1. Armstrong, Neil
Neil A. Armstrong was an American astronaut. He was the first person to set foot on the
moon.
Neil A. Armstrong was an American astronaut. He was the first person to set foot on the
moon. Image credit: NASA
Born in 1930, Neil A. Armstrong, a United States astronaut, was the first person to set
foot on the moon. On July 20, 1969, Armstrong and Buzz Aldrin landed the Apollo 11
lunar module Eagle on the moon. Armstrong left the module and explored the lunar
surface. Upon taking his first step onto the moon, he said: "That's one small step for a
man, one giant leap for mankind." But the word a was lost in radio transmission.
Armstrong was born on Aug. 5, 1930, on his grandparents' farm in Auglaize County,
Ohio. He moved with his family to several Ohio communities before they settled in
Wapakoneta when Neil was 13 years old. Armstrong developed an interest in flying at an
early age. His love of airplanes grew when he went for his first plane ride in a Ford Tri-
Motor, a "Tin Goose," at the age of 6. From then on, he was fascinated by aviation.
In 1947, Armstrong entered Purdue University. He began studies in aeronautical
engineering. But in 1949, the United States Navy called him to active duty. Armstrong
became a Navy pilot and was sent to Korea in 1950, near the start of the Korean War. In
Korea, he flew 78 combat missions in Navy Panther jets.
In 1952, Armstrong returned to Purdue. He earned a bachelor's degree in aeronautical
engineering there in 1955.
Armstrong was a civilian test pilot assigned to test the X-15 rocket airplane before
becoming an astronaut in 1962. He made his first space flight in 1966 on Gemini 8 with
David R. Scott. The two men performed the first successful docking of two vehicles in
space -- the Gemini 8 and an uninhabited Agena rocket.
Armstrong resigned from the United States astronaut program in 1970. Also in 1970, he
earned a master's degree in aerospace engineering at the University of Southern
California. From 1971 to 1979, Armstrong was a professor of aerospace engineering at
the University of Cincinnati. In 1986, he was named vice chairman of a presidential
commission investigating the breakup of the space shuttle Challenger. From 1982 to
1992, Armstrong served as chairman of the board of Computing Technologies for
Aviation, a company that develops software for flight scheduling.
Aurora
An aurora is a natural display of light in the sky that can be seen with the unaided eye
only at night. An auroral display in the Northern Hemisphere is called the aurora borealis,

2. or the northern lights. A similar phenomenon in the Southern Hemisphere is called the
aurora australis. Auroras are the most visible effect of the sun's activity on the earth's
atmosphere.
Most auroras occur in far northern and southern regions. They appear chiefly as arcs,
clouds, and streaks. Some move, brighten, or
flicker suddenly. The most common color in an
aurora is green. But displays that occur extremely
high in the sky may be red or purple. Most
auroras occur about 60 to 620 miles (97 to 1,000
kilometers) above the earth. Some extend
lengthwise across the sky for thousands of miles
or kilometers.
A bar magnet has a magnetic field like
Auroral displays are associated with the solar that of the sun. Field lines, which
wind, a continuous flow of electrically charged represent the field, exit the north pole
particles from the sun. When these particles reach and enter the south pole. Image credit:
the earth's magnetic field, some get trapped. World Book diagram by Precision
Many of these particles travel toward the earth's Graphics
magnetic poles. When the charged particles strike
atoms and molecules in the atmosphere, energy is released. Some of this energy appears
in the form of auroras.
Auroras occur most frequently during the most intense phase of the 11-year sunspot
cycle. During this phase, dark patches on the sun's surface, called sunspots, increase in
number. Violent eruptions on the sun's surface, known as solar flares, are associated with
sunspots. Electrons and protons released by solar flares add to the number of solar
particles that interact with the earth's atmosphere. This increased interaction produces
extremely bright auroras. It also results in sharp variations in the earth's magnetic field
called magnetic storms. During these storms, auroras may shift from the polar regions
toward the equator.

3. Comet
A comet (KOM iht) is an icy body that releases gas
or dust. Most of the comets that can be seen from
Earth travel around the sun in long, oval orbits. A
comet consists of a solid nucleus (core) surrounded
by a cloudy atmosphere called the coma and one or
two tails. Most comets are too small or too faint to
be seen without a telescope. Some comets,
however, become visible to the unaided eye for
several weeks as they pass close to the sun. We can
see comets because the gas and dust in their comas
and tails reflect sunlight. Also, the gases release Halley's Comet becomes visible to
energy absorbed from the sun, causing them to the unaided eye about every 76 years
glow. as it nears the sun. Image credit:
Lick Observatory
Astronomers classify comets according to how long they take to orbit the sun. Short-
period comets need less than 200 years to complete one orbit, while long-period comets
take 200 years or longer.
Astronomers believe that comets are leftover debris from a collection of gas, ice, rocks,
and dust that formed the outer planets about 4.6 billion years ago. Some scientists believe
that comets originally brought to Earth some of the water and the carbon-based molecules
that make up living things.
Parts of a comet
The nucleus of a comet is a ball of ice and rocky dust particles that resembles a dirty
snowball. The ice consists mainly of frozen water but may include other frozen
substances, such as ammonia, carbon dioxide, carbon monoxide, and methane. Scientists
believe the nucleus of some comets may be fragile because several comets have split
apart for no apparent reason.
As a comet nears the inner solar system, heat from the sun vaporizes some of the ice on
the surface of the nucleus, spewing gas and dust particles into space. This gas and dust
forms the comet's coma. Radiation from the sun pushes dust particles away from the
coma. These particles form a tail called the dust tail. At the same time, the solar wind --
that is, the flow of high-speed electrically charged particles from the sun-converts some
of the comet's gases into ions (charged particles). These ions also stream away from the

4. coma, forming an ion tail. Because comet
tails are pushed by solar radiation and the
solar wind, they always point away from the
sun.
Most comets are thought to have a nucleus
that measures about 10 miles (16 kilometers)
or less across. Some comas can reach
diameters of nearly 1 million miles (1.6
million kilometers). Some tails extend to
distances of 100 million miles (160 million
kilometers).
The life of a comet
Comets that pass near the sun come from
Scientists think that short-period comets two groups of comets near the outer edge of
come from a band of objects called the the solar system, according to astronomers.
Kuiper belt, which lies beyond the orbit of The disk-shaped Kuiper belt contributes
Pluto. The gravitational pull of the outer comets that orbit the sun in fewer than 200
planets can nudge objects out of the Kuiper years. The Kuiper belt lies beyond Pluto's
belt and into the inner solar system, where orbit, which extends to about 4.6 billion
they become active comets. Long-period miles (7.4 billion kilometers) from the sun.
comets come from the Oort cloud, a nearly The Oort cloud provides comets that take
spherical collection of icy bodies about longer to complete their orbits. The outer
1,000 times farther away from the sun than edge of the Oort cloud may be 1,000 times
Pluto's orbit. Gravitational interactions with farther than the orbit of Pluto. Image credit:
passing stars can cause icy bodies in the World Book diagram by Terry Hadler,
Oort cloud to enter the inner solar system Bernard Thornton Artists
and become active comets.
Comets lose ice and dust each time they return to the inner solar system, leaving behind
trails of dusty debris. When Earth passes through one of these trails, the debris become
meteors that burn up in the atmosphere. Eventually, some comets lose all their ices. They
break up and dissipate into clouds of dust or turn into fragile, inactive objects similar to
asteroids.
The long, oval-shaped orbits of comets can cross the almost circular orbits of the planets.
As a result, comets sometimes collide with planets and their satellites. Many of the
impact craters in the solar system were caused by collisions with comets.
Studying comets
Scientists learned much about comets by studying Halley's Comet as it passed near Earth
in 1986. Five spacecraft flew past the comet and gathered information about its
appearance and chemical composition. Several probes flew close enough to study the

5. nucleus, which is normally concealed by the comet's coma. The spacecraft found a
roughly potato-shaped nucleus measuring about 9
miles (15 kilometers) long. The nucleus contains
equal amounts of ice and dust. About 80 percent
of the ice is water ice, and frozen carbon
monoxide makes up another 15 percent. Much of
the remainder is frozen carbon dioxide, methane,
and ammonia. Scientists believe that other comets
are chemically similar to Halley's Comet.
Scientists unexpectedly found the nucleus of
Halley's Comet to be extremely dark black. They
The space probe Giotto passed near
now believe that the surface of the comet, and
Halley's Comet on March 14, 1986.
perhaps most other comets, is covered with a
Giotto returned dramatic close-up
black crust of dust and rock that covers most of
images of the comet, including this
the ice. These comets release gas only when holes
one. Image credit: European Space
in this crust rotate toward the sun, exposing the
Agency
interior ice to the warming sunlight.
Another comet nucleus that has been seen by spacecraft cameras is that of Comet
Borrelly. During a flyby in 2001, the Deep Space 1 spacecraft observed a nucleus about
half the size of the nucleus of Halley's Comet. Borrelly's nucleus was also potato-shaped
and had a dark black surface. Like Halley's Comet, Comet Borrelly only released gas
from small areas where holes in the crust exposed the ice to sunlight.
In 1994, astronomers observed a comet named Shoemaker-Levy 9, which had split into
more than two dozen pieces, crashing into the planet Jupiter. One of the most active
comets seen in more than 400 years was Comet Hale-Bopp, which came within 122
million miles (197 million kilometers) of Earth in 1997. This was not an especially close
approach for a comet. However, Hale-Bopp appeared bright to the unaided eye because
its unusually large nucleus gave off a great deal of dust and gas. The nucleus was
estimated to be about 18 to 25 miles (30 to 40 kilometers) across.
In 2004, the U.S. spacecraft Stardust passed near the nucleus of Comet Wild 2 and
gathered samples from the comet's coma. Stardust was scheduled to return the samples to
Earth in 2006. Also in 2004, the European Space Agency launched the Rosetta
spacecraft, which was to go into orbit around Comet Churyumov-Gerasimenko in 2014.
Rosetta carried a small probe designed to land on the comet's nucleus.

6. Europa
Europa, (yu ROH puh), is a large moon of Jupiter. Its
surface is made of ice, which may have an ocean of water
beneath it. Such an ocean could provide a home for living
things. The surface layer of ice or ice and water is 50 to
The surface of Europa, a moon
100 miles (80 to 160 kilometers) deep. The satellite has
of Jupiter, consists mostly of
an extremely thin atmosphere. Electrically charged
huge blocks of ice that have
particles from Jupiter's radiation belts continuously
cracked and shifted about,
bombard Europa.
suggesting that there may be
an ocean of liquid water
Europa is one of the smoothest bodies in the solar system.
underneath. Image credit:
Its surface features include shallow cracks, valleys,
NASA
ridges, pits, blisters, and icy flows. None of them extend
more than a few hundred yards or meters upward or downward. In some places, huge
sections of the surface have split apart and separated. The surface of Europa has few
impact craters (pits caused by collisions with asteroids or comets). The splitting and
shifting of the surface and disruptions from below have destroyed most of the old craters.
Europa's interior is hotter than its surface. This internal heat comes from the gravitational
forces of Jupiter and Jupiter's other large satellites, which pull Europa's interior in
different directions. As a result, the interior flexes, producing heat in a process known as
tidal heating. The core of Europa may be rich in iron, but most of the satellite is made of
rock.
Europa's diameter is 1,940 miles (3,122 kilometers), slightly smaller than Earth's moon.
Europa takes 3.55 days to orbit Jupiter at a distance of 416,900 miles (670,900
kilometers). The Italian astronomer Galileo discovered Europa in 1610. Much of what is
known about it comes from data gathered by a space probe, also named Galileo, that
orbited Jupiter from 1995 to 2003.

7. Global Warming
Global warming is an increase in the average temperature of Earth's surface. Since the
late 1800's, the global average temperature has increased about 0.7 to 1.4 degrees F (0.4
to 0.8 degrees C). Many experts estimate that the average temperature will rise an
additional 2.5 to 10.4 degrees F (1.4 to 5.8 degrees C) by 2100. That rate of increase
would be much larger than most past rates of increase.
Scientists worry that human societies and natural ecosystems might not adapt to rapid
climate changes. An ecosystem consists of the living organisms and physical
environment in a particular area. Global warming could cause much harm, so countries
throughout the world drafted an agreement called the Kyoto Protocol to help limit it.
Causes of global warming
Climatologists (scientists who study climate) have analyzed the global warming that has
occurred since the late 1800's. A majority of climatologists have concluded that human
activities are responsible for most of the warming. Human activities contribute to global
warming by enhancing Earth's natural greenhouse effect. The greenhouse effect warms
Earth's surface through a complex process involving sunlight, gases, and particles in the
atmosphere. Gases that trap heat in the atmosphere are known as greenhouse gases.
The main human activities that contribute to global warming are the burning of fossil
fuels (coal, oil, and natural gas) and the clearing of land. Most of the burning occurs in
automobiles, in factories, and in electric power plants that provide energy for houses and
office buildings. The burning of fossil fuels creates carbon dioxide, whose chemical
formula is CO2. CO2 is a greenhouse gas that slows the escape of heat into space. Trees
and other plants remove CO2 from the air during photosynthesis, the process they use to
produce food. The clearing of land contributes to the buildup of CO2 by reducing the rate
at which the gas is removed from the atmosphere or by the decomposition of dead
vegetation.
A small number of scientists argue that the increase in greenhouse gases has not made a
measurable difference in the temperature. They say that natural processes could have
caused global warming. Those processes include increases in the energy emitted (given
off) by the sun. But the vast majority of climatologists believe that increases in the sun's
energy have contributed only slightly to recent warming.
The impact of global warming

8. Continued global warming could have many
damaging effects. It might harm plants and
animals that live in the sea. It could also force
animals and plants on land to move to new
habitats. Weather patterns could change, causing
flooding, drought, and an increase in damaging
storms. Global warming could melt enough polar
ice to raise the sea level. In certain parts of the
world, human disease could spread, and crop Thousands of icebergs float off the
yields could decline. coast of the Antarctic Peninsula after
1,250 square miles (3,240 square
Harm to ocean life kilometers) of the Larsen B ice shelf
disintegrated in 2002. The area of the
Through global warming, the surface waters of ice was larger than the state of Rhode
the oceans could become warmer, increasing the Island or the nation of Luxembourg.
stress on ocean ecosystems, such as coral reefs. Antarctic ice shelves have been
High water temperatures can cause a damaging shrinking since the early 1970's
process called coral bleaching. When corals because of climate warming in the
bleach, they expel the algae that give them their region. Image credit: NASA/Earth
color and nourishment. The corals turn white and, Observatory
unless the water temperature cools, they die.
Added warmth also helps spread diseases that affect sea creatures.
Changes of habitat
Widespread shifts might occur in the natural habitats of animals and plants. Many species
would have difficulty surviving in the regions they now inhabit. For example, many
flowering plants will not bloom without a sufficient period of winter cold. And human
occupation has altered the landscape in ways that would make new habitats hard to reach
or unavailable altogether.
Weather damage
Extreme weather conditions might become more frequent and therefore more damaging.
Changes in rainfall patterns could increase both flooding and drought in some areas.
More hurricanes and other tropical storms might occur, and they could become more
powerful.
Rising sea level
Continued global warming might, over centuries, melt large amounts of ice from a vast
sheet that covers most of West Antarctica. As a result, the sea level would rise throughout
the world. Many coastal areas would experience flooding, erosion, a loss of wetlands, and
an entry of seawater into freshwater areas. High sea levels would submerge some coastal
cities, small island nations, and other inhabited regions.

9. Threats to human health
Tropical diseases, such as malaria and dengue, might spread to larger regions. Longer-
lasting and more intense heat waves could cause more deaths and illnesses. Floods and
droughts could increase hunger and malnutrition.
Changes in crop yields
Canada and parts of Russia might benefit from an increase in crop yields. But any
increases in yields could be more than offset by decreases caused by drought and higher
temperatures -- particularly if the amount of warming were more than a few degrees
Celsius. Yields in the tropics might fall disastrously because temperatures there are
already almost as high as many crop plants can tolerate.
Limited global warming
Climatologists are studying ways to limit global warming. Two key methods would be
(1) limiting CO2 emissions and (2) carbon sequestration -- either preventing carbon
dioxide from entering the atmosphere or removing CO2 already there.
Limiting CO2 emissions
Two effective techniques for limiting CO2 emissions would be (1) to replace fossil fuels
with energy sources that do not emit CO2, and (2) to use fossil fuels more efficiently.
Alternative energy sources that do not emit CO2 include the wind, sunlight, nuclear
energy, and underground steam. Devices known as wind turbines can convert wind
energy to electric energy. Solar cells can convert sunlight to electric energy, and various
devices can convert solar energy to useful heat. Geothermal power plants convert energy
in underground steam to electric energy.
Alternative sources of energy are more expensive to use than fossil fuels. However,
increased research into their use would almost certainly reduce their cost.
Carbon sequestration could take two forms: (1) underground or underwater storage and
(2) storage in living plants.
Underground or underwater storage would involve injecting industrial emissions of CO2
into underground geologic formations or the ocean. Suitable underground formations
include natural reservoirs of oil and gas from which most of the oil or gas has been
removed. Pumping CO2 into a reservoir would have the added benefit of making it easier
to remove the remaining oil or gas. The value of that product could offset the cost of
sequestration. Deep deposits of salt or coal could also be suitable.

10. The oceans could store much CO2. However, scientists have not yet determined the
environmental impacts of using the ocean for carbon sequestration.
Storage in living plants
Green plants absorb CO2 from the atmosphere as they grow. They combine carbon from
CO2 with hydrogen to make simple sugars, which they store in their tissues. After plants
die, their bodies decay and release CO2. Ecosystems with abundant plant life, such as
forests and even cropland, could tie up much carbon. However, future generations of
people would have to keep the ecosystems intact. Otherwise, the sequestered carbon
would re-enter the atmosphere as CO2.
Agreement on global warming
Delegates from more than 160 countries met in Kyoto, Japan, in 1997 to draft the
agreement that became known as the Kyoto Protocol. That agreement calls for decreases
in the emissions of greenhouse gases.
Emissions targets
Thirty-eight industrialized nations would have to restrict their emissions of CO2 and five
other greenhouse gases. The restrictions would occur from 2008 through 2012. Different
countries would have different emissions targets. As a whole, the 38 countries would
restrict their emissions to a yearly average of about 95 percent of their 1990 emissions.
The agreement does not place restrictions on developing countries. But it encourages the
industrialized nations to cooperate in helping developing countries limit emissions
voluntarily.
Industrialized nations could also buy or sell emission reduction units. Suppose an
industrialized nation cut its emissions more than was required by the agreement. That
country could sell other industrialized nations emission reduction units allowing those
nations to emit the amount equal to the excess it had cut.
Several other programs could also help an industrialized nation earn credit toward its
target. For example, the nation might help a developing country reduce emissions by
replacing fossil fuels in some applications.
Approving the agreement
The protocol would take effect as a treaty if (1) at least 55 countries ratified (formally
approved) it, and (2) the industrialized countries ratifying the protocol had CO2
emissions in 1990 that equaled at least 55 percent of the emissions of all 38 industrialized
countries in 1990.

11. In 2001, the United States rejected the Kyoto Protocol. President George W. Bush said
that the agreement could harm the U.S. economy. But he declared that the United States
would work with other countries to limit global warming. Other countries, most notably
the members of the European Union, agreed to continue with the agreement without
United States participation.
By 2004, more than 100 countries, including nearly all the countries classified as
industrialized under the protocol, had ratified the agreement. However, the agreement
required ratification by Russia or the United States to go into effect. Russia ratified the
protocol in November 2004. The treaty was to come into force in February 2005.
Analyzing global warming
Scientists use information from several sources to analyze global warming that occurred
before people began to use thermometers. Those sources include tree rings, cores
(cylindrical samples) of ice drilled from Antarctica and Greenland, and cores drilled out
of sediments in oceans. Information from these sources indicates that the temperature
increase of the 1900's was probably the largest in the last 1,000 years.
Computers help climatologists analyze past climate changes and predict future changes.
First, a scientist programs a computer with a set of mathematical equations known as a
climate model. The equations describe how various factors, such as the amount of CO2 in
the atmosphere, affect the temperature of Earth's surface. Next, the scientist enters data
representing the values of those factors at a certain time. He or she then runs the program,
and the computer describes how the temperature would vary. A computer's representation
of changing climatic conditions is known as a climate simulation.
In 2001, the Intergovernmental Panel on Climate Change (IPCC), a group sponsored by
the United Nations (UN), published results of climate simulations in a report on global
warming. Climatologists used three simulations to determine whether natural variations
in climate produced the warming of the past 100 years. The first simulation took into
account both natural processes and human activities that affect the climate. The second
simulation took into account only the natural processes, and the third only the human
activities.
The climatologists then compared the temperatures predicted by the three simulations
with the actual temperatures recorded by thermometers. Only the first simulation, which
took into account both natural processes and human activities, produced results that
corresponded closely to the recorded temperatures.
The IPCC also published results of simulations that predicted temperatures until 2100.
The different simulations took into account the same natural processes but different
patterns of human activity. For example, scenarios differed in the amounts of CO2 that
would enter the atmosphere due to human activities.

12. The simulations showed that there can be no "quick fix" to the problem of global
warming. Even if all emissions of greenhouse gases were to cease immediately, the
temperature would continue to increase after 2100 because of the greenhouse gases
already in the atmosphere.
Hurricane
A hurricane is a powerful, swirling storm that
begins over a warm sea. Hurricanes form in
waters near the equator, and then they move
toward the poles.
The winds of a hurricane swirl around a calm
central zone called the eye surrounded by a band Hurricane winds swirl about the eye, a
of tall, dark clouds called the eyewall. The eye is calm area in the center of the storm.
usually 10 to 40 miles (16 to 64 kilometers) in The main mass of clouds shown in this
diameter and is free of rain and large clouds. In photograph measures almost 250 miles
the eyewall, large changes in pressure create the (400 kilometers) across. The
hurricane's strongest winds. These winds can hurricane, named Andrew, struck the
reach nearly 200 miles (320 kilometers) per hour. Bahamas, Florida, and Louisiana in
Damaging winds may extend 250 miles (400 1992, killing 65 people and causing
kilometers) from the eye. billions of dollars in damage. Image
credit: NASA
Hurricanes are referred to by different labels, depending on where they occur. They are
called hurricanes when they happen over the North Atlantic Ocean, the Caribbean Sea,
the Gulf of Mexico, or the Northeast Pacific Ocean. Such storms are known as typhoons
if they occur in the Northwest Pacific Ocean, west of an imaginary line called the
International Date Line. Near Australia and in the Indian Ocean, they are referred to as
tropical cyclones.
Hurricanes are most common during the summer and early fall. In the Atlantic and the
Northeast Pacific, for example, August and September are the peak hurricane months.
Typhoons occur throughout the year in the Northwest Pacific but are most frequent in
summer. In the North Indian Ocean, tropical cyclones strike in May and November. In
the South Indian Ocean, the South Pacific Ocean, and off the coast of Australia, the
hurricane season runs from December to March. Approximately 85 hurricanes, typhoons,
and tropical cyclones occur in a year throughout the world. In the rest of this article, the
term hurricane refers to all such storms.
Hurricane conditions
Hurricanes require a special set of conditions, including ample heat and moisture, that
exist primarily over warm tropical oceans. For a hurricane to form, there must be a warm

13. layer of water at the top of the sea with a surface temperature greater than 80 degrees F
(26.5 degrees C).
Warm seawater evaporates and is absorbed by the surrounding air. The warmer the
ocean, the more water evaporates. The warm, moist air rises, lowering the atmospheric
pressure of the air beneath. In any area of low atmospheric pressure, the column of air
that extends from the surface of the water -- or land -- to the top of the atmosphere is
relatively less dense and therefore weighs relatively less.
Air tends to move from areas of high pressure to areas of low pressure, creating wind. In
the Northern Hemisphere, the earth's rotation causes the wind to swirl into a low-pressure
area in a counterclockwise direction. In the Southern Hemisphere, the winds rotate
clockwise around a low. This effect of the rotating earth on wind flow is called the
Coriolis effect. The Coriolis effect increases in intensity farther from the equator. To
produce a hurricane, a low-pressure area must be more than 5 degrees of latitude north or
south of the equator. Hurricanes seldom occur closer to the equator.
For a hurricane to develop, there must be little wind shear -- that is, little difference in
speed and direction between winds at upper and lower elevations. Uniform winds enable
the warm inner core of the storm to stay intact. The storm would break up if the winds at
higher elevations increased markedly in speed, changed direction, or both. The wind
shear would disrupt the budding hurricane by tipping it over or by blowing the top of the
storm in one direction while the bottom moved in another direction.
The life of a hurricane
Meteorologists (scientists who study weather) divide the life of a hurricane into four
stages: (1) tropical disturbance, (2) tropical depression, (3) tropical storm, and (4)
hurricane.
Tropical disturbance is an area where rain clouds are building. The clouds form when
moist air rises and becomes cooler. Cool air cannot hold as much water vapor as warm air
can, and the excess water changes into tiny droplets of water that form clouds. The clouds
in a tropical disturbance may rise to great heights, forming the towering thunderclouds
that meteorologists call cumulonimbus clouds.
Cumulonimbus clouds usually produce heavy rains that end after an hour or two, and the
weather clears rapidly. If conditions are right for a hurricane, however, there is so much
heat energy and moisture in the atmosphere that new cumulonimbus clouds continually
form from rising moist air.
Tropical depression is a low-pressure area surrounded by winds that have begun to blow
in a circular pattern. A meteorologist considers a depression to exist when there is low
pressure over a large enough area to be plotted on a weather map. On a map of surface
pressure, such a depression appears as one or two circular isobars (lines of equal

14. pressure) over a tropical ocean. The low pressure near the ocean surface draws in warm,
moist air, which feeds more thunderstorms.
The winds swirl slowly around the low-pressure area at first. As the pressure becomes
even lower, more warm, moist air is drawn in, and the winds blow faster.
Tropical storm
When the winds exceed 38 miles (61 kilometers) per hour, a tropical storm has
developed. Viewed from above, the storm clouds now have a well-defined circular shape.
The seas have become so rough that ships must steer clear of the area. The strong winds
near the surface of the ocean draw more and more heat and water vapor from the sea. The
increased warmth and moisture in the air feed the storm.
A tropical storm has a column of warm air near its center. The warmer this column
becomes, the more the pressure at the surface falls. The falling pressure, in turn, draws
more air into the storm. As more air is pulled into the storm, the winds blow harder.
Each tropical storm receives a name. The names help meteorologists and disaster
planners avoid confusion and quickly convey information about the behavior of a storm.
The World Meteorological Organization (WMO), an agency of the United Nations, issues
four alphabetical lists of names, one for the North Atlantic Ocean and the Caribbean Sea,
and one each for the Eastern, Central, and Northwestern Pacific. The lists include both
men's and women's names that are popular in countries affected by the storms.
Except in the Northwestern and Central Pacific, the first storm of the year gets a name
beginning with A -- such as Tropical Storm Alberto. If the storm intensifies into a
hurricane, it becomes Hurricane Alberto. The second storm gets a name beginning with
B, and so on through the alphabet. The lists do not use all the letters of the alphabet,
however, since there are few names beginning with such letters as Q or U. For example,
no Atlantic or Caribbean storms receive names beginning with Q, U, X, Y, or Z.
Because storms in the Northwestern Pacific occur throughout the year, the names run
through the entire alphabet instead of starting over each year. The first typhoon of the
year might be Typhoon Nona, for example. The Central Pacific usually has fewer than
five named storms each year.
The system of naming storms has changed since 1950. Before that year, there was no
formal system. Storms commonly received women's names and names of saints of both
genders. From 1950 to 1952, storms were given names from the United States military
alphabet -- Able, Baker, Charlie, and so on. The WMO began to use only the names of
women in 1953. In 1979, the WMO began to use men's names as well.

15. Hurricane
A storm achieves hurricane status when its
winds exceed 74 miles (119 kilometers) per
hour. By the time a storm reaches hurricane
intensity, it usually has a well-developed eye
at its center. Surface pressure drops to its Hurricane winds on the ocean surface swirl
lowest in the eye. counterclockwise around a calm eye in the
Northern Hemisphere. Image credit: World
In the eyewall, warm air spirals upward, Book illustrations by Bruce Kerr
creating the hurricane's strongest winds. The
speed of the winds in the eyewall is related to the diameter of the eye. Just as ice skaters
spin faster when they pull their arms in, a hurricane's winds blow faster if its eye is small.
If the eye widens, the winds decrease.
Heavy rains fall from the eyewall and bands of dense clouds that swirl around the
eyewall. These bands, called rainbands, can produce more than 2 inches (5 centimeters)
of rain per hour. The hurricane draws large amounts of heat and moisture from the sea.
The path of a hurricane
Hurricanes last an average of 3 to 14 days. A long-lived storm may wander 3,000 to
4,000 miles (4,800 to 6,400 kilometers), typically moving over the sea at speeds of 10 to
20 miles (16 to 32 kilometers) per hour.
Hurricanes in the Northern Hemisphere usually begin by traveling from east to west. As
the storms approach the coast of North America or Asia, however, they shift to a more
northerly direction. Most hurricanes turn gradually northwest, north, and finally
northeast. In the Southern Hemisphere, the storms may travel westward at first and then
turn southwest, south, and finally southeast. The path of an individual hurricane is
irregular and often difficult to predict.
All hurricanes eventually move toward higher latitudes where there is colder air, less
moisture, and greater wind shears. These conditions cause the storm to weaken and die
out. The end comes quickly if a hurricane moves over land, because it no longer receives
heat energy and moisture from warm tropical water. Heavy rains may continue, however,
even after the winds have diminished.
Hurricane damage
Hurricane damage results from wind and water. Hurricane winds can uproot trees and
tear the roofs off houses. The fierce winds also create danger from flying debris. Heavy
rains may cause flooding and mudslides.

16. The most dangerous effect of a hurricane, however, is a rapid rise in sea level called a
storm surge. A storm surge is produced when winds drive ocean waters ashore. Storm
surges are dangerous because many coastal areas are densely populated and lie only a few
feet or meters above sea level. A 1970 cyclone in East Pakistan (now Bangladesh)
produced a surge that killed about 266,000 people. A hurricane in Galveston, Texas, in
1900 produced a surge that killed about 6,000 people, the worst natural disaster in United
States history.
Hurricane watchers rate the intensity of storms on a scale called the Saffir-Simpson scale,
developed by American engineer Herbert S. Saffir and meteorologist Robert H. Simpson.
The scale designates five levels of hurricanes, ranging from Category 1, described as
weak, to Category 5, which can be devastating. Category 5 hurricanes have included
Hurricane Camille, which hit the United States in 1969; Hurricane Gilbert, which raked
the West Indies and Mexico in 1988; and Hurricane Andrew, which struck the Bahamas,
Florida, and Louisiana in 1992.
Forecasting hurricanes
Meteorologists use weather balloons, satellites, and radar to watch for areas of rapidly
falling pressure that may become hurricanes. Specially equipped airplanes called
hurricane hunters investigate budding storms.
If conditions are right for a hurricane, the National Weather Service issues a hurricane
watch. A hurricane watch advises an area that there is a good possibility of a hurricane
within 36 hours. If a hurricane watch is issued for your location, check the radio or
television often for official bulletins. A hurricane warning means that an area is in danger
of being struck by a hurricane in 24 hours or less. Keep your radio tuned to a news station
after a hurricane warning. If local authorities recommend evacuation, move quickly to a
safe area or a designated hurricane shelter.

17. Moon
Moon is Earth's only natural satellite and the only
astronomical body other than Earth ever visited by human
beings. The moon is the brightest object in the night sky The moon's surface shows
but gives off no light of its own. Instead, it reflects light striking contrasts of light and
from the sun. Like Earth and the rest of the solar system, dark. The light areas are
the moon is about 4.6 billion years old. rugged highlands. The dark
zones were partly flooded by
The moon is much smaller than Earth. The moon's lava when volcanoes erupted
average radius (distance from its center to its surface) is billions of years ago. The lava
1,079.6 miles (1,737.4 kilometers), about 27 percent of froze to form smooth rock.
the radius of Earth. Image credit: Lunar and
Planetary Institute
The moon is also much less massive than Earth. The moon has a mass (amount of matter)
of 8.10 x 1019 tons (7.35 x 1019 metric tons). Its mass in metric tons would be written
out as 735 followed by 17 zeroes. Earth is about 81 times that massive. The moon's
density (mass divided by volume) is about 3.34 grams per cubic centimeter, roughly 60
percent of Earth's density.
Because the moon has less mass than Earth, the force due to gravity at the lunar surface is
only about 1/6 of that on Earth. Thus, a person standing on the moon would feel as if his
or her weight had decreased by 5/6. And if that person dropped a rock, the rock would
fall to the surface much more slowly than the
same rock would fall to Earth.
Despite the moon's relatively weak
gravitational force, the moon is close enough
to Earth to produce tides in Earth's waters.
The average distance from the center of
Earth to the center of the moon is 238,897
miles (384,467 kilometers). That distance is
growing -- but extremely slowly. The moon
is moving away from Earth at a speed of
about 1 1/2 inches (3.8 centimeters) per year.
The distance to the moon is measured to an
The temperature at the lunar equator ranges accuracy of 5 centimeters by a laser beam
from extremely low to extremely high -- sent from Earth. The beam bounces off a
from about -280 degrees F (-173 degrees C) laser reflector placed on the moon by
at night to +260 degrees F (+127 degrees C) astronauts, and returns to Earth. Image
credit: World Book diagram by Bensen
Studios

18. in the daytime. In some deep craters near the moon's poles, the temperature is always
near -400 degrees F (-240 degrees C).
The moon has no life of any kind. Compared with Earth, it has changed little over billions
of years. On the moon, the sky is black -- even during the day -- and the stars are always
visible.
A person on Earth looking at the moon with the unaided eye can see light and dark areas
on the lunar surface. The light areas are rugged, cratered highlands known as terrae
(TEHR ee). The word terrae is Latin for lands. The highlands are the original crust of the
moon, shattered and fragmented by the impact of meteoroids, asteroids, and comets.
Many craters in the terrae exceed 25 miles (40 kilometers) in diameter. The largest is the
South Pole-Aitken Basin, which is 1,550 miles (2,500 kilometers) in diameter.
The dark areas on the moon are known as maria (MAHR ee uh). The word maria is Latin
for seas; its singular is mare (MAHR ee). The term comes from the smoothness of the
dark areas and their resemblance to bodies of water. The maria are cratered landscapes
that were partly flooded by lava when volcanoes erupted. The lava then froze, forming
rock. Since that time, meteoroid impacts have created craters in the maria.
The moon has no substantial atmosphere, but small amounts of certain gases are present
above the lunar surface. People sometimes refer to those gases as the lunar atmosphere.
This "atmosphere" can also be called an exosphere, defined as a tenuous (low-density)
zone of particles surrounding an airless body. Mercury and some asteroids also have an
exosphere.
In 1959, scientists began to explore the
moon with robot spacecraft. In that year,
the Soviet Union sent a spacecraft called
Luna 3 around the side of the moon that
faces away from Earth. Luna 3 took the
first photographs of that side of the moon.
The word luna is Latin for moon.
On July 20, 1969, the U.S. Apollo 11 The first people on the moon were U.S.
lunar module landed on the moon in the astronauts Neil A. Armstrong, who took this
first of six Apollo landings. Astronaut picture, and Buzz Aldrin, who is pictured next
Neil A. Armstrong became the first to a seismograph. A television camera and a
human being to set foot on the moon. United States flag are in the background.
Their lunar module, Eagle, stands at the right.
In the 1990's, two U.S. robot space Image credit: NASA
probes, Clementine and Lunar Prospector, detected evidence of frozen water at both of
the moon's poles. The ice came from comets that hit the moon over the last 2 billion to 3
billion years. The ice apparently has lasted in areas that are always in the shadows of

19. crater rims. Because the ice is in the shade, where the temperature is about -400 degrees F
(-240 degrees C), it has not melted and evaporated.
This article discusses Moon (The movements of the moon) (Origin and evolution of the
moon) (The exosphere of the moon) (Surface features of the moon) (The interior of the
moon) (History of moon study).
The movements of the moon
The moon moves in a variety of ways. For example, it rotates on its axis, an imaginary
line that connects its poles. The moon also orbits Earth. Different amounts of the moon's
lighted side become visible in phases because of the moon's orbit around Earth. During
events called eclipses, the moon is positioned in line with Earth and the sun. A slight
motion called libration enables us to see about 59 percent of the moon's surface at
different times.
Rotation and orbit
The moon rotates on its axis once every 29 1/2 days. That is the period from one sunrise
to the next, as seen from the lunar surface, and so it is known as a lunar day. By contrast,
Earth takes only 24 hours for one rotation.
The moon's axis of rotation, like that of Earth, is tilted. Astronomers measure axial tilt
relative to a line perpendicular to the ecliptic plane, an imaginary surface through Earth's
orbit around the sun. The tilt of Earth's axis is about 23.5 degrees from the perpendicular
and accounts for the seasons on Earth. But the tilt of the moon's axis is only about 1.5
degrees, so the moon has no seasons.
Another result of the smallness of the moon's tilt is that certain large peaks near the poles
are always in sunlight. In addition, the floors of some craters -- particularly near the south
pole -- are always in shadow.
The moon completes one orbit of Earth with respect to the stars about every 27 1/3 days,
a period known as a sidereal month. But the moon revolves around Earth once with
respect to the sun in about 29 1/2 days, a period known as a synodic month. A sidereal
month is slightly shorter than a synodic month because, as the moon revolves around
Earth, Earth is revolving around the sun. The moon needs some extra time to "catch up"
with Earth. If the moon started on its orbit from a spot between Earth and the sun, it
would return to almost the same place in about 29 1/2 days.
A synodic month equals a lunar day. As a result, the moon shows the same hemisphere --
the near side -- to Earth at all times. The other hemisphere -- the far side -- is always
turned away from Earth.

20. People sometimes mistakenly use the term dark side to refer to the far side. The moon
does have a dark side -- it is the hemisphere that is turned away from the sun. The
location of the dark side changes constantly, moving with the terminator, the dividing
line between sunlight and dark.
The lunar orbit, like the orbit of Earth, is shaped like a slightly flattened circle. The
distance between the center of Earth and the moon's center varies throughout each orbit.
At perigee (PEHR uh jee), when the moon is closest to Earth, that distance is 225,740
miles (363,300 kilometers). At apogee (AP uh jee), the farthest position, the distance is
251,970 miles (405,500 kilometers). The moon's orbit is elliptical (oval-shaped).
Phases
As the moon orbits Earth, an observer on Earth can see the moon appear to change shape.
It seems to change from a crescent to a circle and back again. The shape looks different
from one day to the next because the observer sees different parts of the moon's sunlit
surface as the moon orbits Earth. The different appearances are known as the phases of
the moon. The moon goes through a complete cycle of phases in a synodic month.
The moon has four phases: (1) new moon, (2) first quarter, (3) full moon, and (4) last
quarter. When the moon is between the sun and Earth, its sunlit side is turned away from
Earth. Astronomers call this darkened phase a new moon.
The next night after a new moon, a thin crescent of light appears along the moon's eastern
edge. The remaining portion of the moon that faces Earth is faintly visible because of
earthshine, sunlight reflected from Earth to the moon. Each night, an observer on Earth
can see more of the sunlit side as the terminator, the line between sunlight and dark,
moves westward. After about seven days, the observer can see half a full moon,
commonly called a half moon. This phase is known as the first quarter because it occurs
one quarter of the way through the synodic month. About seven days later, the moon is
on the side of Earth opposite the sun. The entire sunlit side of the moon is now visible.
This phase is called a full moon.
About seven days after a full moon, the observer again sees a half moon. This phase is
the last quarter, or third quarter. After another seven days, the moon is between Earth and
the sun, and another new moon occurs.
As the moon changes from new moon to full moon, and more and more of it becomes
visible, it is said to be waxing. As it changes from full moon to new moon, and less and
less of it can be seen, it is waning. When the moon appears smaller than a half moon, it is
called crescent. When it looks larger than a half moon, but is not yet a full moon, it is
called gibbous (GIHB uhs).
Like the sun, the moon rises in the east and sets in the west. As the moon progresses
through its phases, it rises and sets at different times. In the new moon phase, it rises with

21. the sun and travels close to the sun across the sky. Each successive day, the moon rises an
average of about 50 minutes later.
Eclipses occur when Earth, the sun, and the moon are in a straight line, or nearly so. A
lunar eclipse occurs when Earth gets directly -- or almost directly -- between the sun and
the moon, and Earth's shadow falls on the moon. A lunar eclipse can occur only during a
full moon. A solar eclipse occurs when the moon gets directly -- or almost directly --
between the sun and Earth, and the moon's shadow falls on Earth. A solar eclipse can
occur only during a new moon.
During one part of each lunar orbit, Earth is between the sun and the moon; and, during
another part of the orbit, the moon is between the sun and Earth. But in most cases, the
astronomical bodies are not aligned directly enough to cause an eclipse. Instead, Earth
casts its shadow into space above or below the moon, or the moon casts its shadow into
space above or below Earth. The shadows extend into space in that way because the
moon's orbit is tilted about 5 degrees relative to Earth's orbit around the sun.
Libration
People on Earth can sometimes see a small part of the far side of the moon. That part is
visible because of lunar libration, a slight rotation of the moon as viewed from Earth.
There are three kinds of libration: (1) libration in longitude, (2) diurnal (daily) libration,
and (3) libration in latitude. Over time, viewers can see more than 50 percent of the
moon's surface. Because of libration, about 59 percent of the lunar surface is visible from
Earth.
Libration in longitude occurs because the moon's orbit is elliptical. As the moon orbits
Earth, its speed varies according to a law
discovered in the 1600's by the German astronomer
Johannes Kepler. When the moon is relatively
close to Earth, the moon travels more rapidly than
its average speed. When the moon is relatively far
from Earth, the moon travels more slowly than
average. But the moon always rotates about its own
axis at the same rate. So when the moon is
traveling more rapidly than average, its rotation is
too slow to keep all of the near side facing Earth.
And when the moon is traveling more slowly than
average, its rotation is too rapid to keep all of the
near side facing Earth.
Diurnal libration is caused by a daily change in the
position of an observer on Earth relative to the
moon. Consider an observer who is at Earth's
equator when the moon is full. As Earth rotates Diurnal libration enables an observer
on Earth to see around one edge of
the moon, then the other, during a
single night. The libration occurs
because Earth's rotation changes the
observer's viewpoint by a distance
equal to the diameter of Earth. Image
credit: World Book illustration

22. from west to east, the observer first sees the moon when it rises at the eastern horizon and
last sees it when it sets at the western horizon. During this time, the observer's viewpoint
moves about 7,900 miles (12,700 kilometers) -- the diameter of Earth -- relative to the
moon. As a result, the moon appears to rotate slightly to the west.
While the moon is rising in the east and climbing to its highest point in the sky, the
observer can see around the western edge of the near side. As the moon descends to the
western horizon, the observer can see around the eastern edge of the near side.
Libration in latitude occurs because the moon's axis of rotation is tilted about 6 1/2
degrees relative to a line perpendicular to the moon's orbit around Earth. Thus, during
each lunar orbit, the moon's north pole tilts first toward Earth, then away from Earth.
When the lunar north pole is tilted toward Earth, people on Earth can see farther than
normal along the top of the moon. When that pole is tilted away from Earth, people on
Earth can see farther than normal along the bottom of the moon.
Origin and evolution of the moon
Scientists believe that the moon formed as a result of a collision known as the Giant
Impact or the "Big Whack." According to this idea, Earth collided with a planet-sized
object 4.6 billion years ago. As a result of the impact, a cloud of vaporized rock shot off
Earth's surface and went into orbit around Earth. The cloud cooled and condensed into a
ring of small, solid bodies, which then gathered together, forming the moon.
The rapid joining together of the small bodies released much energy as heat.
Consequently, the moon melted, creating an "ocean" of magma (melted rock).
The magma ocean slowly cooled and solidified. As it cooled, dense, iron-rich materials
sank deep into the moon. Those materials also cooled and solidified, forming the mantle,
the layer of rock beneath the crust.
As the crust formed, asteroids bombarded it heavily,
shattering and churning it. The largest impacts may have
stripped off the entire crust. Some collisions were so
powerful that they almost split the moon into pieces. One
such collision created the South Pole-Aitken Basin, one
of the largest known impact craters in the solar system.
About 4 billion to 3 billion years ago, melting occurred in A basalt rock that astronauts
the mantle, probably caused by radioactive elements deep brought to Earth from the
in the moon's interior. The resulting magma erupted as moon formed from lava that
dark, iron-rich lava, partly flooding the heavily cratered erupted from a lunar volcano.
surface. The lava cooled and solidified into rocks known Escaping gases created the
as basalts (buh SAWLTS). holes before the lava solidified
into rock. Image credit: Lunar
and Planetary Institute

23. Small eruptions may have continued until as recently as 1 billion years ago. Since that
time, only an occasional impact by an asteroid or comet has modified the surface.
Because the moon has no atmosphere to burn up meteoroids, the bombardment continues
to this day. However, it has become much less intense.
Impacts of large objects can create craters. Impacts of micrometeoroids (tiny meteoroids)
grind the surface rocks into a fine, dusty powder known as the regolith (REHG uh lihth).
Regolith overlies all the bedrock on the moon. Because regolith forms as a result of
exposure to space, the longer a rock is exposed, the thicker the regolith that forms on it.
The exosphere of the moon
The lunar exosphere -- that is, the materials surrounding the moon that make up the lunar
"atmosphere" -- consists mainly of gases that arrive as the solar wind. The solar wind is a
continuous flow of gases from the sun -- mostly hydrogen and helium, along with some
neon and argon.
The remainder of the gases in the exosphere form on the moon. A continual "rain" of
micrometeoroids heats lunar rocks, melting and vaporizing their surface. The most
common atoms in the vapor are atoms of sodium and potassium. Those elements are
present in tiny amounts -- only a few hundred atoms of each per cubic centimeter of
exosphere. In addition to vapors produced by impacts, the moon also releases some gases
from its interior.
Most gases of the exosphere concentrate about halfway between the equator and the
poles, and they are most plentiful just before sunrise. The solar wind continuously sweeps
vapor into space, but the vapor is continuously replaced.
During the night, the pressure of gases at the lunar surface is about 3.9 x 10-14 pound per
square inch (2.7 x 10-10 pascal). That is a stronger vacuum than laboratories on Earth can
usually achieve. The exosphere is so tenuous -- that is, so low in density -- that the rocket
exhaust released during each Apollo landing temporarily doubled the total mass of the
entire exosphere.
The surface of the moon is covered with bowl-shaped holes called craters, shallow
depressions called basins, and broad, flat plains known as maria. A powdery dust called
the regolith overlies much of the surface of the moon.
Craters

24. The vast majority of the moon's craters are formed by the
impact of meteoroids, asteroids, and comets. Craters on
the moon are named for famous scientists. For example, Euler Crater has central peaks
Copernicus Crater is named for Nicolaus Copernicus, a and slumped walls. The peaks
Polish astronomer who realized in the 1500's that the almost certainly formed
planets move about the sun. Archimedes Crater is named quickly after the impact that
for the Greek mathematician Archimedes, who made produced the crater
many mathematical discoveries in the 200's B.C. compressed the ground. The
ground rebounded upward,
The shape of craters varies with their size. Small craters forming the peaks. The crater
with diameters of less than 6 miles (10 kilometers) have walls are slumped because the
relatively simple bowl shapes. Slightly larger craters original walls were too steep
cannot maintain a bowl shape because the crater wall is to withstand the force of
too steep. Material falls inward from the wall to the floor. gravity. Material fell inward,
As a result, the walls become scalloped and the floor away from the walls. This
becomes flat. crater, in Mare Imbrium (Sea
of Rains), is about 17 1/2
Still larger craters have terraced walls and central peaks. miles (28 kilometers) across.
Terraces inside the rim descend like stairsteps to the Image credit: Lunar and
floor. The same process that creates wall scalloping is Planetary Institute
responsible for terraces. The central peaks almost
certainly form as did the central peaks of impact craters on Earth. Studies of the peaks on
Earth show that they result from a deformation of the ground. The impact compresses the
ground, which then rebounds, creating the peaks. Material in the central peaks of lunar
craters may come from depths as great as 12 miles (19 kilometers).
Surrounding the craters is rough, mountainous material -- crushed and broken rocks that
were ripped out of the crater cavity by shock pressure. This material, called the crater
ejecta blanket, can extend about 60 miles (100 kilometers) from the crater.
Farther out are patches of debris and, in many cases, irregular secondary craters, also
known as secondaries. Those craters come in a range of shapes and sizes, and they are
often clustered in groups or aligned in rows. Secondaries form when material thrown out
of the primary (original) crater strikes the surface. This material consists of large blocks,
clumps of loosely joined rocks, and fine sprays of ground-up rock. The material may
travel thousands of miles or kilometers.
Crater rays are light, wispy deposits of powder that can extend thousands of miles or
kilometers from the crater. Rays slowly vanish as micrometeoroid bombardment mixes
the powder into the upper surface layer. Thus, craters that still have visible rays must be
among the youngest craters on the moon.
Craters larger than about 120 miles (200 kilometers) across tend to have central
mountains. Some of them also have inner rings of peaks, in addition to the central peak.

25. The appearance of a ring signals the next major transition in crater shape -- from crater to
basin.
Basins are craters that are 190 miles (300 kilometers) or more across. The smaller basins
have only a single inner ring of peaks, but the larger ones typically have multiple rings.
The rings are concentric -- that is, they all have the same center, like the rings of a
dartboard. The spectacular, multiple-ringed basin called the Eastern Sea (Mare Orientale)
is almost 600 miles (1,000 kilometers) across. Other basins can be more than 1,200 miles
(2,000 kilometers) in diameter -- as large as the entire western United States.
Basins occur equally on the near side and far side. Most basins have little or no fill of
basalt, particularly those on the far side. The difference in filling may be related to
variations in the thickness of the crust. The far side has a thicker crust, so it is more
difficult for molten rock to reach the surface there.
In the highlands, the overlying ejecta blankets of the basins make up most of the upper
few miles or kilometers of material. Much of this material is a large, thick layer of
shattered and crushed rock known as breccia (BREHCH ee uh). Scientists can learn about
the original crust by studying tiny fragments of breccia.
Maria, the dark areas on the surface of the moon, make up about 16 percent of the surface
area. Some maria are named in Latin for weather terms -- for example, Mare Imbrium
(Sea of Rains) and Mare Nubium (Sea of Clouds). Others are named for states of mind, as
in Mare Serenitatus (Sea of Serenity) and Mare Tranquillitatis (Sea of Tranquility).
Landforms on the maria tend to be smaller than those of the highlands. The small size of
mare features relates to the scale of the processes that formed them -- volcanic eruptions
and crustal deformation, rather than large impacts.
The chief landforms on the maria include wrinkle
ridges and rilles and other volcanic features.
Wrinkle ridges are blisterlike humps that wind
across the surface of almost all maria. The ridges
are actually broad folds in the rocks, created by
compression. Many wrinkle ridges are roughly
circular, aligned with small peaks that stick up
through the maria and outlining interior rings.
Circular ridge systems also outline buried
features, such as rims of craters that existed
before the maria formed.
Rilles are snakelike depressions that wind across A lunar rover is parked near the edge
many areas of the maria. Scientists formerly of Hadley Rille, a long channel
thought the rilles might be ancient riverbeds. probably formed by lava 4 billion to 3
However, they now suspect that the rilles are billion years ago. The slopes in the
background are part of a formation
called the Swann Hills. This photo
was taken during the Apollo 15
mission in 1971. Astronaut David R.
Scott is reaching under a seat to get a
camera. Image credit: NASA

26. channels formed by running lava. One piece of evidence favoring this view is the dryness
of rock samples brought to Earth by Apollo astronauts; the samples have almost no water
in their molecular structure. In addition, detailed photographs show that the rilles are
shaped somewhat like channels created by flowing lava on Earth.
Volcanic features
Scattered throughout the maria are a variety of other features formed by volcanic
eruptions. Within Mare Imbrium, scarps (lines of cliffs) wind their way across the
surface. The scarps are lava flow fronts, places where lava solidified, enabling lava that
was still molten to pile up behind them. The presence of the scarps is one piece of
evidence indicating that the maria consist of solidified basaltic lava.
Small hills and domes with pits on top are probably little volcanoes. Both dome-shaped
and cone-shaped volcanoes cluster together in many places, as on Earth. One of the
largest concentrations of cones on the moon is the Marius Hills complex in Oceanus
Procellarum (Ocean of Storms). Within this complex are numerous wrinkle ridges and
rilles, and more than 50 volcanoes.
Large areas of maria and terrae are covered by dark material known as dark mantle
deposits. Evidence collected by the Apollo missions confirmed that dark mantling is
volcanic ash.
Much smaller dark mantles are associated with small craters that lie on the fractured
floors of large craters. Those mantles may be cinder cones -- low, broad, cone-shaped
hills formed by explosive volcanic eruptions.
The interior of the moon
The moon, like Earth, has three interior zones -- crust, mantle, and core. However, the
composition, structure, and origin of the zones on the moon are much different from
those on Earth.
Most of what scientists know about the interior of Earth and the moon has been learned
by studying seismic events -- earthquakes and moonquakes, respectively. The data on
moonquakes come from scientific equipment set up by Apollo astronauts from 1969 to
1972.
Crust
The average thickness of the lunar crust is about 43 miles (70 kilometers), compared with
about 6 miles (10 kilometers) for Earth's crust. The outermost part of the moon's crust is
broken, fractured, and jumbled as a result of the large impacts it has endured. This
shattered zone gives way to intact material below a depth of about 6 miles. The bottom of

27. the crust is defined by an abrupt increase in rock density at a depth of about 37 miles (60
kilometers) on the near side and about 50 miles (80 kilometers) on the far side.
Mantle
The mantle of the moon consists of dense rocks that are rich in iron and magnesium. The
mantle formed during the period of global melting. Low-density minerals floated to the
outer layers of the moon, while dense minerals sank deeper into it.
Later, the mantle partly melted due to a build-up of heat in the deep interior. The source
of the heat was probably the decay (breakup) of uranium and other radioactive elements.
This melting produced basaltic magmas -- bodies of molten rock. The magmas later made
their way to the surface and erupted as the mare lavas and ashes. Although mare
volcanism occurred for more than 1 billion years -- from at least 4 billion years to fewer
than 3 billion years ago -- much less than 1 percent of the volume of the mantle ever
remelted.
Core
Data gathered by Lunar Prospector confirmed that the moon has a core and enabled
scientists to estimate its size. The core has a radius of only about 250 miles (400
kilometers). By contrast, the radius of Earth's core is about 2,200 miles (3,500
kilometers).
The lunar core has less than 1 percent of the mass of the moon. Scientists suspect that the
core consists mostly of iron, and it may also contain large amounts of sulfur and other
elements.
Earth's core is made mostly of molten iron and nickel. This rapidly rotating molten core
is responsible for Earth's magnetic field. A magnetic field is an influence that a magnetic
object creates in the region around it. If the core of a planet or a satellite is molten,
motion within the core caused by the rotation of the planet or satellite makes the core
magnetic. But the small, partly molten core of the moon cannot generate a global
magnetic field. However, small regions on the lunar surface are magnetic. Scientists are
not sure how these regions acquired magnetism. Perhaps the moon once had a larger,
more molten core.
There is evidence that the lunar interior formerly contained gas, and that some gas may
still be there. Basalt from the moon contains holes called vesicles that are created during
a volcanic eruption. On Earth, gas that is dissolved in magma comes out of solution
during an eruption, much as carbon dioxide comes out of a carbonated beverage when
you shake the drink container. The presence of vesicles in lunar basalt indicates that the
deep interior contained gases, probably carbon monoxide or gaseous sulfur. The
existence of volcanic ash is further evidence of interior gas; on Earth, volcanic eruptions
are largely driven by gas.

28. History of moon study
Ancient ideas
Some ancient peoples believed that the moon was a rotating bowl of fire. Others thought
it was a mirror that reflected Earth's lands and seas. But philosophers in ancient Greece
understood that the moon is a sphere in orbit around Earth. They also knew that
moonlight is reflected sunlight.
Some Greek philosophers believed that the moon was a world much like Earth. In about
A.D. 100, Plutarch even suggested that people lived on the moon. The Greeks also
apparently believed that the dark areas of the moon were seas, while the bright regions
were land.
In about A.D. 150, Ptolemy, a Greek astronomer who lived in Alexandria, Egypt, said
that the moon was Earth's nearest neighbor in space. He thought that both the moon and
the sun orbited Earth. Ptolemy's views survived for more than 1,300 years. But by the
early 1500's, the Polish astronomer Nicolaus Copernicus had developed the correct view
-- Earth and the other planets revolve about the sun, and the moon orbits Earth.
Early observations with telescopes
The Italian astronomer and physicist Galileo wrote the first scientific description of the
moon based on observations with a telescope. In 1609, Galileo described a rough,
mountainous surface. This description was quite different from what was commonly
believed -- that the moon was smooth. Galileo noted that the light regions were rough and
hilly and the dark regions were smoother plains.
The presence of high mountains on the moon fascinated Galileo. His detailed description
of a large crater in the central highlands -- probably Albategnius -- began 350 years of
controversy and debate about the origin of the "holes" on the moon.
Other astronomers of the 1600's mapped and cataloged every surface feature they could
see. Increasingly powerful telescopes led to more detailed records. In 1645, the Dutch
engineer and astronomer Michael Florent van Langren, also known as Langrenus,
published a map that gave names to the surface features of the moon, mostly its craters. A
map drawn by the Bohemian-born Italian astronomer Anton M. S. de Rheita in 1645
correctly depicted the bright ray systems of the craters Tycho and Copernicus. Another
effort, by the Polish astronomer Johannes Hevelius in 1647, included the moon's libration
zones.
By 1651, two Jesuit scholars from Italy, the astronomer Giovanni Battista Riccioli and
the mathematician and physicist Francesco M. Grimaldi, had completed a map of the
moon. That map established the naming system for lunar features that is still in use.

29. Determining the origin of craters
Until the late 1800's, most astronomers thought that volcanism formed the craters of the
moon. However, in the 1870's, the English astronomer Richard A. Proctor proposed
correctly that the craters result from the collision of solid objects with the moon. But at
first, few scientists accepted Proctor's proposal. Most astronomers thought that the
moon's craters must be volcanic in origin because no one had yet described a crater on
Earth as an impact crater, but scientists had found dozens of obviously volcanic craters.
In 1892, the American geologist Grove Karl Gilbert argued that most lunar craters were
impact craters. He based his arguments on the large size of some of the craters. Those
included the basins, which he was the first to recognize as huge craters. Gilbert also noted
that lunar craters have only the most general resemblance to calderas (large volcanic
craters) on Earth. Both lunar craters and calderas are large circular pits, but their
structural details do not resemble each other in any way.
In addition, Gilbert created small craters experimentally. He studied what happened when
he dropped clay balls and shot bullets into clay and sand targets.
Gilbert was the first to recognize that the circular Mare Imbrium was the site of a gigantic
impact. By examining photographs, Gilbert also determined which nearby craters formed
before and after that event. For example, a crater that is partially covered by ejecta from
the Imbrium impact formed before the impact. A crater within the mare formed after the
impact.
Describing lunar evolution
Gilbert suggested that scientists could determine the relative age of surface features by
studying the ejecta of the Imbrium impact. That suggestion was the key to unraveling the
history of the moon. Gilbert recognized that the moon is a complex body that was built
up by innumerable impacts over a long period.
In his book The Face of the Moon (1949), the American astronomer and physicist Ralph
B. Baldwin further described lunar evolution. He noted the similarity in form between
craters on the moon and bomb craters created during World War II (1939-1945) and
concluded that lunar craters form by impact.
Baldwin did not say that every lunar feature originated with an impact. He stated
correctly that the maria are solidified flows of basalt lava, similar to flood lava plateaus
on Earth. Finally, independently of Gilbert, he concluded that all circular maria are
actually huge impact craters that later filled with lava.
In the 1950's, the American chemist Harold C. Urey offered a contrasting view of lunar
history. Urey said that, because the moon appears to be cold and rigid, it has always been
so. He then stated -- correctly -- that craters are of impact origin. However, he concluded

30. falsely that the maria are blankets of debris scattered by the impacts that created the
basins. And he was mistaken in concluding that the moon never melted to any significant
extent. Urey had won the 1934 Nobel Prize in chemistry and had an outstanding scientific
reputation, so many people took his views seriously. Urey strongly favored making the
moon a focus of scientific study. Although some of his ideas were mistaken, his support
of moon study was a major factor in making the moon an early goal of the U.S. space
program.
In 1961, the U.S. geologist Eugene M. Shoemaker founded the Branch of Astrogeology
of the U.S. Geological Survey (USGS). Astrogeology is the study of celestial objects
other than Earth. Shoemaker showed that the moon's surface could be studied from a
geological perspective by recognizing a sequence of relative ages of rock units near the
crater Copernicus on the near side. Shoemaker also studied the Meteor Crater in Arizona
and documented the impact origin of this feature. In preparation for the Apollo missions
to the moon, the USGS began to map the geology of the moon using telescopes and
pictures. This work gave scientists their basic understanding of lunar evolution.
Apollo missions
Beginning in 1959, the Soviet Union and the United States sent a series of robot
spacecraft to examine the moon in detail. Their ultimate goal was to land people safely on
the moon. The United States finally reached that goal in 1969 with the landing of the
Apollo 11 lunar module. The United States conducted six more Apollo missions,
including five landings. The last of those was Apollo 17, in December 1972.
The Apollo missions revolutionized the understanding of the moon. Much of the
knowledge gained about the moon also applies to Earth and the other inner planets --
Mercury, Venus, and Mars. Scientists learned, for
example, that impact is a fundamental geological process
operating on the planets and their satellites.
After the Apollo missions, the Soviets sent four Luna
robot craft to the moon. The last, Luna 24, returned
samples of lunar soil to Earth in August 1976.
Recent exploration
The Clementine orbiter used
No more spacecraft went to the moon until January 1994, radar signals to find evidence
when the United States sent the orbiter Clementine. From of a large deposit of frozen
February to May of that year, Clementine's four cameras water on the moon. The orbiter
took more than 2 million pictures of the moon. A laser sent radar signals to various
device measured the height and depth of mountains, target points on the lunar
craters, and other features. Radar signals that Clementine surface. The targets reflected
some of the signals to Earth,
where they were received by
large antennas and analyzed.
Image credit: Lunar and
Planetary Institute

31. bounced off the moon provided evidence of a large deposit of frozen water. The ice
appeared to be inside craters at the south pole.
The U.S. probe Lunar Prospector orbited the moon from January 1998 to July 1999. The
craft mapped the concentrations of chemical elements in the moon, surveyed the moon's
magnetic fields, and found strong evidence of ice at both poles. Small particles of ice are
apparently part of the regolith at the poles.
The SMART-1 spacecraft, launched by the European Space Agency in 2003, went into
orbit around the moon in 2004. The craft's instruments were designed to investigate the
moon's origin and conduct a detailed survey of the chemical elements on the lunar
surface.
Planets
A planet is a large, round heavenly body that orbits a star
and shines with light reflected from the star. Eight planets
orbit the sun in our solar system. In order of increasing
distance from the sun, they are: (1) Mercury, (2) Venus,
(3) Earth, (4) Mars, (5) Jupiter, (6) Saturn, (7) Uranus,
and (8) Neptune. Many nearly planet-sized objects, called
dwarf planets, also orbit the sun. Dwarf planets include
The sun blazes with energy.
Pluto and Ceres. Since 1992, astronomers have
On its surface, magnetic forces
discovered many planets orbiting other stars.
create loops and streams of gas
Traditionally, the term planet has had no formal definition
that extend tens of thousands
in astronomy. Millions of objects orbit the sun—the most
of miles or kilometers into
basic characteristic of a planet. But scholars have
space. This image was made
struggled to devise a simple classification system that
by photographing ultraviolet
distinguishes the smallest worlds from the largest comets,
radiation given off by atoms of
asteroids, and other bodies.
iron gas that are hotter than 9
million degrees F (5 million
The International Astronomical Union (IAU), the
degrees C). Image credit:
recognized authority in naming heavenly bodies, divides
NASA/Transition Region &
Coronal Explorer

32. objects that orbit the sun into three major classes: (1) planets, (2) dwarf planets, and (3)
small solar system bodies. A planet orbits the sun and no other body. It has so much mass
(amount of matter) that its own gravitational pull compacts it into a round shape. In
addition, a planet has a strong enough gravitational pull to sweep the region of its orbit
relatively free of other objects. A dwarf planet also orbits the sun and is large enough to
be round. However, it does not have a strong enough gravitational pull to clear the region
of its orbit. Small solar system bodies, including most asteroids and comets, have too
little mass for gravity to round their irregular shapes. Many planets, dwarf planets, and
other bodies have smaller objects orbiting them called satellites or moons.
The planets in our solar system can be divided into two groups. The innermost four
planets—Mercury, Venus, Earth, and Mars—are
small, rocky worlds. They are called the terrestrial (earthlike) planets, from the Latin
word for Earth, terra. Earth is the largest terrestrial planet. The other Earthlike planets
have from 38 to 95 percent of Earth's diameter and from 5.5 to 82 percent of Earth's
mass.
The outer four planets—Jupiter, Saturn, Uranus, and Neptune—are called gas giants or
Jovian (Jupiterlike) planets. They have gaseous atmospheres and no solid surfaces. All
four Jovian planets consist mainly of hydrogen and helium. Smaller amounts of other
materials also occur, including traces of ammonia and methane in their atmospheres.
They range from 3.9 times to 11.2 times Earth's diameter and from 15 times to 318 times
Earth's mass. Jupiter, Saturn, and Neptune give off more energy than they receive from
the sun. Most of this extra energy takes the form of infrared radiation, which is felt as
heat, instead of visible light. Scientists think the source of some of the energy is probably
the slow compression of the planets by their own gravity.
From its discovery in 1930, Pluto was generally considered a planet. However, its small
size and irregular orbit led many astronomers to question whether Pluto should be
grouped with worlds such as Earth and Jupiter. Pluto more closely resembles other icy
objects found in a region of the outer solar system called the Kuiper belt. In the early
2000’s, astronomers found several such Kuiper belt objects (KBO’s) comparable in size
to Pluto. The IAU created the “dwarf planet” classification to describe Pluto and other
nearly planet-sized objects.
Observing the planets
People have known the inner six planets of our solar system for thousands of years
because they are visible from Earth without a telescope. The outermost three planets—
Uranus and Neptune—were discovered by astronomers, beginning in the 1780's. These
planets can be seen from Earth with a telescope.
To the unaided eye, the planets look much like the background stars in the night sky.
However, the planets move slightly from night to night in relation to the stars. The name

33. planet comes from a Greek word meaning to wander. The planets and the moon follow
the same apparent path through the sky. This path, known as the zodiac, is about 16°
wide. At its center is the ecliptic, the apparent path of the sun. If you see a bright object
near the ecliptic at night or near sunrise or sunset, it is most likely a planet. You can even
see the brightest planets in the daytime, if you know where to look.
Planets and stars also differ in the steadiness of their light when viewed from Earth's
surface. Planets shine with a steady light, but stars seem to twinkle.
The twinkling is due to the moving layers of air that surround Earth. Stars are so far away
that they are mere points of light in the sky, even when viewed through a telescope. The
atmosphere bends the starlight passing through it. As small regions of the atmosphere
move about, the points of light seem to dance and change in brightness.
Planets, which are much closer, look like tiny disks through a telescope. The atmosphere
scatters light from different points on a planet's disk. However, enough light always
arrives from a sufficient number of points to provide a steady appearance.
Orbits
Viewed from Earth's surface, the planets of the solar system and the stars appear to move
around Earth. They rise in the east and set in the west each night. Most of the time, the
planets move westward across the sky slightly more slowly than the stars do. As a result,
the planets seem to drift eastward relative to the background stars. This motion is called
prograde. For a while each year, however, the planets seem to reverse their direction.
This backward motion is called retrograde.
In ancient times, most scientists thought that the moon, sun, planets, and stars actually
moved around Earth. One puzzle that ancient scientists struggled to explain was the
annual retrograde motion of the planets. In about A.D. 150, the Greek astronomer
Ptolemy developed a theory that the planets orbited in small circles, which in turn orbited
Earth in larger circles. Ptolemy thought that retrograde motion was caused by a planet
moving on its small circle in an opposite direction from the motion of the small circle
around the big circle.
In 1543, the Polish astronomer Nicolaus Copernicus showed that the sun is the center of
the orbits of the planets. Our term solar system is based on Copernicus's discovery.
Copernicus realized that retrograde motion occurs because Earth moves faster in its orbit
than the planets that are farther from the sun. The planets that are closer to the sun move
faster in their orbits than Earth travels in its orbit. Retrograde motion occurs whenever
Earth passes an outer planet traveling around the sun or an inner planet passes Earth.
In the 1600's, the German astronomer Johannes Kepler used observations of Mars by the
Danish astronomer Tycho Brahe to figure out three laws of planetary motion. Although

34. Kepler developed his laws for the planets of our solar system, astronomers have since
realized that Kepler's laws are valid for all heavenly bodies that orbit other bodies.
Kepler's first law says that planets move in elliptical (oval-shaped) orbits around their
parent star—in our solar system, the sun. An ellipse is a closed curve formed around two
fixed points called foci. The ellipse is formed by the path of a point moving so that the
sum of its distances from the two foci remains the same. The orbital paths of the planets
form such curves, with the parent star at one focus of the ellipse. Before Kepler, scientists
had assumed that the planets moved in circular orbits.
Kepler's second law says that an imaginary line joining the parent star to its planet
sweeps across equal areas of space in equal amounts of time. When a planet is close to its
star, it moves relatively rapidly in its orbit. The line therefore sweeps out a short, fat,
trianglelike figure. When the planet is farther from its star, it moves relatively slowly. In
this case, the line sweeps out a long, thin figure that resembles a triangle. But the two
figures have equal areas.
Kepler's third law says that a planet's period (the time it takes to complete an orbit around
its star) depends on its average distance from the star. The law says that the square of the
planet's period—that is, the period multiplied by itself—is proportional to the cube of the
planet's average distance from its star—the distance multiplied by itself twice—for all
planets in a solar system.
The English scientist, astronomer, and mathematician Isaac Newton presented his theory
of gravity and explained why Kepler's laws work in a treatise published in 1687. Newton
showed how his expanded version of Kepler's third law could be used to find the mass of
the sun or of any other object around which things orbit. Using Newton's explanation,
astronomers can determine the mass of a planet by studying the period of its moon or
moons and their distance from the planet.
Rotation
Planets rotate at different rates. One day is defined as how long it takes Earth to rotate
once. Jupiter and Saturn spin much faster, in only about 10 hours. Venus rotates much
slower, in about 243 Earth days.
Most planets rotate in the same direction in which they revolve around the sun, with their
axis of rotation standing upright from their orbital path. A law of physics holds that such
rotation does not change by itself. So astronomers think that the solar system formed out
of a cloud of gas and dust that was already spinning.
Uranus is tipped on its side, however, so that its axes lies nearly level with its paths
around the sun. Venus is tipped all the way over. Its axis is almost completely upright,
but the planet rotates in the direction opposite from the direction of its revolution around

35. the sun. Most astronomers think that some other objects in the solar
system must have collided with Uranus, Pluto, and Venus and tipped
them.
The planets of our solar system
The planet
Astronomers measure distances within the solar system in Mercury was first
astronomical units (AU). One astronomical unit is the average photographed in
distance between Earth and the sun, which is about 93 million miles detail on March
(150 million kilometers). The inner planets have orbits whose 29, 1974, by the
diameters are 0.4, 0.7, 1.0, and 1.5 AU, respectively. The orbits of the U.S. probe
gas giants are much larger: 5, 10, 20, and 30 AU, respectively. Mariner 10. Image
Because of their different distances from the sun, the temperature, credit: NASA
surface features, and other conditions on the planets vary widely.
Mercury, the innermost planet, has no moon and almost no atmosphere. It orbits so close
to the sun that temperatures on its surface can climb as high as 800 degrees F (430
degrees C). But some regions near the planet's poles may be always in shadow, and
astronomers speculate that water or ice may remain there. No spacecraft has visited
Mercury since the 1970's, when Mariner 10 photographed about half the planet's surface
at close range. The Messenger spacecraft, launched in 2004, was scheduled to fly by
Mercury three times before going into orbit around the planet in 2011.
Venus is known as Earth's twin because it resembles Earth in size and
mass, though it has no moon. Venus has a dense atmosphere that
consists primarily of carbon dioxide. The pressure of the atmosphere
on Venus's surface is 90 times that of Earth's atmosphere. Venus's
thick atmosphere traps energy from the sun, raising the surface
temperature on Venus to about 870 degrees F (465 degrees C), hot
enough to melt lead. This trapping of heat is Thick clouds of
known as the greenhouse effect. Scientists have sulfuric acid cover
warned that a similar process on Earth is causing Venus. Image
permanent global warming. Several spacecraft credit: NASA
have orbited or landed on Venus. In the 1990's, the
Magellan spacecraft used radar -- radio waves
bounced off the planet -- to map Venus in detail.
Earth, our home
Earth, our home planet, has an atmosphere that is planet, has oceans
mostly nitrogen with some oxygen. Earth has of liquid water,
oceans of liquid water and continents that rise and continents
above sea level. Many measuring devices on the that rise above sea
surface and in space monitor conditions on our level. Image
planet. In 1998, the National Aeronautics and credit:
Space Administration (NASA) launched the first of NASA/Goddard
a series of satellites called the Earth Observing Space Flight
Center

36. System (EOS). The EOS satellites will carry remote-sensing instruments to measure
climate changes and other conditions on Earth's surface.
Mars is known as the red planet because of its reddish-brown
appearance, caused by rusty dust on the Martian surface. Mars is a
cold, dry world with a thin atmosphere. The atmospheric pressure
(pressure exerted by the weight of the gases in the atmosphere) on the
Martian surface is less than 1 percent the atmospheric pressure on
Earth. This low surface pressure has enabled most of the water that
Mars may once have had to escape into space.
The planet Mars
has clouds in its The surface of Mars has giant volcanoes, a huge system of canyons,
atmosphere and a and stream beds that look as if water flowed through them in the past.
deposit of ice at its Mars has two tiny moons, Phobos and Deimos. Many spacecraft have
north pole. Image landed on or orbited Mars.
credit:
NASA/JPL/Malin Jupiter, the largest planet in our solar system, has more mass than the
Space Science other planets combined. Like the other Jovian planets, it has gaseous The layers of
Systems outer layers and may have a rocky core. A huge storm system called dense clouds
the Great Red Spot in Jupiter's atmosphere is larger than Earth and has raged for around Jupiter
hundreds of years. appear in a
photograph of the
Jupiter's four largest moons -- Io, Europa, Ganymede, and Callisto -- are larger than planet taken by
Pluto, and Ganymede is also bigger than Mercury. Circling Jupiter's equator are three the Voyager 1
thin rings, consisting mostly of dust particles. A pair of Voyager spacecraft flew by space probe.
Jupiter in 1979 and sent back close-up pictures. In 1995, the Galileo spacecraft dropped a Image credit: JPL
probe into Jupiter's atmosphere. Galileo orbited Jupiter from 1995 to 2003.
Saturn, another giant planet, has a magnificent set of gleaming
rings. Its gaseous atmosphere is not as colorful as Jupiter's,
however. One reason Saturn is relatively drab is that its hazy upper
atmosphere makes the cloud patterns below difficult to see. Another
Saturn is encircled reason is that Saturn is farther than Jupiter from the sun. Because of
by seven major the difference in distance, Saturn is colder than Jupiter. Due to the
rings. Image credit: temperature difference, the kinds of chemical reactions that color
NASA/JPL/Space Jupiter's atmosphere occur too slowly to do the same on Saturn.
Science Institute
Saturn's moon Titan is larger than Pluto and Mercury. Titan has a thick atmosphere of
nitrogen and methane. In 1980 and 1981, the Voyager 2 spacecraft sent back close-up
views of Saturn and its rings and moons.
The Cassini spacecraft began orbiting Saturn in 2004. It carried a small probe that was
designed to be dropped into Titan's atmosphere.

37. Uranus was the first planet discovered with a telescope. German-
born English astronomer William Herschel found it in 1781. He Uranus appears in true
at first thought he had discovered a comet. Almost 200 years colors, left, and false
later, scientists detected 10 narrow rings around Uranus when the colors, right, in images
planet moved in front of a star and the rings became visible. produced by
Voyager 2 studied Uranus and its rings and moons close-up in combining numerous
1986. pictures taken by the
Voyager 2 spacecraft.
Neptune was first observed in 1846 by German Image credit: JPL
astronomer Johann G. Galle after other astronomers predicted its
position by studying how it affected Uranus's orbit. In 1989, Voyager 2
found that Neptune had a storm system called the Great Dark Spot,
similar to Jupiter's Great Red Spot. But five years later, in 1994, the
Hubble Space Telescope found that the Great Dark Spot had vanished.
The blue clouds Neptune has four narrow rings, one of which has clumps of
of Neptune are matter. Neptune's moon Triton is one of the largest in the solar
mostly frozen system and has volcanoes that emit plumes of frozen nitrogen.
methane. The
other object The Dwarf Planets
shown is
Pluto is so far from
Neptune's moon The solar system’s dwarf planets consist primarily of rock and ice
Earth that even
Triton. Image and feature little or no atmosphere. They lack the mass to sweep powerful telescopes
credit: their orbits clear, so they tend to be found among populations of reveal little detail of its
NASA/JPL similar, smaller bodies. surface. The Hubble
Space Telescope
Ceres ranks as the largest of millions of asteroids found between the orbits of Mars gathered the light for
and Jupiter. Ceres has a rocky composition and resembles a slightly squashed the pictures of Pluto
sphere. Its longest diameter measures 596 miles (960 kilometers). The Italian shown here. Image
astronomer Giuseppe Piazzi discovered Ceres in 1801. As with Pluto, people once credit: NASA
widely considered Ceres a planet.
The outer dwarf planets generally lie beyond the orbit of Neptune. Astronomers have had
difficulty studying bodies in this region because they are extremely far from Earth.
Dozens of them probably fit the IAU’s definition of a dwarf planet. Most of these bodies
belong to the Kuiper belt.
Some astronomers have suggested calling the outer dwarf planets plutonians in honor of
Pluto, the first one discovered. A body designated 2003 UB313 ranks as the largest dwarf
planet, with a diameter of around 1,500 miles (2,400 kilometers). Quaoar «KWAH oh
wahr» , a KBO discovered in 2002, measures roughly half the size of Pluto. Sedna,
discovered in 2004, measures about three-fourths the size of Pluto and lies nearly three
times as far from the sun. Some scientists think Sedna belongs to population of cometlike
objects called the Oort cloud, which lies beyond the Kuiper belt.