按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
d where the very stuff of life falls out of the skies onto the unknown surface below。 It is so far away that light takes more than an hour to get there from the Sun。 Spacecraft take years。 Much about it is still a mystery—including whether it holds great oceans。 We know just enough； though； to recognize that within reach may be a place where certain processes ate today working themselves out that aeons ago on Earth led to the origin of life。
On our own world a long…standing—and in some respects quite promising—experiment has been under way on the evolution of matter。 The oldest known fossils are about 3。6 billion years old。 Of course； the origin of life had to have happened well before that。 But 4。2 or 4。3 billion years ago the Earth was being so ravaged by the final stages of its formation that life could not yet have e into being： Massive collisions were melting the surface； turning the oceans into steam and driving any atmosphere that had accumulated since the last impact off into space。 So around 4 billion years ago； there was a fairly narrow window—perhaps only a hundred million years wide in which our most distant ancestors came to be。 Once conditions permitted； life arose fast。 Somehow。 The first living things very likely were inept； far less capable than the most humble microbe alive today—perhaps just barely able to make crude copies of themselves。 But natural selection； the key process first coherently described by Charles Darwin； is an instrument of such enormous power that from the most modest beginnings there can emerge all the richness and beauty of the biological world。
Those first living things were made of pieces； parts； building blocks which had to e into being on their own—that is； driven by the laws of physics and chemistry on a lifeless Earth。 The building blocks of all terrestrial life are called organic molecules； molecules based on carbon。 Of the stupendous ＇lumber of possible organic molecules； very few are used at the heart of life。 The two most important classes are the amino acids； the building blocks of proteins； and the nucleotide bases； the building blocks of the nucleic acids。 Mist before the origin of life； where did these molecules e from？ There are only two possibilities： from the outside or from the inside。 We know that vastly more ets and asteroids were hitting the Earth than do so today； that these small worlds are rich storehouses of plex organic molecules； and that some of these molecules escaped being fried on impact。 Here I'm describing homemade； not imported； goods： the organic molecules generated in the air and waters of the primitive Earth。
Unfortunately； we don't know very much about the position of the early air； and organic molecules are far easier to make in some atmospheres than in others。 There couldn't have been much oxygen； because oxygen is generated by green plants and there weren't any green plants yet。 There was probably more hydrogen； because hydrogen is very abundant in the Universe and escapes from the upper atmosphere of the Earth into space better than any other atom （because it's so light）。 If we can imagine various possible early atmospheres； we can duplicate them in the laboratory； supply some energy； and see which organic molecules are made and in what amounts。 Such experiments have over the years proved provocative and promising。 But our ignorance of initial conditions limits their relevance。
What we need is a real world whose atmosphere still retains some of those hydrogen…rich gases； a world in other respects something like the Earth； a world in which the organic building blocks of life are being massively generated in our own time； a world we can go to to seek our own beginnings。 There is only one such world in the Solar System。* That world is Titan； the big moon of Saturn。 It's about 5；150 kilometers （3；200 miles） in diameter； a little less than half the size of the Earth。 It takes 16 of our days to plete one orbit of Saturn。
* There could have been none。 We're very lucky that there is such a world study。 The others ill have too much hydrogen； or not enough； or no atmosphere at all。
No world is a perfect replica of any other； and in at least one important respect Titan is very different from the primitive Earth： Being so far from the Sun； its surface is extremely cold； far below the freezing point of water； around 180° below zero Celsius。 So while the Earth at the time of the origin of life was； as now； mainly ocean…covered； plainly there can be no oceans of liquid water on Titan。 （Oceans made of other stuff are a different story； as we shall see。） The low temperatures provide an advantage； though； because once molecules are synthesized on Titan； they tend to stick around： The higher the temperature； the faster molecules fall to pieces。 On Titan the molecules that have been raining down like manna from heaven for the last 4 billion years might still be there； largely unaltered； deep…frozen； awaiting the chemists from Earth。
THE INVENTION OF THE TELESCOPE In the seventeenth century led to the discovery of many new worlds。 In 1610 Galileo first spied the four large satellites of Jupiter。 It looked like a miniature solar system； the little moons racing around Jupiter as the planets were thought by Copernicus to orbit the Sun。 It was another blow to the geocentrists。 Forty…five years later； the celebrated Dutch physicist Christianus Huygens discovered a moon moving about the planet Saturn and named it Titan。* It was a dot of light a billion miles away； gleaming in reflected sunlight。 From the time of its discovery； when European men wore long curly wigs； to world War II； when American men cut their hair down to stubble； almost nothing more was discovered about Titan except the fact it had a curious； tawny color。 Ground…based telescopes could； even in principle； barely make out some enigmatic detail。 The Spanish astronomer J。 as Sola reported at the turn of the twentieth century some faint and indirect evidence of an atmosphere。
* Not because he thought it remarkably large。 but because in Greek mythology members of the generation 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 t