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s 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 surface and measured directly—essentially by sticking out a thermometer—the surface and near…surface temperatures。 They turn out to be about 470°C； almost 900°F。 When such factors as calibration errors of terrestrial radio telescopes and surface emissivity are taken into account； the old radio observations and the new direct spacecraft measurements turn out to be in good accord。
Early Soviet landers were designed for an atmosphere somewhat like our own。 They were crushed by the high pressures like a tin can in the grasp of a champion arm wrestler； or a World War II submarine in the Tonga Trench。 Thereafter； Soviet Venus entry vehicles were heavily reinforced； like modern submarines； and successfully landed on the searing surface。 When it became clear how deep the atmosphere is and how thick the clouds； Soviet designers became concerned that the surface might be pitch…black。 Veneras 9 and 10 were equipped with floodlights。 They proved unnecessary。 A few percent of the sunlight that falls on the top of the clouds makes it through to the surface； and Venus is about as bright as on a cloudy day on Earth。
The resistance to the idea of a hot surface on Venus can； I suppose； be attributed to our reluctance to abandon the notion that the nearest planet is hospitable for life； for future exploration； and perhaps even； in the longer term； for human settlement。 As it turns out there are no Carboniferous swamps no global oil or seltzer oceans。 Instead； Venus is a stifling； brooding inferno。 There are some deserts； but it's mainly a world of frozen lava seas。 Our hopes are unfulfilled。 The call of this world is now more muted than in the early days of spacecraft exploration； when almost anything was possible and our most romantic notions about Venus might； for all we then knew； be realized。
MANY SPACECRAFT CONTRIBUTED to our present understanding of Venus。 But the pioneering mission was Mariner 2。 Mariner 1 failed at launch and—as they say of a racehorse with a broken leg—had to be destroyed。 Mariner 2 worked beautifully and provided the key early radio data on the climate of Venus。 It made infrared observations of the properties of the clouds。 On its way from Earth to Venus； it discovered and measured the solar wind—the stream of charged particles that flows outward from the Sun； filling the magnetospheres of any planets in its way； blowing back the tails of ets； and establishing the distant heliopause。 Mariner 2 was the first successful planetary probe； the ship that ushered in the age of planetary exploration。
It's still in orbit around the Sun； every few hundred days still approaching； more or less tangentially； the orbit of Venus。 Each time that happens； Venus isn't there。 But if we wait long enough； Venus will one day be nearby and Mariner 2 will be accelerated by the planet's gravity into some quite different orbit。 Ultimately； Mariner 2； like some planetesimal from ages past； will be swept up by another planet； fall into the Sun； or be ejected from the Solar System。
Until then； this harbinger of the age of planetary exploration； this minuscule artificial planet； will continue silently orbiting the Sun。 It's a little as if Columbus's flagship； the Santa Maria； were still making regular runs with a ghostly crew across the Atlantic between Cadiz and Hispaniola。 In the vacuum of interplanetary space； Mariner 2 should be in mint condition for many generations。
My wish on the evening and morning star is this： that late in the twenty…first century some great ship； on its regular gravity…assisted transit to the outer Solar System； intercepts this ancient derelict and heaves it aboard； so it can be displayed in a museum of early space technology—on Mars； perhaps； or Europa； or Iapetus。
CHAPTER 12 THE GROUND MELTS
Midway between Thera and Therasia； fires broke forth from the sea and continued for four days； so that the whole sea boiled and blazed； and the fires cast up an island which was gradually elevated as though by levers 。 。 。 After the cessation of the eruption； the Rhodians； at the time of their maritime supremacy； were first to venture upon the scene and to erect on the island a temple。
—STRABO； GEOGRAPHY （CA。 7 B。C）
All over the Earth； you can find a kind of mountain with one striking and unusual feature。 Any child can recognize it： The top seems sheared or squared off： If you climb to the summit or fly over it； you discover that the mountain has a hole or crater at its peak。 In some mountains of this sort； the craters are small； in others； they are almost as big as the mountain itself。 Occasionally； the craters are filled with water。 Sometimes they're filled with a more amazing liquid： You tiptoe 10 the edge； and see vast； glowing lakes of yellow…red liquid and fountains of fire。 These holes in the tops of mountains are called calderas； after the word 〃caldron；〃 and the mountains on which they sit are known； of course； as volcanos—after Vulcan； the Roman god of fire。 There are perhaps 600 active volcanos discovered on Earth。 Some； beneath the oceans； are yet to be found。
A typical volcanic mountain looks safe enough。 Natural vegetation runs up its sides。 Terraced fields decorate its flanks。 Hamlets and shrines nestle at its base。 And yet； without warning； after centuries of lassitude； the mountain may explode。 Barrages of boulders； torrents of ash drop out of the sky。 Rivers of molten rock e pouring down its sides。 All over the Earth people imagined that an active volcano was an imprisoned giant or demon struggling to get out。
The eruptions of Mt。 St。 Helens and Mt。 Pinatubo are recent reminders； but examples can be found throughout history。 In 1902 a hot； glowing volcanic cloud swept down the slopes of Mt。 Pelee and killed 35；000 people in the city of St。 Pierre on the Caribbean island of Martinique。 Massive mudflows from the eruption of the Nevado del Ruiz volcano in 1985 killed more than 25；000 Colombians。 The eruption of Mt。 Vesuvius in the first century buried in ash the hapless inhabitants of Pompeii and Herculaneum and killed the intrepid naturalist Plin