Geography Reference
In-Depth Information
Venus
The planet closest in size to Earth, Venus is shrouded by clouds that presumably create a green-
house effect. The temperature on Venus is hot enough at 880˚F (500˚C) to melt lead. It is one of two planets
on which spacecraft have landed (the Soviet Union having done it three times).
Earth
(and its moon) The largest of the four terrestrial planets, Earth is the only one with liquid water
on the surface and apparently the only one with tectonic activity.
Mars
With about half the diameter of the earth, Mars has been called the Red Planet because the iron in
its rocky surface has rusted, giving it a distinctive reddish patina. There are polar ice caps of dry ice (frozen
carbon dioxide), and Mars also boasts the solar system's largest mountain, an extinct volcano called Olym-
pus Mons that is 17 miles high and 370 miles across at its base. The notorious “canals” of Mars, first seen
in 1877 and once believed to be a sign of life on the planet, simply don't exist and are most likely an op-
tical illusion.
Between Mars and the other planets is a belt of interplanetary landfill, with celestial bodies ranging in
size from tiny space pebbles to Ceres, an asteroid several hundred miles across. This is the asteroid belt
formed of materials that under the right conditions might have formed another planet. Now it's a big mess
of outer-space cookie crumbs, some of which occasionally cross paths with the earth.
Beyond the belt are the
outer planets
, called the
Jovian planets
, after the Roman name for Jupiter. Our
knowledge of them has been transformed with the discoveries made by four unmanned space probes, the
Pioneers 10
and
11
and the
Voyagers 1
and
2.
We now know that these four large planets differ from the
inner planets in that they may have a solid core but are surrounded by layers of liquid and gas.
Jupiter
The largest planet spins rapidly—its “day” is only ten hours long. Scientists have speculated
that since, like the sun, Jupiter is composed mainly of hydrogen and helium, and it has many
moons—sixteen have been sighted so far—and is so large, it possibly was almost a star itself.
Saturn
With its famous rings, Saturn is probably the most recognizable planet to laymen. The second-
largest planet, it has twenty-one moons, one of which, Titan, is one of the largest satellites in the solar
system and has its own atmosphere. Those seven notable ring systems are debris, mostly ice and rocks
ranging in size from grains of dust to house-sized blocks.
Uranus
Invisible to the naked eye, Uranus was not discovered until 1781 and was the first planet to be
observed by telescope. The third-largest planet, it has fifteen moons, ten of them discovered by
Voyager 2
,
and a series of dark rings.
Neptune
First seen in 1846 and named for the Roman sea god, Neptune is the fourth-largest planet.
With eight known moons and its own set of rings, Neptune also features the highest surface winds in the
solar system, clocked at 1500 miles per hour. Its large moon, Triton, has an atmosphere and is the coldest
known place in the solar system, measured at -390˚F.
Pluto
The ninth planet in our solar system is normally the planet farthest from the sun, usually about
3.7 billion miles. But an eccentricity in its orbit brought Pluto temporarily into a position inside Neptune's
orbit during the 1990s. Along with its strange orbit, Pluto's small size (less than one-fifth the diameter of
Earth) led some astronomers to contend that it is not a full-fledged planet. On August 24, 2006, the Inter-
national Astronomical Union passed a new definition for planets and designated Pluto a “dwarf planet.”Its
one moon is called Charon.
Our solar system is centered, of course, on the sun, a whirling body of hydrogen and helium burning up
in constant thermonuclear detonations. These explosions, which produce the light and heat that made life
possible and sustain the earth today, start within the sun's center. The solar energy we now receive took
thirty thousand years to work its way out from the solar core. Things get easier after that, though. Having
reached the sun's surface, the solar energy takes but eight minutes to find its way across 93 million miles
of empty space and down to earth. (It takes about five and a half hours for the sun's light to reach Pluto.)
