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Solar System
Astronomy 1-1
Lecture 07-1
Solar System
Consists of
Sun
8 Planets
Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, Neptune, and
(Pluto)
165 Moons
Asteroid Belt between Mars and Jupiter
Comets
Astronomy 1-1
Lecture 07-2
Measuring the Planets
Orbital Period - Can be observed
Distance from Sun - Known by Kepler’s laws
Radius - Known from angular size and distance
Masses - From Newton’s laws
Rotation Period - From observations
Density - Can be calculated knowing radius and
mass
Astronomy 1-1
Lecture 07-3
Overall Layout of the Solar System
All orbits but
Mercury’s are close to
same plane
Astronomy 1-1
Lecture 07-4
Overall Layout of the Solar System
Because the planet’s
orbits are close to being
in a plane, it is possible
for them to appear in a
straight line as viewed
from Earth
This photograph was
taken in April 2002
Astronomy 1-1
Lecture 07-5
Relative Size Comparison
Major Planetary Points
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
(Pluto
Scorched, heavily cratered
Dense, Corrosive cloud cover
Life Supporting
Great dust storms
Great Red Spot
Spectacular ring systems
Tilted on its side
Interesting Moon
Highly eccentric orbit)
With the exception of Pluto, all the planets are effectively
in the equatorial plane of the Sun
Astronomy 1-1
Lecture 07-8
Planetary Motions
All the planets are moving counterclockwise
around the Sun, when viewed from the Solar
North Pole
With the exception of Venus and Uranus, all the
planets, are rotating counterclockwise about
their own rotation axes
Venus is rotating clockwise about its own axis
Uranus' rotation axis is in ecliptic plane
Astronomy 1-1
Lecture 07-9
Statistical Data
The planets can be divided into two groups:
The Terrestrials
Mercury, Venus, Earth, and Mars
These have densities > 3.9 gm/cm3
This is a density consistent with that of rock
The Gas Giants
Jupiter, Saturn, Uranus, and Neptune
These have densities < 2.0 gm/cm3
This is a density consistent with that of ices
(Water, Carbon Dioxide, Ammonia, Methane)
Astronomy 1-1
Lecture 07-10
Comparison of Terrestrial and
Jovian Planets
Terrestrial
Jovian
Close to the Sun
Closely Spaced Orbits
Small Masses
Small Radii
Predominantly Rocky
Solid Surface
High Density
Slow Rotation
Weak Magnetic Fields
Few Moons
No Rings
Far from Sun
Widely Spaced Orbits
Large Masses
Large Radii
Predominantly Gaseous
No Solid Surface
Low Density
Fast Rotation
Strong Magnetic Fields
Many Moons
Many Rings
Astronomy 1-1
Lecture 07-11
Differences Among the Terrestrial
Planets
All have atmospheres, but they are very different
Surface conditions vary as well
Only Earth has oxygen in its atmosphere and
liquid water on its surface
Earth and Mars spin at about the same rate;
Mercury is much slower, Venus is slow and
retrograde
Only Earth and Mars have moons
Only Earth and Mercury have magnetic fields
Astronomy 1-1
Lecture 07-12
Chemical Composition
Terrestrial Planets
Rocks and Metals
Inner Asteroids
Rocks and Metals
Outer Asteroids
Rocks, Metals, and Ices
Jovian Planets
Mostly Gas with Rock, Metal and Ice cores
Jovian Satellites
Rocks, Metals, and Ices
Pluto
Rocks, Metals, and Ices
Comets
Mostly Ices, some Rocks and Metals
Astronomy 1-1
Lecture 07-13
Solar System Formation
Nebular Hypothesis
Solar system formed from a
condensing cloud of gas and
dust which collapsed under
gravitational forces
Condensation of smaller
particles into larger
particles and then objects
called planetesimals
The process of gathering up
material is called accretion.
Astronomy 1-1
Lecture 07-14
Planetary Formation
Nebular Hypothesis
In inner region of disc temperatures high enough to drive
away the lighter elements such as hydrogen and
helium, leaving behind the heavier elements.
Planetesimals collide at small relative velocities and merge
forming even larger objects. These objects became
spherical in shape as this is the most stable.
Kinetic and potential energy are converted into heat
energy which allows material to become partially
molten allowing the heavier elements to sink inwards.
This process of the heavier elements sinking inwards is
called differentiation.
Astronomy 1-1
Lecture 07-15
Solar System Formation
The observation of disks surrounding newly formed
stars supports this theory
Astronomy 1-1
Lecture 07-16
Heat Sources for a Planet
The conversion of kinetic energy of the planetesimals into
heat energy is known as accretionary heating
As the heavier elements sink into the core region potential
energy is converted into heat. This heat is from core
formation
As the radioactive elements decay additional heat is
released
This is known as radiogenic heating
The interaction of two orbiting bodies will also cause tidal
bulges. This stretching and contracting of the planetary
mass yields heat energy
This is called tidal heating.
Astronomy 1-1
Lecture 07-17
Effects of Heating
Internal heating keeps some planets like the
Earth warm on the inside
Warm enough to have the internal material
molten or plasticized
The material on the surface, the crust, is solidified
This crust lies on a plasticized, molten layer
called the mantle
The molten rock is called magma when below the
surface and lava when above the surface
Astronomy 1-1
Lecture 07-18
Crustal Layer
Often fragmented with the fragments being
called plates
These plates drift around on the mantle
This activity is called plate tectonics
Volcanic activity often occurs along the
boundary between two plates
Astronomy 1-1
Lecture 07-19
Heat Transfer
Energy is transferred by three different means.
These are:
Convection
The transfer of energy by actual movement of material
from one point to another
Conduction
The transfer of energy from one point to another by
directly passing energy from atom to atom
Radiation
The transfer of energy by the emission of
electromagnetic radiation
Astronomy 1-1
Lecture 07-20
Rock Forms
There are various forms of rocks that occur
throughout the solar system. These are:
Igneous
These are rocks that are formed when molten
materials solidify
Sedimentary
These are rocks that have been formed from deposited
layers of mud and clay
Metamorphic
These are rocks that have been altered by heat and
pressure
On Earth there is a continuous cycling between
these three forms of rock
Astronomy 1-1
Lecture 07-21
Continuous Cycling
Astronomy 1-1
Lecture 07-22
Composition of Rocks
Rocks are made from various types of minerals
Minerals are either single elements or molecules
90% of the rocks on Earth contain silicon
The most common silicon bearing rocks are:
Quartz Feldspar
Mica Hornblade
Other minerals that are in rocks are
Oxides
Sulfides
Carbonates Halides
Astronomy 1-1
Lecture 07-23
Planetary Atmospheres
With the exception of Mercury, all of the planets have
atmospheres
There are several distinctions between the terrestrial
planets and the gas giants:
1) The terrestrials have a distinct boundary between
the atmosphere and the planet, whereas the gas
giants do not have a clear boundary
2) The atmospheres that the terrestrial planets have
are not their original atmospheres, while the
atmospheres on the gas giants are their original
atmospheres
Astronomy 1-1
Lecture 07-24
When the Sun ignited, it threw off its outer tenuous layers
as a wind, the T-Taurus wind. This wind blew off the
original atmospheres of the terrestrials planets
While the terrestrial planets were partially molten, they
outgassed through volcanic like processes
Since the initial outgassing, the atmospheres of Venus,
Earth, and Mars have undergone changes
On Venus, the temperature of the atmosphere and also
the surface has increased
On Earth, the original atmosphere contained very little, if
any, oxygen
On Mars, the atmosphere has either escaped or been
frozen out
Astronomy 1-1
Lecture 07-25
Atmospheric Circulation
The atmospheres on the planets are all in motion.
The reasons for this motion are:
Differential Heating
Equators receive more heat than polar regions.
Atmosphere rises at equator and sinks at poles
Planetary Rotation
Causes currents to be deflected away from path
towards the poles
Astronomy 1-1
Lecture 07-26
Tidal Forces
The gravitational interaction between the planets and
their respective moons is not necessarily uniform
The resulting forces are known as a tidal forces. These
forces act both on the planet and on the moon
This is seen on the Earth as the rising and lowering of the
oceans - the tides
The greatest tides occur when the Sun, Earth, and Moon lie in a
straight line
These are called spring tides
The tides are the least at first and last quarter moon when the
Moon, Earth, and Sun form a right angle
These are called neap tides
Not only do the oceans rise and fall but so does the land
Astronomy 1-1
Lecture 07-27
Tidal Effects
The Earth's tides are not aligned with the Earth-Moon
direction
The tides point slightly ahead of the moon
This is because the Earth carries the oceans with it
This "leading" bulge causes the Moon to accelerate
The Moon, because of its increasing velocity, moves into a higher
orbit
The Moon's orbital radius is increasing at a rate of – 4 cm/year
The Earth's own rotational rate is also slowing down due
to internal frictional forces
The day is becoming longer at a rate of - 0.002 sec/year
This process will stabilize when the Moon's rotation rate
about the Earth matches the Earth's rotation rate
about its own axis
This will occur when Earth's "day" will be 47 days long!
Astronomy 1-1
Lecture 07-28
Synchronous Rotation
The tidal forces acting between the Earth and the
Moon are responsible for the Moon always
keeping the same face towards the Earth
The Moon's sidereal rotation rate about the
Earth matches its sidereal period about its own
axis
This is known as Synchronous rotation
Most of the moons in the solar system are in
synchronous rotation about their respective
planets
Astronomy 1-1
Lecture 07-29
Spacecraft Exploration - Mercury
Mariner 10: Flew by
Mercury, 1974–1975
Next visit to Mercury:
Messenger, 2011
Astronomy 1-1
Lecture 07-30
Spacecraft Exploration - Venus
Soviet Venera probes landed on Venus from 1970 to
1978
Astronomy 1-1
Lecture 07-31
Spacecraft Exploration - Venus
The most recent Venus expedition from the United
States was the Magellan orbiter, 1990 – 1994
Astronomy 1-1
Lecture 07-32
Spacecraft Exploration - Mars
Viking landers arrived at Mars in 1976
Astronomy 1-1
Lecture 07-33
Spacecraft Exploration - Mars
Typical orbital path to Mars:
Astronomy 1-1
Lecture 07-34
Spacecraft Exploration - Mars
Sojourner was deployed on Mars in 1997
Astronomy 1-1
Lecture 07-35
Spacecraft Exploration - Mars
Spirit and Opportunity Rovers launched June and July 2003
Spirit currently stuck in the sand
Opportunity still “roving”
Astronomy 1-1
Lecture 07-36
Spacecraft Exploration - Jovians
Pioneer and Voyager flew through outer solar
system. This is Voyager:
Astronomy 1-1
Lecture 07-37
Gravitational “Slingshots”
Gravitational “slingshots” can change direction of
spacecraft, and also accelerate it
Astronomy 1-1
Lecture 07-38
Spacecraft Exploration - Saturn
Cassini mission arrived at Saturn in 2004, will stay 4
years
Astronomy 1-1
Lecture 07-39