pale blue dot -carl sagan-第19节
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preceding the Olympian gods—Saturn； his siblings； and his cousins—were called Titans。
In a way； I grew up with Titan。 I did my doctoral dissertation at the University of Chicago under the guidance of Gerard P。 Kuiper； the astronomer who made the definitive discovery that Titan has an atmosphere。 Kuiper was Dutch and in a direct line of intellectual descent from Christianus Huygens。 In 1914； while making a spectroscopic examination of Titan； Kuiper was astonished to find the characteristic spectral features of the gas methane。 When he pointed the telescope at Titan； there was the signature of methane。 When he pointed it away； not a hint of methane。* But moons were not supposed to hold onto sizable atmospheres； and the Earth's Moon certainly doesn't。 Titan could retain an atmosphere； Kuiper realized； even though its gravity was less than Earth's； because its upper atmosphere is very cold。 The molecules simply aren't moving fast enough for significant numbers to achieve escape velocity and trickle away to space。
* Titan's atmosphere has no detectable oxygen； so methane is not wildly out of chemical equilibrium—as it is on Earth—and its presence is in no way a sign of life。
Daniel Harris； a student of Kuiper's； showed definitively that Titan is red。 Maybe we were looking at a rusty surface； like that of Mars。 If you wanted to learn more about Titan； you could also measure the polarization of sunlight reflected off it。 Ordinary sunlight is unpolarized。 Joseph Veverka； now a fellow faculty member at Cornell University； was my graduate student at Harvard University； and therefore； so to speak； a grandstudent of Kuiper's。 In his doctoral work； around 1970； he measured the polarization of Titan and found that it changed as the relative positions of Titan； the Sun； and the Earth changed。 But the change was very different from that exhibited by； say； the Moon。 Veverka concluded that the character of this variation was consistent with extensive clouds or haze on Titan。 When we looked at it through the telescope； we weren't seeing its surface。 We knew nothing about what the surface was like。 We had no idea how fat below the clouds the surface was。
So； by the early 1970s； as a kind of legacy from Huygens and his line of intellectual descent； we knew at least that Titan has a dense methane…rich atmosphere； and that it's probable enveloped by a reddish cloud veil or aerosol haze。 But what kind of cloud is red？ By the early 1970s my colleague Bishun Khare and I had been doing experiments at Cornell in which we irradiated various methane…rich atmospheres with Ultraviolet light or electrons and were generating reddish or brownish solids； the stuff would coat the interiors of our reaction vessels。 It seemed to me that； if methane…rich Titan had red…brown clouds； those clouds might very well be similar to what we were making in the laboratory。 We called this material tholin； after a Greek word for 〃muddy。〃 At the beginning we had yen little idea what it was made of。 It was some organic stew made by breaking apart our starting molecules； and allowing the atoms—carbon； hydrogen； nitrogen—and molecular fragments to rebine。
The word 〃organic〃 carries no imputation of biological origin； following long…standing chemical usage dating back mots than a century； it merely describes molecules built out of car bon atoms （excluding a few very simple ones such as carbon monoxide； CO； and carbon dioxide； CO2）。 Since life on Earth is based oil organic molecules； and since there was a time before there was life on Earth； some process must have made organic molecules on our planet before the time of the first organism。 Something sitar； I proposed； might be happening on Titan today。
The epochal event in our understanding of Titan was the arrival in 1980 and 1981 of the Voyager 1 and 2 spacecraft in the Saturn system。 The ultraviolet； infrared； and radio instruments revealed the pressure and temperature through the atmosphere—from the hidden surface to the edge of space。 We learned how high the cloud tops are。 We found that the air on Titan is posed mainly of nitrogen； N2； as on the Earth today。 The other principal constituent is； as Kuiper found； methane。 CH4 the starting material from which carbon…based organic molecules are generated there。
A variety of simple organic molecules was found； present as gases； mainly hydrocarbons and nitriles。 The most plex of them have four 〃heavy〃 （carbon and/or nitrogen） atoms。 Hydrocarbons are molecules posed of carbon and hydrogen atoms only； and are familiar to us as natural gas； petroleum； and waxes。 （They're quite different from carbohydrates； such as sugars and starch； which also have oxygen atoms。） Nitriles are molecules with a carbon and nitrogen atom attached in a particular way。 The best known nitrile is HCN； hydrogen cyanide； a deadly gas for humans。 But hydrogen cyanide is implicated in the steps that on Earth led to the origin of life。
Finding these simple organic molecules in Titan's upper atmosphere—even if present only in a part per million or a part per billion—is tantalizing。 Could the atmosphere of the primeval Earth have been similar？ There's about ten times more air oil Titan than there is on Earth today； but the early Earth may well have had a denser atmosphere。
Moreover； Voyager discovered an extensive region of energetic electrons and protons surrounding Saturn； trapped by the planet's magnetic field。 During the course of its orbital motion around Saturn； Titan bobs in and out of this magnetosphere。 Beams of electrons （plus solar ultraviolet light） fall on the upper air of Titan； just as charged particles （plus solar ultraviolet light） were intercepted by the atmosphere of the primitive Earth。
So it's a natural thought to irradiate the appropriate mixture of nitrogen and methane with ultraviolet light or electrons at very low pressures； and find out what more plex molecules can be made。 Can we simulate what's going on in Titan's high atmosphere？ In our laboratory at Cornell—with my colleague W。 Reid Thompson playing a key role—we've replicated some of Titan's manufacture 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； th
