Science 3210 001 : Introduction to Astronomy

Lecture 4 : Overview of the Solar System, The

Sun, and the Inner Planets

Robert Fisher

Items

! First Midterm in two weeks.

! Homeworks / textbooks

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 warmrocky bodies, and thin to non-existent atmospheres.

! Outer planets are further from the sun -- they have rocky cores andenormous gaseous atmospheres which constitute the bulk of theplanet.

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 100known 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 orbitingthe sun, it would be classified as a planet.

Some of the largest moons (most noteably Saturn’s Titan) haveatmospheres. Others show signs of active geological activity,including one (Jupiter’s Io) that shows direct signs of volcanicactivity.

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 tomeet 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 :

Planet

Star

Disk (in greyscale)

Gap

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’sorbit intersects a comet’s trajectory, we experience a meteoroidshower on Earth. These happen at regular dates on the calendareach year.

! The collision of rocky bodies over the course of history of thesolar system produces smaller bodies, down to dust-sizedparticles. These dust-like particles are responsible for thezodiacal light effect.

! The outer layers of the sun continuously blow away a stream ofcharged particles, referred to as the solar wind.

Magnetic Field

! The magnetic field surrounding a magnet can be visualized usingiron filings.

Solar Atmosphere

! The outer layers of the sun are incredibly active, powered by the

Sun’s intense magnetic field.

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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.

H2

O2

Gravity

Atmospheric Physics and the Giant Planets

! The fate of a molecule is determined by the planet’s mass andthe temperature of the atmosphere -- the larger the mass, themore species it can retain.

! What about the most common element, hydrogen, and itsmolecular counterpart H2?

! The Earth, and all inner solar system planets, lack the sufficientmass to retain H2.

! The outer solar planets do have sufficient mass to retain H2.

! This observation explains how the outer solar system planetsgrew 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.

Planet 2

Planet 1

Space Probe

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.

Surface of the Moon

Daedelus Crater on Far Side of Moon

Venus

! Venus is often referred to as Earth’s “sister planet” because of thesimilarity 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 equalsurface 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 inother respects :

! Venus’s thick atmospheric surface pressure is some 90 times that of theEarth, made up almost entirely of carbon dioxide.

! Venus’s surface temperature is incredibly hot -- nearly 900 degreesFahrenheit (!) -- hot enough to melt lead. Clearly, no surface waterexists.

! Venus has no moon.

! Venus’s rotational axis is has a tilt of just 3.4 degrees as opposed to 23.5degrees 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!