Pale Blue Dot ( PDFDrive.com ) (1)

there is one previously unknown planet, there may be many more—in this solar system and in others.
Who can tell what might be found if a multitude of new worlds are hiding in the dark?
The discovery was made not even by a professional astronomer but by William Herschel, a
musician whose relatives had come to Britain with the family of another anglified German, the
reigning monarch and future oppressor of the American colonists, George III. It became
Herschel's wish to call the planet George ("George's Star," actually), after his patron. but,
providentially, the name didn't stick. (Astronomers seem to have been very busy buttering up
kings.) Instead, the planet that Herschel found is called Uranus (an inexhaustible source of
hilarity renewed in each generation of English-speaking nine-year-olds). It is named after the
ancient sky god who, according to Greek myth, was Saturn's father and thus the grandfather of the
Olympian gods.
We no longer consider the Sun and Moon to be planets, and ignoring the comparatively
insignificant asteroids and comets, count Uranus as the seventh planet in order from the Sun
(Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto). It is the first planet
unknown to the ancients. The four outer, Jovian, planets turn out to be very different from the
four inner, terrestrial, planets. Pluto 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
becomes 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.