sagan-第20节
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sphere onto the surface of Titan。 If it's all deep…frozen and unchanged in the intervening aeons； the amount accumulated should be at least tens of meters （a hundred feet） thick； outside estimates put it at a kilometer deep。
But at 180°C below the freezing point of water； you might very well think that amino acids will never be made。 Dropping tholins into water may be relevant to the early Earth； but not； it would seem； to Titan。 However； ets and asteroids must on occasion e crashing into the surface of Titan。 （The other nearby moons of Saturn show abundant impact craters； and the atmosphere of Titan isn't thick enough to prevent large； high…speed objects from reaching the surface。） Although we've never seen the surface of Titan； planetary scientists nevertheless know something about its position。 The average density of Titan lies between the density of ice and the density of rock。 Plausibly it contains both。 Ice and rock are abundant on nearby worlds； some of which are made of nearly pure rice。 If the surface of Titan is icy； a high…speed etary impact will temporarily melt the ice。 Thompson and I estimate that any given spot on Titan's surface has a better than 50…50 chance of having once been melted； with an average lifetime of the impact melt and slurry of almost a thousand years。
This makes for a very different story。 The origin of life on Earth seems to have occurred in oceans and shallow tidepools。 Life on Earth is made mainly of water； which plays an essential physical and chemical role。 Indeed； it's hard； for us water…besotted creatures to imagine life without water。 If on our planet the origin of life took less than a hundred million years； is there any chance that on Titan it took a thousand？ With tholins mixed into liquid water…even for only a thousand years the surface of Titan may be much further along toward the origin of life than we thought。
DESPITE ALL THIS we understand pitifully little about Titan。 This was brought home forcefully to me at a scientific symposium on Titan held in Toulouse； France； and sponsored by the European Space Agency （ESA）。 While oceans of liquid water are impossible on Titan； oceans of liquid hydrocarbons are not。 Clouds of methane （CH4）； the most abundant hydrocarbon； are expected not far above the surface。 Ethane （C2H6）； the next most abundant hydrocarbon； must condense out at the surface in the same way that water vapor bees a liquid near the surface of the Earth； where the temperature is generally between the freezing and melting points。 Vast oceans of liquid hydrocarbons should have accumulated over the lifetime of Titan。 They would lie far beneath the haze and clouds。 But that doesn't mean they would be wholly inaccessible to us—because radio waves readily penetrate the atmosphere of Titan and its suspended； slowly falling fine particles。
In Toulouse； Duane O。 Muhleman of the California Institute of Technology described to us the very difficult technical feat of transmitting a set of radio pulses from a radio telescope in California's Mojave Desert； so they reach Titan； penetrate through the haze and clouds to its surface； are reflected back into space； and then returned to Earth。 Here； the greatly enfeebled signal is picked up by an array of radio telescopes near Socorro； New Mexico。 Great。 If Titan has a rocky or icy surface； a radar pulse reflected off its surface should be detectable on Earth。 But if Titan were covered with hydrocarbon oceans； Muhleman shouldn't see a thing： Liquid hydrocarbons are black to these radio waves； and no echo would have been returned to Earth。 In fact； Muhleman's giant radar system sees a reflection when some longitudes of Titan are turned toward Earth； and not at other longitudes。 All right； you might say； so Titan has oceans and continents； and it was a continent that reflected the signals back to Earth。 But if Titanic in this respect like the Earth—for some meridians （through Europe and Africa； say） mainly continent； and for others （through the central Pacific； say） mainly ocean—then we must confront another problem：
The orbit of Titan around Saturn is not a perfect circle。 it's noticeably squashed out； or elliptical。 If Titan has extensive oceans； though； the giant planet Saturn around which it orbits will raise substantial tides on Titan； and the resulting tidal friction will circularize Titan's orbit in much less than the age of the Solar System。 In a 1982 scientific paper called 〃The Tide in the Seas of Titan；〃 Stanley Dermott； now at the University of Florida； and I argued that for this reason Titan must be either an all…ocean or an all…land world。 Otherwise the tidal friction in places where the ocean is shallow would have taken its toll。 Lakes and islands might be permitted； but anything more and Titan would have a very different orbit than the one we see。
We have； then； three scientific arguments—one concluding that this world is almost entirely covered with hydrocarbon oceans； another that it's a mix of continents and oceans； and a third requiring us to choose； counseling that Titan can't have extensive oceans 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。 Pe
