# Chapter 17 - "The Solar System"

```• THE SOLAR SYSTEM
This view of the
rising Earth was
seen by the
Apollo 11
astronauts after
they entered
orbit around the
Moon. Earth is
just above the
lunar horizon in
this photograph.
• Ideas about the Solar System
• The Geocentric Model
– This model saw the solar system as perfect spheres with
attached celestial bodies rotating around a fixed Earth.
– The planets rotated around the Earth in perfect circles.
– This model grew out of the ideas that:
• Humans were at the center of a perfect universe created
just for them.
• Since Heaven was a perfect place, then everything that
existed in the Heavens would be perfect.
– This naturally meant that the solar system must be a
place of perfect epicycles moving on perfect spheres,
which were moving in perfect circles.
The paths of Venus and the Sun at equal time intervals
according to the Ptolemaic system. The combination of
epicycle and Sun movement explains retrograde motion
with a stationary Earth.
(A)The Ptolemaic system
explanation of Venus as a
morning star and evening
star. (B) The heliocentric
system explanation of
Venus as a morning star
and evening star.
• The Heliocentric Model
– Nicolas Copernicus in 1543 suggested that the Earth
revolves around the Sun.
– In this model each planet moved around the Sun in
perfect circles at different distances and at faster speeds
the closer to the Sun a planet was.
– Tycho Brahe
• Made precise measurements of the Sun, Moon, planets, and the
stars.
– Johannes Kepler
• Found that the planets did not move in perfect circles.
• Planets move in the path of a ellipse.
The heliocentric system explanation of retrograde motion.
• Kepler’s Laws of Planetary Motion
– Kepler’s First Law.
• Each planet moves in an orbit that has the shape of a ellipse,
with the Sun located at one focus.
– Kepler’s Second Law
• An imaginary line between the Sun and a planet moves over
equal areas of the ellipse during equal time intervals.
• The velocity of a planet would therefore vary as it is not always
the same distance from the focus.
• The point where the planet comes closet to the Sun is the
perihelion and the point at which it is farthest from the Sun is
the aphelion.
Kepler's first law describes the shape of planetary orbit as
an ellipse, which is exaggerated in this figure. The Sun is
located at one focus of the ellipse.
Kepler's second law. A line from the Sun to a planet at
point A sweeps over a certain area as the planet moves to
point B in a given time interval. A line from the Sun to a
planet at point C will sweep over the same area as the
planet moves to point D during the same time interval.
The time required to move from point A to point B is the
same as the time required to move from point C to point
D, so the planet moves faster in its orbit at perihelion.
– Kepler’s Third Law.
• The square of the period of a planet’s orbit is
proportional to the cube of that planet’s semimajor
axis t2d3
Kepler's third law describes a relationship between the
time required for a planet to move around the Sun and its
average distance from the Sun. The relationship is that the
time squared is proportional to the distance cubed.
• Origin of the Solar System
• Heliocentric model of the solar system
– Commonly the most accepted model of the solar system
• Protoplanet nebular model – most accepted theory
of the origin of the solar system.
Formation of the solar system
according to the protoplanet
nebular model, not drawn to
scale.
(A) The process starts with a nebula of gas, dust, and
chemical elements from previously existing stars.
(B) The nebula is pulled together by gravity, collapsing
into the protosun and protoplanets.
(C) As the planets form, they revolve around the Sun in
orbits that are described by Kepler's laws of planetary
motion.
– Stage A
• All the elements that made up the current solar system
were derived from stars that disappeared billions of
year ago, even before our Sun was born.
• Hydrogen fusion in the core of large stars results in the
formation of elements through iron.
• Elements that are heavier that iron are formed in rare
supernova explosions of dying massive stars.
– Stage B
• All of the elements from Stage A began to form large,
slowly rotating nebula
• Under the influence of gravity, the size of this nebula
begin to decrease, which increased its rate of spin
• Eventually this spinning nebula formed an accretion
disk which is an enormous, bulging disk of gases and
elements formed as nebula condense.
• Eventually this accretion disk became compressed into
protoplanets and a protosun
– Stage C
• An initial flare-up of the Sun from the warming and
condensing established the protosun as a star and it
became our Sun.
– Between the orbits of Mars and Jupiter there is an
Asteroid belt that some think was formed by the breakup
of a larger planet.
• The Planets
• Introduction
– The solar system consists of a middle aged main
sequence G type star called the Sun with nine planets,
nearly fifty moons, thousands of asteroids, and many
comets revolving around it.
– This is all held together by the force of gravitational
attraction
– Terrestrial planets are those which have a composition
very similar to the Earth’s composition
• these planets are composed mostly of rocky material
with iron
• These planets have a density of 4 – 5.5 g/cm3.
– The Jovian (Giant) planets are those that are composed
mostly of hydrogen, helium, and methane with a density
of 2g/cm3.
• The probably contain a core of material much like the
makeup of the terrestrial planets which is surrounded
by a thick layer of gases.
The order of the planets out from the Sun. The orbits and
the planet sizes are not drawn to scale.
Earth's rotation around its axis results in day and night.
One year is required for one revolution.
(A)The early secondary
atmosphere of Venus lost
hydrogen to space and
oxygen became
combined with rocks as
decomposed water
molecules. (B) On Earth,
water formed the oceans,
carbon dioxide was
removed, and plants
released oxygen, which
in turn formed an ozone
layer in the atmosphere,
protecting the water and
life below.
• Mercury
–
–
–
–
Innermost planet
Period of revolution is 88 days.
Rotation once every 59 days.
High kinetic energy of gas molecules and a low
gravitational pull
– Surface temperature 427OC (800OF)in the sunlight to –
180OC (-350OF)in the dark
– Surface covered with craters
– Presence of magnetic fields and high density so must
have a high iron content with at least a partial molten
core
Mercury is close to the Sun and visible only briefly before
or after sunrise or sunset, showing phases. Mercury
actually appears much smaller in an orbit that is not tilted
as much as shown in this figure.
A photomosaic of Mercury made from pictures taken by
the Mariner 10 spacecraft. The surface of Mercury is
heavily cratered, looking much like the surface of Earth's
Moon. All the interior planets and the Moon were
bombarded early in the life of the solar system.
• Venus
–
–
–
–
–
–
–
–
Very bright in morning and evening sky.
revolves around the Sun once every 225 days.
Rotates on its axis once every 243 days.
Exhibits retrograde rotation where the rotation of the
planet is opposite its direct of revolution around the Sun
and opposite most other planets.
Average surface temperature is 480 OC (900 OF)
Atmospheric pressure approximately 100 times that
experienced on Earth.
Clouds and rain consisting of sulfuric acid.
No satellites and no magnetic field.
This is an image of an 8 km
(5 mile) high volcano on the
surface of Venus. The image
was created by a computer
simulating a viewpoint
elevation of 1.7 km (1 mile)
above the surface. The lava
flows extend for hundreds
of km across the fractured
plains shown in the
foreground. The simulated
colors are based on color
images recorded by the
Soviet Venera 13 and 14
spacecraft.
• Mars
– Unique, bright reddish color which exhibits a swift
retrograde motion against the background of stars.
– Revolves around the Sun in 687 days.
– Rotates on its axis once every 24 hours, 37 minutes
– Has an atmosphere
– A geologically active past and is divided into 4
provinces:
•
•
•
•
Volcanic regions
Systems of canyons
Terraced plateaus near the poles
Flat regions pitted with impact craters.
– average temperature is –53 OC (-63 OF)
– Atmosphere is 95% CO
– Atmospheric pressure 0.6 percent of Earth’s atmospheric
pressure
– Two satellites
• Deimos – 13 km across
• Phobos – 22 km across
• Both are thought to be captured asteroids
Surface picture
taken by the
Viking 1
lander found
reddish, finegrained
material, rocks
coated with a
reddish stain,
and groups of
blue-black
volcanic rocks.
A view of the surface of Mars taken by the Viking Orbiter 1
cameras. The scene shows three volcanoes that rise an average of
17 km (about 11 mile) above the top of a 10 km (about 6 mile)
high ridge. Clouds can be seen in the upper portion of the
photograph, and haze is present in the valleys at the lower right.
• Jupiter
– Largest of all planets
• Twice as massive as all other planets combined and
about 318 times as massive as the Earth.
– Radiates twice as much energy as it gets from the sun due
to slow gravitational compression
– Average density of 1.3 g/cm3
– Made mostly of hydrogen and helium with some rocky
substances
– A solid core with a radius of about 14,000 km (8,500
miles)
The interior structure of Jupiter.
Photos of Jupiter taken
by Voyager 1. (A) From
million mi). (B) A closer
view, from the Great Red
Spot to the South Pole,
showing organized cloud
patterns. In general, dark
features are warmer, and
light features are colder.
The Great Red spot soars
mi) above the
surrounding clouds and
is the coldest place on
the planet.
– Above this core is about 35,000 km (22,00 mi) of liquid
hydrogen, which is compressed so tightly by millions of
atmospheres of pressure that it is able to conduct
electricity and is termed metallic hydrogen.
– Above this is 20,000 km (12,000 mi) of liquid hydrogen
under much less pressure.
– The outer layer is about 500 km (300 mi) of hydrogen,
helium, ammonia gas, and crystalline compounds with a
mixture of ice and water.
– Sixteen satellites – four are called Galilean moons as they
were discovered by Galileo in 1610
• Io
• Europa
• Ganymede
• Callisto
The four Galilean
moons pictured by
Voyager 1.
Clockwise from
upper left, Io,
Europa, Ganymede,
and Callisto. Io and
size of Earth's Moon;
Ganymede and
Callisto are larger
than Mercury.
(A)This image, made by the Hubble Space Telescope, clearly
shows the large impact site made by fragment G of former
Comet Shoemaker-Levy 9 when it collided with Jupiter.
(B) This is a picture of Comet Shoemaker-Levy 9 after it
broke into twenty-two pieces, lined up in this row, then
proceeded to plummet into Jupiter during July, 1994. The
picture was made by Hubble Space Telescope.
• Saturn
– Slightly smaller and less massive than Jupiter with a system of
rings
– The rings are made up of particles, some which are meters across
and some that are dust sized particles.
– 10 moons
•
•
•
•
•
•
•
•
•
•
Janus
Mimas
Tethys
Dione
Rhea
Titan
Hyperion
Iapetus
Phoebe
– Titan is the only moon in the solar system with an atmosphere and
is larger than the planet Mercury
A part of
Saturn's system
of rings, pictured
by Voyager 2
from a distance
million km
mi). More than
sixty bright and
dark ringlets are
seen here;
different colors
indicate different
surface
compositions.
• Uranus, Neptune, and Pluto
– Uranus revolves around the Sun once about every 84
years.
– Both Uranus and Neptune have a core of rocky material
surrounded by water and ice.
– Uranus and Neptune have an atmosphere of hydrogen
and helium
– Average temperature on Uranus is-210 OC (-350 OF)
– Average temperature on Neptune is –235 OC (-391 OF)
– Uranus tilts 82O on its axis, which is different from the
less that 30O for other planets.
– 15 Known satellites around Uranus
– Uranus has 10 narrow rings and a number of dusty bands.
–
–
–
–
–
Neptune has 8 satellites
Neptune also has a series of rings
Pluto is the smallest planet in the solar system.
Pluto has a density of 2 g/cm3
Pluto’s atmosphere is probably mostly nitrogen, with
some methane and CO2
– Pluto orbits around the Sun once every 248 years.
This is a photo
image of
Neptune taken
by Voyager.
Neptune has a
turbulent
atmosphere
over a very
cold surface
of frozen
hydrogen and
helium.
The interior structure of Uranus and Neptune.
The orbital planes of the planets. Notice the exceptional
tilt of the orbit of Pluto.
• Smaller bodies of the Solar System
• Comets
– Comets
• Small relative to other bodies in the solar system
• Made of frozen CO2, NH3, CH4 and particles of dust
and rock mixed in.
• Originate approximately 30 A.U. from the Sun
– Oort Cloud
• A spherical cloud of objects that occur out past the
orbit of Pluto made of water ice, frozen methane,
frozen ammonia, dry ice, and dust.
• Occurs about 30,0000 A.U. out to a light year or more
from the Sun
– Kuiper Belt
• A disk shaped region of smaller icy bodies from 30 to
100 A.U from the Sun.
• This is the source of short period comets.
– Centaurs
• Kuiper Belt Objects that orbit between Jupiter and
Neptune
– Comets have two types of tails
• Ionized gases.
• Dust
– The comets tail always points away from the Sun, so it
follows the comet as it approaches the Sun and leads the
comet as if moves away from the Sun
Comets originate from the Oort cloud, a large spherical cloud of icy
aggregates, or from the Kuiper Belt, a disk-shaped region of icy
bodies. The Kuiper Belt is illustrated here, extending from 30 to
100 A.U. from the Sun. The Oort cloud is a spherical &quot;cloud&quot; that
ranges from about 30,000 A.U. to a light-year or more from the
Sun.
As a comet nears the Sun
it grows brighter, with the
tail always pointing away
from the Sun.
• Asteroids
– The asteroid belt lies between Mars and Jupiter
– The asteroids in this belt are between 1 km up to the
largest (Ceres) which is 1,000 km (600 mi)
– The asteroids in the inside of the belt are made of a stony
material and the asteroids on the outside of the belt are
– Some other asteroids are metallic, made of iron and
nickel
Most of the asteroids in the asteroid belt are about
halfway between the Sun and Jupiter
• Meteors and Meteorites
– Meteroids
• Remnants of asteroids and comets after the heat of the Sun and
collisions.
– Meteor
• The streak of light and smoke left in the sky by a meteoroid is
called a meteor.
• These are the shooting stars that we see in the sky
– Meteor shower
• Occurs when the Earth passes through a stream of particles that
are left by a comet.
• There is an intense meteor shower the third week in October as
the Earth crosses the path of Halley’s Comet.
– Meteorite
• A meteorite is what is left of a meteoroid if it survives the flight
through the Earth’s atmosphere.
• Classified according to their makeup
• Iron meteorites
– made up mostly of iron and nickel material
• Stony meteorites
– composed mostly of rocky material much like that found on
Earth
– chondrites – have a structure of small spherical lumps of silicate
materials.
– Achondrites – do not contain this silicate clumps
• Stony-iron meteorites
– Made up of a conglomerate of both types of materials.
(A)A stony meteorite.
The smooth, black
surface was melted by
friction with the
atmosphere.
(B) An iron meteorite that
has been cut, polished, and
etched with acid. The
pattern indicates that the
original material cooled
from a molten material over
millions of years.
```