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THE
Planets
By
Anthony Riggins
Main point
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Mars
Neptune
Uranus
Saturn
Earth
Mercury
Venus
Jupiter
Neptune
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Neptune is one of the two planets that cannot be
seen without a telescope. The other is Pluto.
Neptune is about 30 times as far from the sun as
is Earth. Pluto is the only planet farther from the
sun than Neptune. But every 248 years Pluto
moves inside Neptune's orbit for about a 20-year
period, during which it is closer to the sun than
Neptune. Pluto last crossed Neptune's orbit on
Jan. 23, 1979, and remained within it until Feb. 11,
1999.
Neptune's diameter at the equator is 30,775 miles
(49,528 kilometers), or almost 4 times that of
Earth. It is about 17 times as massive (heavy) as
Earth, but is not so dense as Earth. Neptune has
11 satellites (moons) and several rings around it.
Neptune travels around the sun in an elliptical
(oval-shaped) orbit. Its average distance from the
sun is about 2,793,100,000 miles (4,495,060,000
kilometers). Neptune goes around the sun once
about every 165 Earth years, compared with once
a year for Earth. As Neptune orbits the sun, it
spins on its axis, an imaginary line through its
center. Neptune's axis is not perpendicular (at an
angle of 90 degrees) to the planet's path around
the sun. The axis tilts about 28 degrees from the
perpendicular position. Neptune spins around
once in about 16 hours and 7 minutes
Mars
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Along the equator lies one of the most striking features on the planet, a system of canyons
known as the Valleys Mariners. The name is Latin for Valleys of Mariner; a space probe called
Mariner 9 discovered the canyons in 1971. The canyons run roughly east-west for about 2,500
miles (4,000 kilometers), which is close to the width of Australia or the distance from
Philadelphia to San Diego. Scientists believe that the Valleys Mariners formed mostly by rifting,
a splitting of the crust due to being stretched.
Individual canyons of the Valleys Mariners are as much as 60 miles (100 kilometers) wide. The
canyons merge in the central part of the system, in a region that is as much as 370 miles (600
kilometers) wide. The depth of the canyons is enormous, reaching 5 to 6 miles (8 to 10
kilometers) in some places.
Large channels emerge from the eastern end of the canyons, and some parts of the canyons
have layered sediments. The channels and sediments indicate that the canyons may once have
been partly filled with water.
Volcanoes
Mars has the largest volcanoes in the solar system. The tallest one, Olympus Mons (Latin for
Mount Olympus), rises 17 miles (27 kilometers) above the surrounding plains. It is about 370
miles (600 kilometers) in diameter. Three other large volcanoes, called Asia Mons, Acres Mons,
and Pagonis Mons, sit atop a broad uplifted region called Thyrsus.
All these volcanoes have slopes that rise gradually, much like the slopes of Hawaiian
volcanoes. Both the Martian and Hawaiian volcanoes are shield volcanoes. They formed from
eruptions of lavas that can flow for long distances before solidifying.
Jupiter
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The layers of dense clouds around Jupiter appear in a photograph of the planet taken by
the Voyager 1 space probe. The large, oval-shaped mark on the clouds is the Great Red
Spot. The spot is believed to be an intense atmospheric disturbance. Image credit: Jet
Propulsion Laboratory
Jupiter is the largest planet in the solar system. Its diameter is 88,846 miles (142,984
kilometers), more than 11 times that of Earth, and about one-tenth that of the sun. It
would take more than 1,000 Earths to fill up the volume of the giant planet. When viewed
from Earth, Jupiter appears brighter than most stars. It is usually the second brightest
planet -- after Venus.
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Jupiter is the fifth planet from the sun. Its mean (average) distance from the sun is about
483,780,000 miles (778,570,000 kilometers), more than five times Earth's distance.
Ancient astronomers named Jupiter after the king of the Roman gods.
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Astronomers have studied Jupiter with telescopes based on Earth and aboard artificial
satellites in orbit around Earth. In addition, the United States has sent six space probes
(crewless exploratory craft) to Jupiter.
Venus
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Venus: Optical View
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The second nearest planet to the Sun,
Venus is shrouded in a thick, highly
corrosive layer of clouds made up
mostly of sulfuric acid.
JPEG Image (22K); Caption and Credits
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A Very Different Atmosphere
Venus's atmosphere is very dense, with
the pressure at the planet's surface
ninety times greater than that on Earth.
Comprising 95 percent carbon dioxide,
small amounts of nitrogen and traces of
water vapor, the atmosphere exhibits a
powerful greenhouse effect. The
temperature on the planet's surface
reaches about 470 degrees Celsius (896
deg. Fahrenheit), high enough to melt
lead.
Mercury
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Mercury is the planet nearest the sun. It has a diameter of 3,032 miles (4,879 kilometers), about two-fifths of
Earth's diameter. Mercury orbits the sun at an average distance of about 36 million miles (58 million
kilometers), compared with about 93 million miles (150 million kilometers) for Earth.
Because of Mercury's size and nearness to the brightly shining sun, the planet is often hard to see from the
Earth without a telescope. At certain times of the year, Mercury can be seen low in the western sky just after
sunset. At other times, it can be seen low in the eastern sky just before sunrise.
Orbit
Mercury travels around the sun in an elliptical (oval-shaped) orbit. The planet is about 28,580,000 miles
(46,000,000 kilometers) from the sun at its closest point, and about 43,380,000 miles (69,820,000 kilometers)
from the sun at its farthest point. Mercury is about 48,000,000 miles (77,300,000 kilometers) from Earth at its
closest approach.
Mercury moves around the sun faster than any other planet. The ancient Romans named it Mercury in honor
of the swift messenger of their gods. Mercury travels about 30 miles (48 kilometers) per second, and goes
around the sun once every 88 Earth days. The Earth goes around the sun once every 365 days, or one year.
Rotation
As Mercury moves around the sun, it rotates on its axis, an imaginary line that runs through its center. The
planet rotates once about every 59 Earth days -- a rotation slower than that of any other planet except Venus.
As a result of the planet's slow rotation on its axis and rapid movement around the sun, a day on Mercury -that is, the interval between one sunrise and the next -- lasts 176 Earth days.
Until the mid-1960's, astronomers believed that Mercury rotated once every 88 Earth days, the same time the
planet takes to go around the sun. If Mercury did this, one side of the planet would always face the sun, and
the other side would always be dark. However, radar studies conducted in 1965 showed that the planet rotates
once in about 59 days.
Phases
When viewed through a telescope, Mercury can be seen going through "changes" in shape and size. These
apparent changes are called phases, and resemble those of the moon. They result from different parts of
Mercury's sunlit side being visible from the Earth at different times
Saturn
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Saturn is the second largest planet. Only Jupiter is larger. Saturn has seven thin, flat rings around it. The rings consist
of numerous narrow ringlets, which are made up of ice particles that travel around the planet. The gleaming rings make
Saturn one of the most beautiful objects in the solar system. Jupiter, Neptune, and Uranus are the only other planets
known to have rings. Their rings are much fainter than those around Saturn. Saturn's diameter at its equator is
about 74,900 miles (120,540 kilometers), almost 10 times that of Earth. The planet can be seen from Earth with
the unaided eye, but its rings cannot. Saturn was the farthest planet from Earth that the ancient astronomers
knew about. They named it for the Roman god of agriculture.
Saturn travels around the sun in an elliptical (oval-shaped) orbit. Its distance from the sun varies from about
941,070,000 miles (1,514,500,000 kilometers) at its farthest point to about 840,440,000 miles (1,352,550,000
kilometers) at its closest point. The planet takes about 10,759 Earth days, or about 29 1/2 Earth years, to go
around the sun, compared with 365 days, or one year, for Earth.
Rotation
As Saturn travels around the sun, it spins on its axis, an imaginary line drawn through its center. Saturn's axis
is not perpendicular (at an angle of 90 degrees) to the planet's path around the sun. The axis tilts at an angle
of about 27 degrees from the perpendicular position.
Saturn rotates faster than any other planet except Jupiter. Saturn spins around once in only 10 hours 39
minutes, compared to about 24 hours, or one day, for Earth. The rapid rotation of Saturn causes the planet to
bulge at its equator and flatten at its poles. The planet's diameter is 8,000 miles (13,000 kilometers) larger at
the equator than between the poles.
Surface and atmosphere
Most scientists believe Saturn is a giant ball of gas that has no solid surface. However, the planet seems to
have a hot solid inner core of iron and rocky material. Around this dense central part is an outer core that
probably consists of ammonia, methane, and water. A layer of highly compressed, liquid metallic hydrogen
surrounds the outer core. Above this layer lies a region composed of hydrogen and helium in a viscous
(syruplike) form. The hydrogen and helium become gaseous near the planet's surface and merge with its
atmosphere, which consists chiefly of the same two elements.