pale blue dot -carl sagan-第24章
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o the public； and unable to answer a single straightforward； well…posed scientific question。 I thought myself that whether there are breaks in the clouds was one such question。 I argued that cameras could also answer questions that we were too dumb even to pose。 I argued that pictures were the only way to show the public—who were； after all； footing the bill—the excitement of robotic missions。 At any rate； no camera was flown； and subsequent missions have； for this particular world； at least partly vindicated that judgment： Even at high resolution from close flybys； in visible light it turns out there are no breaks in the clouds of Venus； any more than in the clouds of Titan。* These worlds are permanently overcast。
* For Titan； imaging revealed a succession of detached hazes above the main layer of aerosols。 So Venus works out to be the only world in the Solar System for which spacecraft cameras working in ordinary visible light haven't discovered something important。 Happily； we've now returned pictures from almost every world we've visited。 （NASA's International etary Explorer； which raced through the tail of et Giacobini…Zimmer in 1985； flew blind； be devoted to charged particles and magnetic fields。）
In the ultraviolet there is detail； but due to transient patches of high…altitude overcast； far above the main cloud deck。 The high clouds race around the planet much faster than the planet itself turns： super…rotation。 We have an even smaller chance of seeing the surface in the ultraviolet。
When it became clear that the atmosphere of Venus was much thicker than the air on Earth—as we now know； the pressure at the surface is ninety times what it is here—it immediately followed that in ordinary visible light we could not possibly see the surface； even if there were breaks in the clouds。 What little sunlight is able to make its tortuous way through the dense atmosphere to the surface would be reflected back； all right； but the photons would be so jumbled by repeated scattering off molecules in the lower air that no image of surface features could be retained。 It would be like a 〃whiteout〃 in polar snowstorm。 However； this effect； intense Rayleigh scattering； declines rapidly with increasing wavelength； in the near…infrared； it was easy to calculate； you could see the surface if there were breaks in the clouds or if the clouds were transparent there。
So in 1970 Jim Pollack； Dave Morrison； and I went to the McDonald Observatory of the University of Texas to try to observe Venus in the near…infrared。 We 〃hypersensitized〃 our emulsions； the good old…fashioned* glass photographic plates were treated with ammonia； and sometimes heated or briefly illuminated； before being exposed at the telescope to light from Venus。 For a time the cellars of McDonald Observatory reeked of ammonia。 We took many pictures。 None showed any detail。 We concluded that either we hadn't gone far enough into the infrared or the clouds of Venus were opaque and unbroken in the near infrared。
* Today many telescopic images are obtained with such electronic contrivances as charge…coupled devices and diode arrays； and processed by puter—all technologies unavailable to astronomers in 1970。
More than 20 years later； the Galileo spacecraft； making a close flyby of Venus； examined it with higher resolution and sensitivity； and at wavelengths a little further into the infrared than we were able to reach with our crude glass emulsions。 Galileo photographed great mountain ranges。 We already knew of their existence； though； a much more powerful technique had earlier been employed： radar。 Radio waves effortlessly penetrate the clouds and thick atmosphere of Venus； bounce off the surface； and return to Earth； where they are gathered in and used to make a picture。 The first work had been done； chiefly； by。 American ground…based radar at JPL's Goldstone tracking station in the Mojave Desert and at the Arecibo Observatory in Puerto Rico； operated by Cornell University。
Then the U。S。 Pioneer 12； the Soviet Venera 15 and ？6 and the U。S。 Magellan missions inserted radar telescopes into orbit around Venus and mapped the place pole to pole。 Each spacecraft would transmit a radar signal to the surface and then catch it as it bounced back。 From how reflective each patch of surface was and how long it took the signal to return （shorter from mountains； longer from valleys）； a detailed map of the entire surface was slowly and painstakingly constructed。
The world so revealed turns out to be uniquely sculpted by lava flows （and； to a much lesser degree； by wind）； as described in the next chapter。 The clouds and atmosphere of Venus have now bee transparent to us； and another world has been visited by the doughty robot explorers from Earth。 Our experience with Venus is now being applied elsewhere—especially to Titan； where once again impenetrable clouds hide an enigmatic surface； and radar is beginning to give us hints of what might lie below。
VENUS HAD LONG BEEN THOUGHT of as our sister world。 It is the nearest planet to the Earth。 It has almost the same mass； size； density； and gravitational pull as the Earth does。 It's a little closer to the Sun than the Earth； but its bright clouds reflect more sunlight back to space than our clouds do。 As a first guess you might very well imagine that； under those unbroken clouds； Venus was rather like Earth。 Early scientific speculation included fetid swamps crawling with monster amphibians； like the Earth in the Carboniferous Period； a world desert； a global petroleum sea； and a seltzer ocean dotted here and there with limestone…encrusted islands。 While based on some scientific data； these “models〃 of Venus—the first dating from the beginnings of the century； the second from the 1930s； and the last two from the raid…1950s—were little more than scientific romances； hardly constrained by the sparse data available。
Then； in 1956； a report was published in The Astrophysical Journal by Cornell H。 Mayer and his colleagues。 They had pointed a newly pleted radio telescope； built in part for classified research； on the roof of the Naval Research Laboratory in Washington； D。C。； at Venus and measured the flux of radio waves arriving at Earth。 This was not radar： No radio waves were bounced off Venus。 This was listening to radio waves that Venus on its own emits to space。 Venus turned out to be much brighter than the background of distant stars and galaxies。 This in itself was not very surprising。 Every object warmer than absolute zero （…273°C） gives off radiation throughout the electromagnetic spectrum； including the radio region。 You； for example； emit radio waves at an effective or 〃brightness〃 temperature of about 35°C； and if you were in surroundings colder than you are； a sensitive radio telescope could detect the faint radio waves you are transmitting in all directions。 Each of us is a source of cold static。
What was surprising about Mayer's discovery was that the brightness temperature of Venus is more than 300°C； far higher than the surface temperature of the Earth or the measured infrared temperature of the clouds of Venus。 Some places on Venus seemed at least 200° hotter than the normal boiling point of water。 What could this mean？
Soon there was a deluge of explanations。 I argued that the high radio brightness temperature was a direct indication of a hot surface； and that the high temperatures were due to a massive carbon dioxide/water vapor greenhouse effect—in which some sunlight is transmitted through the clouds and heats the Surface； but the surface experiences enormous difficulty in radiating back to space because of the high infrared opacity of carbon dioxide and water vapor。 Carbon dioxide absorbs at a range of wavelengths through the infrared； but there seemed to be 〃windows〃 between the CO2 absorption bands through which the surface could readily cool off to space。 Water vapor； though； absorbs at infrared frequencies that correspond in part to the windows in the carbon dioxide opacity。 The two gases together； it seemed to me； could pretty well absorb almost all the infrared emission； even if there was very little water vapor—something like two picket fences； the slats of one being fortuitously positioned to cover the gaps of the other。
There was another very different category of explanation； in which the high brightness temperature of Venus had nothing to do with the ground。 The surface could still be temperate； clement； congenial。 It was proposed that some region in the atmosphere of Venus or in its surrounding magnetosphere emitted these radio waves to space。 Electrical discharges between water droplets in the Venus clouds were suggested。 A glow discharge in which ions and electrons rebined at twilight and dawn in the upper atmosphere was offered。 A very dense ionosphere had its advocates； in which the mutual acceleration of unbound electrons （〃free…free emission〃） gave off radio waves。 （One proponent of this idea even suggested that the high ionization required was due to an average of 10；000 times greater radioactivity on Venus than on Earth—perhaps from a recent nuclear war there。） And； in the light of the discovery of radiation from Jupiter's magnetosphere； it was natural to suggest that the radio emission came from an immense cloud of charged particles trapped by some hypothetical very intense Venusian magnetic field。
In a series of papers I published in the middle 1960s； many in collaboration with Jim Pollack；* these conflicting models of a high hot emitting region and a cold surface were subjected to a critical analysis。 By then we had two important new clues： the radio spectrum of Venus； and the Mariner 2 evidence that the radio emission was more intense at the center of the disk of Venus than toward its edge。 By 1967 we were able to exclude the alternative models with some confidence； and conclude that the surface of Venus was at a scorching and un…Earthlike temperature； in excess of 400°C。 But the argument was inferential； and there were many intermediate steps。 We longed for a more direct measurement。
* James B。 Pollack made important contributions to every area of planetary science。 He was my first graduate student and a colleague ever since。 He converted NASA's Ames Research Center into a world leader in planetary research and the post…doctoral training of planetary scientists。 His gentleness was as extraordinary as his scientific abilities。 He died in 1994 at the height of his powers。
In October 1967—memorating the tenth anniversary of Sputnik 1—the Soviet Venera 4 spacecraft dropped an entry capsule into the clouds of Venus。 It returned data from the hot lover atmosphere； but did not survive to the surface。 One day later； the United States spacecraft Mariner 5 flew by Venus； its radio transmission to Earth skimming the atmosphere at progressively greater depths。 The rate of fading of the signal gave information about atmospheric temperatures。 Although there seemed to be some discrepancies （later resolved） between the two sets of spacecraft data； both clearly indicated that the surface of Venus is very hot。
Since then a progression of Soviet Venera spacecraft and one cluster of American spacecraft from the Pioneer 12 mission have entered the deep atmosphere or landed on the
