pale blue dot -carl sagan-第22章
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re of organic gases。 The simplest hydrocarbons on Titan are manufactured by ultraviolet light from the Sun。 But for all the other gas products； those made most readily by electrons in the laboratory correspond to those discovered by Voyager on Titan； and in the same proportions。 The correspondence is one to one。 The next most abundant gases that we've found in the laboratory will be looked for in future studies of Titan。 The most plex organic gases we make have six or seven carbon and/or nitrogen atoms。 These product molecules are on their way to forming tholins。
WE HAD HOPED FOR A BREAK In the weather as Voyager 1 approached Titan。 A long distance away； it appeared as a tiny disk； at closest approach； our camera's field of view was filled by a small province of Titan。 If there had been a break in the haze and clouds； even only a few miles across； as we scanned the disk we would have seen something of its hidden surface。 But there was no hint of a break。 This world is socked in。 No one on Earth knows what's on Titan's surface。 And an observer there； looking up ill ordinary visible light； would have no idea of the glories that await upon ascending through the haze and beholding Saturn and its magnificent rings。
From measurements by Voyager； by the International Ultraviolet Explorer observatory in Earth orbit； and by ground…based telescopes； we know a fair amount about the orange…brown haze particles that obscure the surface： which colors of light they like to absorb； which colors they pretty much let pass through them； how much they bend； the light that does pass through them； and how big they are。 （They're mostly the size of the particles in cigarette smoke。） The 〃optical properties〃 will depend； of course； on the position of the haze particles。
In collaboration with Edward Arakawa of Oak Ridge National Laboratory in Tennessee； Khare and I have measured the optical properties of Titan tholin。 It turns out to be a dead ringer for the real Titan haze。 No other candidate material； mineral or organic； matches the optical constants of Titan。 So we can fairly claim to have bottled the haze of Titan—formed high in its atmosphere； slowly falling out； and accumulating in copious amounts on its surface。 What is this stuff made of？
It's very hard to know the exact position of a plex organic solid。 For example； the chemistry of coal is still not fully understood； despite a long…standing economic incentive。 But we've found out some things about Titan tholin。 It contains many of the essential building blocks of life on Earth。 Indeed； if you drop Titan tholin into water you make a large number of amino acids； the fundamental constituents of proteins； and nucleotide bases also； the building blocks of DNA and RNA。 Some of the amino acids so formed are widespread in living things on Earth。 Others are of a pletely different sort。 A rich array of other organic molecules is present also； some relevant to life； some not。 During the past four billion years； immense quantities of organic molecules sedimented out of the atmosphere 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 o
