Pale Blue Dot - Carl Sagan [49]
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 circum-planetary 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.
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 8½ 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 cacaphony 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.
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