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o is a separate case。
As the years passed and the quality of astronomical instruments unproved； we began to learn more about distant Uranus。 What reflects the dim sunlight back to us is no solid surface； but atmosphere and clouds just as for Titan； Venus； Jupiter Saturn； and Neptune。 The air on Uranus is made of hydrogen and helium； the two simplest gases。 Methane and other hydrocarbons are also present。 Just below the clouds visible to Earthbound observers is a massive atmosphere with enormous quantities of ammonia； hydrogen sulfide； and； especially； water。
At depth on Jupiter and Saturn； the pressures are so great that atoms sweat electrons； and the air bees a metal。 That does not seem to happen on less massive Uranus； because the pressures at depth are less。 Still deeper； discovered only by its subtle tugs on Uranus' moons； wholly inaccessible to view； under the crushing weight of the overlying atmosphere； is a rocky surface。 A big Earthlike planet is hiding down there； swathed in an immense blanket of air。
The Earth's surface temperature is due to the sunlight it intercepts。 Turn off the Sun and the planet soon chills—not to trifling Antarctic cold； not just so cold that the oceans freeze； but to a cold so intense that the very air precipitates out； forming a ten…meter…thick layer of oxygen and nitrogen snows covering the whole planet。 The little bit of energy that trickles up from the Earth's hot interior would be insufficient to melt these snows。 For Jupiter； Saturn； and Neptune it's different。 There's about as much heat pouring out from their interiors as they acquire from the warmth of the distant Sun。 Turn off the Sun； and they would be only a little affected。
But Uranus is another story。 Uranus is an anomaly among the Jovian planets。 Uranus is like the Earth： There's very little intrinsic heat pouring out。 We have no good understanding of why this should be； why Uranus—which in many respects is so similar to Neptune—should lack a potent source of internal heat。 For this reason； among others； we cannot say we understand what is going on in the deep interiors of these mighty worlds。
Uranus is lying on its side as it goes around the Sun。 In the 1990s。 the south pole is heated by the Sun； and it is this pole that Earthbound observers at the end of the twentieth century see when they look at Uranus。 It takes Uranus 84 Earth years to make one circuit of the Sun。 So in the 2030s； the north pole will be sunward （and Earthward）。 In the 2070s the south pole will be pointing to the Sun once again。 In between； Earthbound astronomers will be looking mainly at equatorial latitudes。
All the other planets spin much more upright in their orbits。 No one is sure of the reason for Uranus' anomalous spin； the most promising suggestion is that sometime in its early history； billions of years ago； it was struck by a rogue planet； about the size of the Earth； in a highly eccentric orbit。 Such a collision； if it ever happened； must have worked much tumult in the Uranus system； for all we know； there may be other vestiges of ancient havoc still left for us to find。 But Uranus' remoteness tends to guard its mysteries。
In 1977 a team of scientists led by James Elliot； then of Cornell University； accidentally discovered that； like Saturn； Uranus has rings。 The scientists were flying over the Indian Ocean in a special NASA airplane—the Kuiper Airborne Observatory—to witness the passage of Uranus in front of a star。 （Such passages； or occultations as they're called； happen from time to time； precisely because Uranus slowly moves with respect to the distant stars。） The observers were surprised to find that the star winked on and off several times just before it passed behind Uranus and its atmosphere； then several times more just after it emerged。 Since the patterns of winking on and off were the same before and after occultation； this finding （and much subsequent work） has led to the discovery of nine very thin； very dark circumplanetary rings； giving Uranus the appearance of a bull's…eye in the sky。
Surrounding the rings； Earthbound observers understood were the concentric orbits of the five moons then known： Miranda； Ariel； Umbriel； Titania； and Oberon。 They're named after characters in Shakespeare's A Midsummer Night's Dream and The Tempest； and in Alexander Pope's The Rape of the Lock。 Two of them were found by Herschel himself。 The innermost of the five； Miranda； was discovered as recently as 1948； by my teacher G。 P。 Kuiper。* I remember how great an achievement the discovery of a new moon of Uranus was considered back then。 The near…infrared light reflected by all five moons subsequently revealed the spectral signature of ordinary water ice on their Surfaces。 And no wonder—Uranus is so far from the Sun that it is no brighter there at noontime than it is after sunset on Earth。 The temperatures are frigid。 Any water must be frozen。
* He so named it because of the words spoken by Miranda； the heroine of The Tempest： 〃O brave new world； That has such people in't。〃 （To Which Prospero replies； 〃'Tis new to thee。〃 Just so。 Like all the other worlds in the Solar System； Miranda is about 4。5 billion years old。）
A REVOLUTION IN OUR UNDERSTANDING of the Uranus system—the planet； its rings； and its moons—began on January 24； 1986。 On that day； after a journey of 8？ years； the Voyager 2 spacecraft sailed very near to Miranda； and hit the bull's…eye in the sky。 Uranus' gravity then flung it on to Neptune。 The spacecraft returned 4；300 close…up pictures of the Uranus system and a wealth of other data。
Uranus was found to be surrounded by an intense radiation belt； electrons and protons trapped by the planet's magnetic field。 Voyager flew through this radiation belt； measuring the magnetic field and the trapped charged particles as it went。 It also detected—in changing timbres； harmonies； and nuance； but mainly in fortissimo—a cacophony of radio waves generated by the speeding； trapped particles。 Something similar was discovered on Jupiter and Saturn and would be later found at Neptune—but always with a theme and counterpoint characteristic of each world。
On Earth the magnetic and geographical poles are quite close together。 On Uranus the magnetic axis and the axis of rotation are tilted away from each other by some 60 degrees。 No one yet understands why： Some have suggested that we are catching Uranus in a reversal of its north and south magnetic poles； as periodically happens on Earth。 Others propose that this too is the consequence of that mighty； ancient collision that knocked the planet over。 But we do not know。
Uranus is emitting much more ultraviolet light than it's receiving from the Sun； probably generated by charged particles leaking out of the magnetosphere and striking its upper atmosphere。 From a vantage point in the Uranus system； the spacecraft examined a bright star winking on and off as the rings of Uranus passed by。 New faint dust bands were found。 From the perspective of Earth； the spacecraft passed behind Uranus； so the radio signals it was transmitting back home passed tangentially through the Uranian atmosphere； probing it—to below its methane clouds。 A vast and deep ocean； perhaps 8；000 kilometers thick； of super…heated liquid water floating in the air is inferred by some。
Among the principal glories of the Uranus encounter were the pictures。 With Voyager's two television cameras； we discovered ten new moons； determined the length of the day in the clouds of Uranus （about 17 hours）； and studied about a dozen rings。 The most spectacular pictures were those returned from the five larger； previously known moons of Uranus； especially the smallest of them； Kuiper's Miranda。 Its surface is a tumult of fault valleys； parallel ridges； sheer cliffs； low mountains； impact craters； and frozen floods of once…molten surface material。 This turmoiled landscape is unexpected for a small； cold； icy world so distant from the Sun。 Perhaps the surface was melted and reworked in some long…gone epoch when a gravitational resonance between Uranus； Miranda； and Ariel pumped energy from the nearby planet into Miranda's interior。 Or perhaps we are seeing the results of the primordial collision that is thought to have knocked Uranus over。 Or； just conceivably； maybe Miranda was once utterly destroyed； dismembered； blasted into smithereens by a wild careening world； with many collision fragments still left in Miranda's orbit。 The shards and remnants； slowly colliding； gravitationally attracting one another； may have re…aggregated into just such a jumbled； patchy； unfinished world as Miranda is today。
For me； there's something eerie about the pictures of dusky Miranda； because I can remember so well when it ‘vas only a faint point of light almost lost in the glare of Uranus； discovered through great difficulty by dint of the astronomer's skills and patience。 In only half a lifetime it has gone from an undiscovered world to a destination whose ancient and idiosyncratic secrets have been at least partially revealed。
CHAPTER 9 AN AMERICAN SHIP AT THE FRONTIERS OF THE SOLAR SYSTEM
。 。 。 by the shore of Triton's Lake 。 。 。
I will clear my breast of secrets。
—EURIPIDES； ION （CA。 413 B。C。）
Neptune was the final port of call in Voyager 2's grand tour of the Solar System。 Usually； it is thought of as the penultimate planet； with Pluto the outermost。 But because of Pluto's stretched…out； elliptical orbit； Neptune has lately been the outermost planet； and will remain so until 1999。 Typical temperatures in its upper clouds are about …240°C； because it is so far from the warming rays of the Sun。 It would be colder still except for the heat welling up from its interior。 Neptune glides along the hem of interstellar night。 It is so far away that； in its sky； the Sun appears as little more than an extremely bright star。
How far？ So far away that it has yet to plete a single trip around the Sun； a Neptunian year； since its discovery in 1846。* It's so far away that it cannot be seen with the naked eye。 It's so far away that it takes light—faster than which nothing can go—more than five hours to get from Neptune to Earth。
* It takes so long to circuit the Sun because its orbit is so vast； 23 billion miles around； and because the force of the Sun's gravity—which keeps it from flying out into interstellar space—is at that distant paratively feeble； less than a thousandth what it is in the Earth's vicinity。
When Voyager 2 raced through the Neptune system in 1989； its cameras； spectrometers； particle and field detectors； and other instruments were feverishly examining the planet； its moons； and its rings。 The planet itself； like its cousins Jupiter； Saturn； and Uranus； is a giant。 Every planet is an Earthlike world at heart—but the four gas giants wear elaborate； cumbersome disguises。 Jupiter and Saturn are great gas worlds with relatively small rocky and icy cores。 But Uranus and Neptune are fundamentally rock and ice worlds swaddled in dense atmospheres that hide them from view。
Neptune is four times bigger than the Earth。 When we look down on its cool； austere blueness； again we are seeing only atmosphere and clouds—no solid surface。 Again； the atmosphere is made mainly of hydrogen and helium； with a little methane and traces of other hydrocarbons。 There may also be some nitrogen。 The bright clouds； which seem to be methane crystals； float above thick； deeper clouds of unknown position。 From the motion of the clouds we discovered fierce winds； approaching the local speed of sound。 A Great Dark Spot was found； curiously at almost the same latitude as the Great Red Spot on Jupiter。 The azure color seems appropriate for a planet named after the god of the sea。
Surrounding this dimly lit； chilly； stormy； remote world is—here also—a system of rings； each posed of innumerable orbiting objects ranging in size from the fine particles in cigarette smoke to small trucks。 Like the rings of the other Jovian planets； those of Neptune seem to be evanescent—it is calculated that gravity and solar radiation will disrupt them in much less than the age of the Solar System。 If they are destroyed quickly； we must see them only because they were made recently。 But how can rings be made？
The biggest moon in th