Download Astronomy Chapter 10 – The Outer Planets A. Main Ideas Beyond

Astronomy
Chapter 10 – The Outer Planets
A. Main Ideas
Beyond Mars, the nature of the planets changes. Instead of small, rocky planets, giant planets of gas
and ice have formed.
1. Jupiter
• Appearance and Physical Properties
⇒ Jupiter is the largest planet both in radius and mass
⇒ Jupiter’s atmosphere is consists mostly of hydrogen, helium, and hydrogen-rich gases
such as methane, ammonia, and water
• Interior
⇒ Jupiter’s density is slightly greater than that of water, 1.3 grams per cubic centimeter, so
the bulk of the planet is composed of light elements
⇒ Jupiter’s immense exerts a tremendous gravitational force on its interior, and as depth
increases the gases of the atmosphere are compressed into metallic liquid hydrogen.
⇒ Jupiter also contains heavier elements like silicon and iron, but because of their high
density they have sunk to the planet’s center forming a rocky core
• Atmosphere
⇒ The combination of Jupiter’s rapid rotation and convection currents in its atmosphere
results in the formation of jet streams moving in the opposite direction of Jupiter’s rotation
⇒ The convection currents in the metallic liquid hydrogen along with Jupiter’s rotation
generates the strongest magnetic field in the Solar System
• Jupiter’s ring
⇒ The ring is thought to be made of tiny particles or rock dust held in orbit by Jupiter’s
immense gravitational attraction
• Jupiter’s Moons
⇒ Jupiter has 63 moons in orbit about it. Most are too small to be easily seen from Earth,
but the four largest—Europa, Callisto, Ganymede, and Io--were first viewed by Galileo in
the 17th century
⇒ The four large moons are thought to have formed in the same manner as the planets,
and each exhibit distinguishing characteristics.
2. Saturn
• Appearance and Physical Properties
⇒ Saturn’s diameter is about 10 times larger than the Earth’s, and has a mass of around 95
times that of Earth’s
⇒ Saturn has a density of 0.7 grams per cubic centimeter, which is less than the density of
water.
⇒ Like Jupiter, Saturn is mostly of hydrogen and hydrogen-rich compounds
• Saturn’s rings
⇒ Were first observed by Galileo who thought they looked liked handles on either side of
the planet
⇒ The rings are very wide but very thin.
⇒ The rings are a swarm of small particles, ranging in size from a few centimeters to a few
meters across
⇒ There are gaps in the rings, the largest is known as Cassini’s division. The gaps are a
result of complex interactions between Saturn’s moons and the particles that make up the
rings
• Origin of planetary rings
⇒ The particles that make up the rings eventually spiral into the planet, so if new material
were not being added to the rings they would disappear in a few million years. Scientists
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think that the source for new material is the small moons close to the planets. The moons
either collide with each other or are torn apart by the gravitational tidal forces generated
by the planet
The Roche Limit
⇒ In 1849, the French scientist M. E. Roche demonstrated mathematically that if a moon
gets too close to its planet, the planet’s gravity could rip the moon apart.
⇒ Roche calculated that the distance at which this would happen is if the approaches within
2.44 planetary radii. This distance is called the Roche limit. All rings lie within the Roche
limit of their planet
Saturn’s moons
⇒ Saturn has one large moon and 60 smaller moons that have been cataloged. Most orbit
around Saturn’s equator forming a mini Solar System
⇒ The moons have a small average density, indicating that their interiors must be mostly ice
3. Uranus
Although small when compared to Jupiter and Saturn, Uranus has a diameter four times that of
Earth’s, and a mass equal to around 15 Earth masses
• Atmosphere
⇒ Uranus’s atmosphere is rich in hydrogen and methane, and it is methane that is
responsible for the planet’s blue color. The methane absorbs the all the colors of the
spectrum except for blue, which is reflected
• Interior
⇒ The density of Uranus is about 1.2 grams per cubic centimeter. This value suggests that
the interior of Uranus consists of water mixed with methane and ammonia
⇒ The core is probably iron and rock, although some scientists thinks it could consist of ice
with a very small amount of rocky material
• Odd tilt
⇒ Uranus’s axis of rotation is tilted so that its equator is nearly perpendicular to its orbit.
Uranus spins on its side.
⇒ One explanation for this phenomenon is that early in its history Uranus was struck by an
enormous planetesimal that tipped the planet over
• Rings and Moons
⇒ Uranus is surrounded by a set of narrow rings composed of small particles, around a
meter in diameter. Compared to Saturn’s, the rings of Uranus are very narrow and are
very dark, perhaps because the particles are composed of carbon-rich molecules
⇒ Uranus has 5 large moons and about 16 smaller ones that probably composed of ice and
rock
4. Neptune
Neptune is very nearly the same size of Uranus in terms of radius and mass.
• Structure
⇒ Neptune is composed mostly of water and has a thin atmosphere rich in hydrogen and
hydrogen compounds
⇒ Neptune’s average density equals 1.67 grams per cubic centimeter, so it must be
composed of light elements, but like the other gas and ice giants has an iron and rocky
core
• Atmosphere
⇒ Neptune’s blue color is caused by the abundance of methane in its atmosphere, just like
Uranus. Unlike Uranus, however, Neptune has cloud belts and high winds caused by the
convection currents that rise to its outer atmosphere
• Rings and Moons
⇒ Neptune has very narrow rings like Uranus, but are composed not of ice but of dust. It is
possible that these rings will be short lived and disappear after a few million years
⇒ Neptune has 6 small moon orbiting close to it in nearly circular orbits and two larger
moons a much greater distance from the planet.
5. Pluto and Beyond
• There are no major planets beyond the orbit of Neptune, but there are many smaller objects
called Trans-Neptunian Objects.
• Pluto was discovered in 1930, and for more than 75 years was considered to be a planet
• There are several hundred other objects a few hundred kilometers in diameter that orbit the
Sun at nearly the same distance as Pluto
• The term dwarf planet was coined to describe objects similar to Pluto mass but are not
considered to be major planets
B. Vocabulary
Roche limit
Shepherding satellites
Trans-Neptunian object
C. Discussion Questions
1. Why are the outer planets so large?
2. Do Jupiter and Saturn have solid surfaces?
3. How do astronomers know what lies inside the outer planets?
4. What are the rings of Saturn made of?
5. How did the discovery of a moon orbiting Pluto help astronomers better understand this object?
6. Where did Pluto and other TNOs form? How did they get to where they are today?