Science 3210 001 : Introduction to Astronomy
Lecture 3 : Overview of the Solar System, The
Sun, and the Inner Planets
Robert Fisher
Items
 Solution sets 1 and 2 have been posted, as well as homework
assignment number 4, due next week (2/23)
 Syllabus typo -- Spring break is the third week of March, not
February. Everything moves up a week, so first midterm is in two
weeks time.
 Homeworks
Review Week 2
 Celestial Sphere
 Zenith, Nadir, Meridian, Equinox, Solstice
 Retrograde Motion
Review Week 3
 Kepler’s Three Laws
 Newton’s Three Laws
 Spectra -- Continuum, Absorption, Emission
Today’s Material
 A Few Comments about the Primary Colors and Color
Photography
 Overview of the Solar System
 Planets, Moons, Rings, Asteroids, Comets…
 Fundamentals of Planetary Physics
 The Inner Solar System
 The Cratered Worlds of Mercury and the Moon
 Venus and Mars
 Earth
A Few Comments About Color Theory and Color
Photography
The Primary Colors
 There are three primary colors precisely because the human eye
has three types of cone photoreceptor cells, each sensitive to one
band of light.
Cone Cell Color Response
 Each of the the three types of cone cells has a different
biochemical makeup with a different color response curve :
Solar Spectrum
 The reason why our rod cells have a peak absorption at roughly
500 nm in wavelength is simply because the solar spectrum
peaks at that same wavelength :
First Color Photograph
 James Clerk Maxwell produced the first color photograph in 1861
using three images photographed on black and white positive
films, filtered through each of the primary colors :
Overview of the Solar System
Overview of Solar System
 The Sun. In a sense, the sun is the solar system. 99.9% of the
total mass of the solar system, and also the source of the vast
majority of energy.
Sun Compared with Planets
 A 3D computer rendering of the sun and the planets, all
compared to-scale :
Overview of Solar System
 The Planets
 Inner planets are close to the sun -- consisting of relatively warm
rocky bodies, and thin to non-existent atmospheres.
 Outer planets are further from the sun -- they have rocky cores and
enormous gaseous atmospheres which constitute the bulk of the
planet.
Comparison of Inner Planets
 A 3D computer rendering of the inner planets and the moon, all
compared to-scale :
Comparison of Outer Planets
 A 3D computer rendering of the outer planets, compared with the
inner planets, all to-scale :
Question
 Which planet can never be seen on the Meridian at midnight?
 A) Mercury
 B) Mars
 C) Jupiter
 D) Saturn
Overview of Solar System
 Moons / Rings
A moon is simply a natural satellite of a planet. There are over 100
known moons -- most around Jupiter and the outer planets.
The largest moon in the solar system (Jupiter’s Ganymede,
discovered by Galileo) is larger than Mercury. If it were orbiting
the sun, it would be classified as a planet.
Some of the largest moons (most noteably Saturn’s Titan) have
atmospheres. Others show signs of active geological activity,
including one (Jupiter’s Io) that shows direct signs of volcanic
activity.
Ganymede
 Ganymede, as photographed by the Galileo space probe.
A Theoretical Model of Interior of Ganymede
Io
 Jupiter’s moon Io is volcanically active, spewing sulfur plumes.
Overview of Solar System - Rings
 All four of the outer solar system planets have ring systems,
though Saturn’s is by far the most spectacular.
 These rings are the remnant material left over from the formation
of the moons surrounding the planet, and are made up of
enormous numbers of icy rocks.
 In a sense, the moon/planet systems are a kind of “mini solar
system,” and the rings are analogous to a scaled-down version of
the asteroid belts in our own solar system (which we will discuss
in just a moment).
Artist’s Conception of Saturn’s RIngs
Saturn Imaged by the Cassini Spacecraft
Saturn’s Rings Imaged by Voyager
Overview of Solar System - Minor Bodies
 Dwarf Planets - a very new category.
 According to the new International Astronomical Union (IAU)
specification laid down last year, planets are officially defined to
meet three criteria :
 Major bodies orbiting the sun (or another star).
 Large enough to be spherical in shape.
 Have swept their neighborhood clear.
 Dwarf planets satisfy the first two criteria, but fail the third.
Examples include Pluto and the asteroid Ceres.
Disk Clearing / Gap Formation
 A large enough body will sweep up the material its neighborhood.
Here, for instance, is the result of a simulation of the early solar
system, before it had been cleared of gas and dust :
Star
Gap
Planet
Disk (in greyscale)
Overview of Solar System - Minor Bodies
 Asteroids
 The asteroids were once thought to have been a “broken-up” planet,
but they actually contain far too little mass to constitute a planet.
They are the left-over products of planet formation.
 They range in size from the spherically-shaped Ceres (almost 1000
km in radius) down to much smaller bodies barely a few kilometers
across.
The Asteroid Belt - Minor Bodies
 Most asteroids are clustered between the orbits of Mars and
Jupiter. It is thought that Jupiter’s enormous gravitational force
kept these bodies from coalescing into a rocky planetary core.
Vesta, Ceres, and the Moon
 Side-by-side comparison of two of the largest asteroids (Vesta
and Ceres) and the Earth’s moon :
Question
 Why are the larger bodies in previous image rounder than the
smaller ones?
The Asteroid Eros
 Eros was visited by the spaceprobe NEAR Shoemaker, which
began its orbit around the asteroid on February 14, 2000.
Eugene Shoemaker (1928 - 1997)
Barringer (or Meteor) Crater in Arizona
 Shoemaker’s great legacy was to establish that catastrophic
impacts do occur throughout the solar system.
Overview of Solar System - Minor Bodies
 In addition to the asteroid belt, another belt of bodies orbits the
sun beyond the orbit of Neptune -- the Kuiper Belt.
 It is thought that Pluto is in fact a Kuiper belt object -- one of the
largest.
 Because of their great distance from the sun, Kuiper Belt Objects
are much harder to detect than asteroids. 800 objects have been
detected to date.
Kuiper Belt Objects
 Like the case of the asteroid belt, the Kuiper Belt is “debris” left
over from the early solar system. In this case, they have been
missed by Neptune’s “sweeping” of its neighborhood.
Overview of Solar System - Meteoroids, Dust,
and the Solar Wind
 Meteoroids, Dust, Solar Wind
 The trail of comets is filled with tiny meteoroids. When the Earth’s
orbit intersects a comet’s trajectory, we experience a meteoroid
shower on Earth. These happen at regular dates on the calendar
each year.
 The collision of rocky bodies over the course of history of the
solar system produces smaller bodies, down to dust-sized
particles. These dust-like particles are responsible for the
zodiacal light effect.
 The outer layers of the sun continuously blow away a stream of
charged particles, referred to as the solar wind.
Magnetic Field
 The magnetic field surrounding a magnet can be visualized using
iron filings.
Solar Atmosphere
 The outer layers of the sun are incredibly active, powered by the
Sun’s intense magnetic field.
Fundamentals of Planetary Physics
 The properties of the planets are largely determined by a few crucial
physical parameters -- its mass, rotational rate/inclination, and surface
temperature.
 The combination of mass and surface temperature, for instance, will
determine the atmospheric content of the planet.
 Larger bodies tend to be more geologically active than smaller bodies.
 More rapidly rotating bodies tend to have stronger magnetic fields.
Planetary Magnetic Fields
 The interior of the planets contains electrically conducting iron
and nickel, which flow like fluids over very long timescales.
 Planetary rotation sets these fluids in motion and generates
currents, which in turn generate magnetic fields surrounding the
planet.
Earth’s Magnetic Field
 Earth has the strongest magnetic field of the inner planets in the
solar system.
 This is believed to be important in protecting life on Earth from
harmful charged particles from the Sun and elsewhere.
Earth’s Aurora
Charged particles from the solar wind become trapped in the Earth’s
magnetic field and stream down to the poles, generating the
phenomenon we see on earth as the Aurora.
Why are Some Moons and Planets Geologically
Active and Others Dormant?
 Geological activity requires a source of energy.
 That source of energy is the heat interior to a body.
 The amount of heat energy contained in a body is proportional to
its volume.
 The rate at which heat energy is lost is proportional to its surface
area.
 Consequently, smaller bodies cool more rapidly than larger
ones. As a result, smaller bodies tend to be less geologically
active.
Examples of Planetary / Lunar Interiors
Atmospheric Physics
 In a gas in thermal equilibrium each molecule shares the same
kinetic energy. This means that lighter molecules must be moving
faster on average to have the same kinetic energy as heavier
molecules.
H2
O2
Does Gravity or Heat Win?
 Gravity exerts an inward pull on atmospheric molecules. Kinetic
energy (in the form of heat) causes them to want to escape.
O2
Gravity
H2
Atmospheric Physics and the Giant Planets
 The fate of a molecule is determined by the planet’s mass and
the temperature of the atmosphere -- the larger the mass, the
more species it can retain.
 What about the most common element, hydrogen, and its
molecular counterpart H2?
 The Earth, and all inner solar system planets, lack the sufficient
mass to retain H2.
 The outer solar planets do have sufficient mass to retain H2.
 This observation explains how the outer solar system planets
grew to become giant planets.
How do Interplanetary Space Probes Work?
 The night-time launch of Voyager 1 on a Titan-Centaur rocket in
1977.
How do Interplanetary Space Probes Work?
 A space probe in orbit about the sun must obey the same laws
laid down by Kepler, which apply to all orbiting bodies.
Space Probe
Planet 2
Planet 1
Sun
Question
 Is it possible to send a satellite to the sun? What would its orbit
look like?
Gravitational Slingshot Effect
 If a spacecraft were to encounter a stationary body, its final speed
must equal its initial speed.
Gravitational Slingshot Effect
 The same encounter, viewed from the reference frame of the sun,
looks very similar, except that the spacecraft has acquired the
motion of the planet. It has gotten an “assist” from the planet.
Planetary Grand Tour
 An rare alignment of the planets (occuring once every 200 years)
permitted the Voyager 1 and 2 space missions a “Grand Tour” of
the solar system using the gravitational slingshot effect.
Cratered Worlds : Mercury and the Moon
 The cratered world of the Mercury and the moon share many
properties in common with one another.
 Both are too small to have any substantial atmosphere.
 Without any atmospheric erosion, their surface records the
earliest period of formation and subsequent history.
Mercury
 Mercury is the smallest of the rocky inner planets, and is the least
similar to Earth
 Orbits the sun at .4 AU, and has a slow rotational speed with
extreme seasons
Mercury Interior
 Knowing the mass of Mercury, scientists have modeled the
interior.
 Among the rocky planets it is unusual for its very large iron core -possibly due to an early impact.
Moon
 The Earth’s moon is the nearest body in the solar system, with a
radius about 25% that of the Earth, and a mass about 1% of the
Earth.
 The surface of the moon has been extensively studied by several
unmanned and manned exploration missions.
 Without weathering, the surface is ancient in comparison to the
Earth -- almost 4 billion years old.
 Scientists have concluded that the moon most likely originated
from a giant impact early in the history of the solar system.
 The cratering history indicates that impacts peaked in the distant
past -- around 4 billion years ago.
Daedelus Crater on Far Side of Moon
Venus
 Venus is often referred to as Earth’s “sister planet” because of the
similarity in orbit, size, mass, and rocky composition.
 Venus orbits the sun at a distance of .73 AU from the sun.
 Venus’s radius is 95% that of the Earth, and its mass is 82% that of Earth.
Both planets have a molten core and a rocky composition, and nearly equal
surface gravities.
Venus Photographed by Magellan Spaceprobe
in Optical
Venus Imaged by the Magellan Spaceprobe in
Radio (false color)
Venus vs. Earth
 Despite these similarities, Venus is greatly dissimilar from the Earth in
other respects :
 Venus’s thick atmospheric surface pressure is some 90 times that of the
Earth, made up almost entirely of carbon dioxide.
 Venus’s surface temperature is incredibly hot -- nearly 900 degrees
Fahrenheit (!) -- hot enough to melt lead. Clearly, no surface water
exists.
 Venus has no moon.
 Venus’s rotational axis is has a tilt of just 3.4 degrees as opposed to 23.5
degrees for the Earth.
 Venus is a very slow rotator -- A Venusian day is one Venusian year
(243 Earth days).
 Venus lacks a magnetic field.
Venusian Atmosphere
 The Venusian Atmosphere is hottest at the surface, but already
by the upper cloud layer the temperature is much higher than on
Earth.
80 F
620 F
940 F
Mars
 The “red planet” Mars is the current focus of NASA’s unmanned
interplanetary missions, because it is believed to have once
harbored a warm, moist Earth-like phase -- possibly even life.
 There are several similarities between Earth and Mars.
 Mars orbits the sun at 1.5 AU.
 Its axis is tilted at 25 degrees.
 Its day is nearly identical to one Earth day.
Mars vs. Earth
 Mars is much smaller than the Earth, with a radius about half that of
Earth, and a mass of about a tenth the Earth’s.
 The surface temperature today is far below freezing.
 Even if one could warm water ice on Mars today, it would go directly into
a gaseous state without becoming liquid because of the thin atmosphere.
 It has two tiny moons, Phobos and Deimos.
 While tilt is similar to that of Earth today, the tilt angle oscillates wildly
over tens and hundreds of millions of years.
 It has only a weak magnetic field in its crust, and lacks a magnetic core.
Next Week -- Global Warming, and the Outer
Planets
 Next week we will continue our discussion of Mars and the Earth,
comparing the geology of the two planets.
 We will discuss the consequences of enriching our atmosphere
with carbon dioxide -- is Earth destined to become like Venus?
 We will also begin to cover the Outer Planets.
 First midterm in two weeks!