Name
Date
The Rocky Planets
By Patti Hutchison
Our solar system is made up of mostly empty space. It contains the sun,
the planets and their moons, and other bodies such as comets and
asteroids. But each of these is much smaller than the space between
them. Scientists believe there are other bodies "out there" that have not
been discovered yet.
Planets are divided into two categories. There are the "rocky" planets
and the "gas giant" planets. Rocky planets include Mercury, Venus,
Earth, and Mars.
Mercury is the closest planet to the sun. It is difficult to see from Earth.
Mercury is about one-third the size of Earth. It completes a trip around the sun every 88 (Earth) days. This makes it
the fastest orbiting planet, moving at about 50 kilometers per second.
Mercury spins slowly on its axis, however. In two of its years (orbits) only three days pass! The length of one
Mercury day is equal to about 58 Earth days.
The daytime temperature on Mercury can reach over 425 degrees Celsius. There is very little atmosphere to absorb
this heat, however. The nighttime temperature goes down to about -170 degrees. This is the largest temperature
difference of all the planets in our solar system.
The surface of Mercury looks almost like our Moon's surface. There are many craters caused by collisions with
meteoroids and comets. There are some areas of smooth surface. In some places there are tall, sharp cliffs.
Mercury's iron core has a radius of about 1800 kilometers. It is surrounded by an outer shell that is only about 500
kilometers thick.
Next in line is Venus. Venus is very similar to Earth. The two planets have about the same size and mass. But
Venus does not look blue, as Earth does. Venus has no ocean. It is covered with a thick mass of clouds. The clouds
spin very fast, trapping surface heat. This makes the surface temperatures very hot and the surface pressure very
high.
Venus revolves around the sun once every 225 (Earth) days. It takes 243 Earth days for it to make one rotation on
its axis. Venus spins in the opposite direction, compared to Earth. If you lived on Venus, the sun would appear to
rise in the west and set in the east!
The surface of Venus is highly eroded. It also has large volcanic ridges. It has an iron core that is about 3000
kilometers in diameter. Scientists think this core might be liquid and extends halfway to the surface of Venus.
The third planet from the sun is Earth. It is the only planet known to have life. This is due to our atmosphere. It
provides oxygen, allowing living things to exist. The "blue" planet also has another feature that supports lifewater. It covers about70 percent of the surface.
Unlike other planets, Earth still has tectonic activity. Plates are constantly in motion. Volcanoes are still erupting.
The surface is constantly changing.
Earth completes one trip around the sun every 365 1/4 days. It spins on its axis once in 24 hours. This is a much
faster rate than most other planets. The tilt of Earth's axis causes the change of seasons. Earth's moon is a familiar
object in the night sky.
The last rocky planet is Mars, the red planet. Its red color is caused by high iron content in the surface soil. Mars is
smaller and less dense than Earth. It is orbited by two small, oddly shaped moons.
Name
Date
Mars' atmosphere is very thin. There is a constant wind on Mars, often causing dust storms that last for weeks. Ice
caps cover both poles. They grow and shrink with the seasons. However, scientists are still searching for liquid
water on the planet.
The two hemispheres of Mars have different surface features. The northern hemisphere is mostly plains, with few
craters. The southern hemisphere has many craters and highlands. The largest volcanic mountain in the solar
system is found on Mars. Scientists believe this planet has a core of iron and nickel, which is probably solid.
Mars' rate of rotation is very similar to Earth's, about once in a little over 24 hours. However, it takes Mars almost
twice as long to orbit around the sun. Its year is about 687 Earth days.
The rocky planets are the closest to the sun in our solar system. They are small and dense. They are totally
different from the "gas giants" that exist farther from the sun.
The Rocky Planets
Questions
1. Name the two categories of planets.
2. Mercury, Venus, Earth, and Mars are:
A. gas giant planets
B. stars
C. rocky planets
3. The fastest orbiting planet is:
A. Mercury
B. Earth
C. Venus
4. Name two features of Earth that support life.
5. The planet most similar to Earth is:
A. Venus
B. Mars
C. Mercury
Name
Date
6. What causes Mars to look red?
Write a paragraph comparing the surfaces of all the rocky planets.
Write a paragraph explaining why Mercury has the largest temperature difference of all the planets.