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The planets
Midterm!
► In
next week Part I (take home exam,
including 10 points from Mastering Astronomy,
50 pts) will be available, due October 26th,
noon
► In 2 weeks, Part II (in class exam, 50 pts.)
 Taken in 3rd hour (week of 10/22 to 10/25)
 Bring SCANTRON (882 form) and #2 pencil
 Based on “Review Questions” handout, available
now!
► Also:
10 of the 25 extra credit points are due
by October 26th, noon.
Lecture 7: A Planetary Overview
Comparative Planetology
► In
this chapter we wish to look at the solar
system as whole and compare the worlds to
each other, seeking to understand their
similarities and differences – comparative
planetology
► Planetology broadly includes moons,
asteroids, and comets as well as the planets.
► We can see that the solar system is not a
random collection of worlds
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Lecture 7: A Planetary Overview
Comparative Planetology
► We
see several values to comparative
planetology (CP):
 CP has revealed similarities and difference in the
planets have helped guide the development of our
theory of solar system formation
 CP gives us deep new insights into the physical
processes that have shaped the Earth and other
worlds
 CP allows us to apply lessons from our own solar
system to the study of other solar systems.
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Lecture 7: A Planetary Overview
Solar System Roll Call
►The
Sun the is largest and brightest
Sun
object in the solar system
►The Sun is hot (5800 K on surface)
►The Sun is gaseous and converts
matter into energy in core
►The Sun has the greatest influence on
the rest of the solar system (light, solar
wind…)
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Lecture 7: A Planetary Overview
Solar System Roll Call
► Mercury
is the smallest planet in the solar
Mercury
system
► It rotates every 58.6 days and revolves every
88 days and is tidally locked to the Sun
Tidal
► The produces 88 days of daylight and 88
days of night, making temperatures extreme
(425°C to -150°C; 800°F to -240°F).
► One spacecraft has visited Mercury and
another is on its way
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Lecture 7: A Planetary Overview
Solar System Roll Call
Venus
Venus is often called Earth’s “twin” because it is
nearly the same size as the Earth. But it’s nothing like
the Earth…
► It rotates backwards (or upside down) very slowly
► It is covered with an atmosphere of mostly CO2 which
allows a runaway greenhouse effect to occur raising
the temperature to 470°C (880°F) planetwide
► Its surface pressure in 90 times greater than the
Earth and there are clouds of sulfuric acid near the
surface of the planet
► The Venus Express currently orbits Venus
►
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Lecture 7: A Planetary Overview
Solar System Roll Call
Earth
► Earth
is only world that we know of that has or
had life on it
► It is the only world with a significant amount of
oxygen in the atmosphere
► It is the only world with significant amounts of
liquid water
► It is the closest planet to the Sun to have a
moon and our Moon is quite large compared to
the Earth
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Lecture 7: A Planetary Overview
Solar System Roll Call
Mars
►
Mars may bear the closest resemblance to the
Earth
► It has a thin atmosphere of mostly CO2
► It has polar caps made of CO2 and water-ice
► In the past, water very likely flowed on the surface
► It has great geological wonders such as a great
canyon and the largest volcano in the solar system
moons
► It has two tiny moons
► It is the most studied extraterrestrial planet and has
several spacecraft present and proposed to land or
orbit Mars.
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Lecture 7: A Planetary Overview
Solar System Roll Call
Jupiter
► Jupiter
is largest planet in the solar system
and is made mostly of gas with a Earth sized
rocky-ice core in the center
Jupiter2
► It has more than 300 times the diameter and
1000 times the volume of the Earth
► Its atmosphere has many storms many of
which have lasted for hundreds of years
Gal Moons
► Its four largest moons (of 63) have
interesting properties too (active volcanoes,
subsurface water, magnetic fields)
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Lecture 7: A Planetary Overview
Solar System Roll Call
Saturn
► Saturn
is another gaseous giant planet with
a spectacular ring system
► The ring system is made of millions of icedust chunks orbiting around the planet
► Saturn has 60 moons, a few of them midsize
moons and one large one, Titan, which has a
significant atmosphere.
► Currently, Cassini is orbiting around Saturn
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Lecture 7: A Planetary Overview
Uranus
Uranus3
Uranus2
Solar System Roll Call
Neptune
Neptune3
Neptune2
►
Uranus (YUR-uh-nus) is a smaller gas giant with a greenblue color due to methane
► It has several dozen moons a few of which are midsize
► The entire system (planet, rings, moons) is tilted on their
side
► Neptune is just a bit smaller than Uranus and bluer in
color
► It has about a dozen moons, one of which is large (Triton).
Triton is the largest moon to go backward (retrograde)
around the planet
► Both Neptune and Uranus has been visited by only one
spacecraft (Voyager 2)
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Lecture 7: A Planetary Overview
Solar System Roll Call
Pluto
►
Pluto (and the other Dwarf Planets) are round
objects which orbit around the Sun
Eris
► Pluto was discovered as a planet in 1930, but was
an oddball world. One of its 3 moons is half its size
(Charon). It will be visited by spacecraft in 2015.
► Soon in the 1990s other objects out where Pluto
lived were being discovered. One of these, Eris,
was found to be a little larger than Pluto
Ceres
► In 2006, the phrase “dwarf planet” was defined for
these objects and asteroids (like Ceres) which were
round but were found “nearby” other solar system
objects
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Lecture 7: A Planetary Overview
Solar System featurs
►
Looking at the general characteristics, there
are 4 features which stand out:
1.
2.
3.
4.
Patterns of motion among large bodies
Two major types of planets
Asteroids and comets
Exceptions to the rules
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Lecture 7: A Planetary Overview
Distances In The Solar System
Measuring Distances in the Solar System
►Copernicus
used geometry to determine
relative distances to the planets.
►Today we measure planetary distances
using radar.
►Average distances to the planets from
the Sun range from .387 AU for Mercury
to 39.53 AU for Pluto.
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Lecture 7: A Planetary Overview
Feature 1: Patterns
of Motion
► All
planetary orbits are ellipses, but all are
nearly circular.
orbits
► Each of the planets revolves around the Sun
in the same direction.
► All planets - except Venus, Uranus - rotate in
a counterclockwise direction.
► Most of the satellites revolving around
planets also move in a counterclockwise
direction, though there are some exceptions.
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Lecture 7: A Planetary Overview
Feature 1: Patterns
of Motion
of a planet’s orbit is the
tilts
angle between the plane of a planet’s
orbit and the ecliptic plane (the plane of
the Earth’s orbit).
►The elliptical paths of all the planets are
very nearly in the same plane
(inclination about 0°), though Mercury’s
orbit is inclined at 7° and Pluto’s at 17°.
►Inclination
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Lecture 7: A Planetary Overview
Planet Diameters
Diameters of Non-Earth Planets
► Diameters are determined from distances (from
the Earth to the planet) and the planet’s angular
size via the small angle formula (Mathematical
Insight 2.1)
► Diameter of Sun (1.39 × 106 km) is over 100
times that of Earth (1.3 × 104 km).
► Jupiter’s diameter is 11 times that of Earth.
► Pluto’s diameter is 1/5 that of Earth.
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Lecture 7: A Planetary Overview
Planet Masses
Mass of the Planets
► Kepler’s
third law was reformulated by Newton
to include masses (Mathematical Insight 4.3):
► Newton’s
a3/p2 = K (M1 + M2)
statement of Kepler’s third law allows
us to calculate the mass of the Sun.
► Consider the orbits of planets around the Sun.
Since one of the masses to the Sun (the other
being a planet), the sum of the two is essentially
equal to the mass of the Sun, and the equation
can be rewritten as:
a3/p2 = KM
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Lecture 7: A Planetary Overview
Planet Masses
► We
can do the same sort of calculation for
planets as long as they have satellites
orbiting them
► The masses of 7 of the 9 known planets can
be calculated based on the distances and
periods of revolution of these planets’ natural
satellites.
► For Mercury and Venus, which do not
possess any natural satellites, accurate
determinations of their respective masses
had to await orbiting or flyby space probes.
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Lecture 7: A Planetary Overview
Feature 2: Classifying
the Planets
► The
planets (except Pluto) fit into two groups:
the inner terrestrial planets and the outer
Jovian planets.
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Size, Mass, and Density
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► The Jovian planets have much bigger
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diameters and even larger masses than the
terrestrial planets.
► Terrestrial planets are more dense, however.
► Earth is the densest planet of them all.
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planets
Lecture 7: A Planetary Overview
Classifying the Planets
Satellites and Rings
► The Jovian planets have more satellites than
the terrestrials.
► 4 Jovian planets: 163 total satellites as of
September 2007 (63 for Jupiter, 60 for
Saturn, 27 for Uranus, and 13 for Neptune).
► 4 terrestrial planets: 3 total satellites.
► Pluto has 3 satellites. Eris has one satellite.
► Each Jovian planet has a ring or ring system.
None of the terrestrial planets do.
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A comparison of planetary characteristics
Terrestrial
Near the Sun
Small
Mostly solid
Low mass
Slow rotation
No rings
High density
Thin atmosphere
Few moons
Jovian
Far from the Sun
Large
Mostly liquid & gas
Great mass
Fast rotation
Rings
Low density
Dense atmosphere
Many moons
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Lecture 7: A Planetary Overview
Feature 3: Asteroids
and Comets
Asteroids
Asteroids
► These
rocky bodies orbit the Sun, but are
much smaller than planets. Most lie between
Mars and Jupiter
Comets
comets
► Small icy (water, ammonia, methane) objects
which occasionally visit the inner solar system
and become visible
► Comets originate from two regions: the Kuiper
Belt and the Öort Cloud
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Lecture 7: A Planetary Overview
Feature 4: Exceptions
to the Rules
Asteroids
►
There are objects in the solar system that are
unusual or have characteristics which are unusual as
compared to the rest of the solar system. Some
comets
examples:
 Venus and Uranus rotate differently (backwards and on its
side, respectively)
 Small moons of Jupiter and Saturn and the large moon
Triton (around Neptune) revolve in the opposite direction of
the rotation of the host planet.
 While other terrestrial planets have no moons (Mercury,
Venus) or tiny moons (Mars) The Earth’s moon is large
compared to the Earth.
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Lecture 7: A Planetary Overview
Spacecraft Exploration of the Solar System
► Our
knowledge of the solar system has been
dramatically increased by telescopic
observations
► We gone to the Moon to directly explore the
surface and bring back moon rock samples
► Other samples of the solar system have
come to use via meteorites
► But most of our recent knowledge of the solar
system has come from robotic spacecraft
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Lecture 7: A Planetary Overview
Spacecraft Exploration of the Solar System
There are 4 broad categories in which robotic
mission may be classified:
► Flyby: spacecraft goes by planet once
► Orbiter: spacecraft orbits planet allowing longer term
study
► Lander or Probe: spacecraft lands on planet (or
sends a probe to explore the planet), some may
have rovers for mobile surveys of the planet
► Sample return mission: spacecraft lands gets a
sample of the surface and takes off to Earth
All these mission carry some sort of radio for
communication to and from the Earth
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Lecture 7: A Planetary Overview
Spacecraft Exploration of the Solar System
Voy 2
Flybys:
► Generally, the cheapest of the spacecraft
types
► Fuel is only used to change the course of the
spacecraft
► Some spacecraft such a Voyager 2 use
gravity assists (gravity slingshots) to help the
spacecraft change direction and increase
speed
► They carry telescopes, cameras and
spectroscopes.
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Lecture 7: A Planetary Overview
Spacecraft Exploration of the Solar System
Orbiters:
►
►
►
►
►
More expensive than flyby mission because they must carry
more fuel so they can get into an orbit
Sometimes an orbit may be very large and elliptical and must
be changed to get it more circular and closer to the planet
Some spacecraft have used a technique of aerobraking or
skimming the atmosphere to shrink the orbit to a smaller size
Orbiters have equipment like flyby spacecraft plus instruments
to detect magnetic fields and radar to measure precise
altitudes
Orbiters have been sent to the Moon, Venus, Mars, Jupiter,
Saturn and to the asteroid Eros
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Lecture 7: A Planetary Overview
Spacecraft Exploration of the Solar System
Landers or Probes:
►
►
►
►
►
Sprit
One can get the closest to a planet by landing on it or sending a
landing
probe through the atmosphere.
Galileo sent a probe through Jupiter’s atmosphere sending
information about temperature, pressure, composition, and radiation
before the signal was too faint to be detected. It presumably was
crushed by the high pressure atmosphere.
Planets with solid surfaces, landers can provide close up views and
local weather monitoring. Some landers may have rovers which can
venture across the surface (like Spirit and Opportunity).
Landers require more fuel since they must land softly on the surface,
but the spacecraft which brought the rovers “crashed-landed” on the
surface using protective airbags.
Landers have been sent to the Moon, Venus, Mars. A probe went
into Jupiter, and on Titan.
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Lecture 7: A Planetary Overview
Spacecraft Exploration of the Solar System
Sample Return Mission:
►
The hardest mission of them all is to land on a
surface, gather sample and return home.
► The only sample return mission are the Apollo
mission to the moon, the Soviet robotic mission to
the moon in the early 1970s
► A slight variation to this occurred with Stardust which
collected comet dust and returned to the Earth
► There are plans to have a sample return mission to
Mars
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Lecture 7: A Planetary Overview
Spacecraft Exploration of the Solar System
Combination spacecraft:
Cassini
►
Many mission have combined more than one type of
spacecraft.
► The Viking missions of the 1970s had an orbiter and
two landers
► The Galileo mission had an orbiter and a probe that
went into the Jupiter atmosphere
► The Cassini mission had an orbiter and a probe
(Huygens) that went to Titan
List of selected robotic missions
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The End
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