pale blue dot -carl sagan-第25章
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！
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 Pliny the Elder as he made his way up the side of the volcano； intent on arriving at a better understanding of its workings。 （Pliny was hardly the last： Fifteen volcanologists have been killed in sundry volcanic eruptions between 1979 and 1993。） The Mediterranean island of Santorin （also called Thera） is in reality the only part above water of the rim of a volcano now inundated by the sea。* The explosion of the Santorin volcano in 1623 B。C。 may； some historians think； have helped destroy the great Minoan civilization on the nearby island of Crete and changed the balance of power in early classical civilization。 This disaster may be the origin of the Atlantis legend as related by Plato； in which a civilization was destroyed 〃in a single day and night of misfortune。〃 It must have been easy back then to think that a god was angry。
* The eruption of a nearby submarine volcano and the rapid construction Of' new island in 197 B。C。 are described by Strabo in the epigraph to this chapter。
Volcanos have naturally been regarded with fear and awe。 When medieval Christians viewed the eruption of Mt。 Hekla in Iceland and saw churning fragments of soft lava suspended over the summit； they imagined they were seeing the souls of the damned awaiting entrance to Hell。 〃Fearful howlings； weeping and gnashing of teeth；〃 〃melancholy cries and loud wailings〃
were dutifully reported。 The glowing red lakes and sulfurous gases within the Hekla caldera were thought to be a real glimpse into the underworld and a confirmation of folk beliefs in Hell sand； by symmetry； in its partner； Heaven）。
A volcano is； in fact； an aperture to an underground realm much vaster than the thin surface layer that humans inhabit； and far more hostile。 The lava that erupts from a volcano is liquid rock—rock raised to its melting point； generally around 1000°C。 The lava emerges from a hole in the Earth； as it cools and solidifies； it generates and later remakes the flanks of a volcanic mountain。
The most volcanically active locales on Earth tend to be along ridges on the ocean floor and island arcs—at the junction of two great plates of oceanic crust—either separating from each other； or one slipping under the other。 On the seafloor there are long zones of volcanic eruptions—acpanied by swarms of earthquakes and plumes of abyssal smoke and hot water—that we a are just beginning to observe with robot and manned submersible vehicles。
Eruptions of lava must mean that the Earth's interior is extremely hot。 Indeed； seismic evidence shows that； only a few hundred kilometers beneath the surface； nearly the entire body of the Earth is at least slightly molten。 The interior of the Earth is hot； in part； because radioactive elements there； such as uranium； give off heat as they decay； and in part because the Earth retains some of the original heat released in its formation； when many small worlds fell together by their mutual gravity to make the Earth； and when iron drifted down to form our planet's core。
The molten rock； or magma； rises through fissures in the surrounding heavier solid rocks。 We can imagine vast subterranean caverns filled with glowing； red； bubbling； viscous liquids that shoot up toward the surface if a suitable channel is by chance provided。 The magma； called lava as it pours out of the summit caldera； does indeed arise from the underworld。 The souls of the damned have so far eluded detection。
Once the volcano is fully built from successive outpourings； and the lava is no longer spewing up into the caldera； then it bees just like any other mountain—slowly eroding because of rainfall and windblown debris and； eventually； the movement of continental plates across the Earth's surface。 〃How many years can a mountain exist before it is washed to the sea？〃 asked Bob Dylan in the ballad 〃Blowing in the Wind。〃 The answer depends on which planet we're talking about。 For the Earth； it's typically about ten million years。 So mountains； volcanic and otherwise； must be built on the same timescale； otherwise the Earth would be everywhere smooth as Kansas。* Volcanic explosions can punch vast quantities of matter—mainly fine droplets of sulfuric acid—into the stratosphere。 There； for a year or two； they reflect sunlight back to space and cool the Earth。 This happened recently with the Philippine volcano； Mt。 Pinatubo； and disastrously in 1815…16 after the eruption of the Indonesian volcano Mt。 Tambora； which resulted in the famine…ridden 〃year without a summer。〃 A volcanic eruption in Taupo； New Zealand； in the year 177 cooled the climate of the Mediterranean； half a world away； and dropped fine particles onto the Greenland ice cap。 The explosion of Mt。 Mazama in Oregon （which left the caldera now called Crater Lake） in 4803 B。C。 had climatic consequences throughout the northern hemisphere。 Studies of volcanic effects on the climate were on the investigative path that eventually led to the discovery of nuclear winter。 They provide important tests of our use of puter models to predi
