James T. ShipmanJerry D. WilsonCharles A. Higgins, Jr.

Moons and Smaller Solar System Bodies

Chapter 17

Sections Covered: 1, 2 & 6

Hmwk: M: a-k, m-t & w-y; MC: 1-14 & 24-26; Ex: 2, 4, 6, 10, 12 & 14

Moons and Small Solar System Bodies

• Moons are natural satellites• 176 are known at present• Many are small (less than 1 km in diameter) and

some are large.

• Some small solar system bodies orbit beyond the orbit of Neptune – Trans Neptunian Objects

• Other objects are• Comets• Asteroids • Interplanetary dust

Intro

The Moon

• July 20, 1969 – humans first landed on moon• Retroreflector – placed on moon and designed

to reflect a laser beam from Earth• Rates of Continental Drift on Earth, change in Earth’s

tilt, distance to moon, Gravitational Constant (G)

• Average distance = 384,000 km (240,000 mi)

Section 17.1

The Moon

• Second brightest object in the sky (what is brightest?)

• “moon” – unknown origin of the word• Many primitive and modern societies base their

religious ceremonies on the cycles of the moon (e.g., new and full moons).

• Our month is based on moon’s cycle.• Human ovarian cycle is also synchronized to the

29.5 day lunar cycle.

Section 17.1

General Features of the Moon

• Fifth-largest moon in solar system• Largest moon of any of the terrestrial planets

(Mercury, Venus, Earth, and Mars)• One complete revolution = 29.5 days (approx.)• The moon rotates on its axis at the same rate as it

revolves around Earth; therefore, we only see one side of the moon.

• Since the moon rotates every 29.5 days, the sun would appear to rise/set every 29.5 days.

• The surface features of the moon formed millions of years ago without erosion to erase them.

Section 17.1

Characteristics of the Moon

• Nearly spherical, with a diameter of 3476 km (2160 mi) – approx. ¼ the Earth’s diameter

• Mass of the moon = 1/81 of the Earth• Average density of 3.3 g/cm3 (Earth is 5.5)• Surface gravity of the moon is only one-sixth of

Earth’s.• Therefore one’s weight on the moon would only

be one-sixth of that on Earth.• Average reflectance (albedo) = only 7% (only

7% of the light received from the sun is reflected)

Section 17.1

Composition and History of the Moon

• Using radiometric dating techniques• Mountain rocks – 4.4 to 3.9 billion years old• Plain rocks – 3.9 to 3.1 billion years old

• No rocks older than 4.4 or younger than 3.1 billion years old have been found.

Section 17.1

Composition of the Moon

• From inside to surface -- Inner core (iron rich solid), outer core (iron rich semi-liquid), rocky mantle, crust, regolith (covers the crust with pulverized dust and rock)

• Crust thickness varies across the surface• Presently does not have a magnetic field (not

rotating fast enough)• Interestingly though, the rocks brought back

show some magnetism, indicating that the moon had a slight magnetic field at the time of rock crystallization.

Section 17.1

Origin of the MoonMost widely accepted theory: great impact

• Planet-sized object (size of Mars) struck Earth with glancing blow 4.4 billion years ago

• Composition of lunar mantle and crust similar to Earth’s mantle

Section 17.1

Prominent Features of the Moon

• Besides phases of the moon, moon’s most prominent features include craters, basins, plains, rays, rills, mountains, and faults.

• Crater – Greek for “bowl-shaped”• Large to small• Believed to have been formed by meteorite impacts

• Highlands: cover 85% of lunar surface• Light colored surface• Extend to several kilometers above lunar surface• Form an older lunar crust

Section 17.1

Lunar Plains (maria- Latin for ‘seas’)

• Dark, flat areas believed to be craters formed by meteorite impact that filled with volcanic lava

• Surface covered by layer of loose debris called regolith

• Rocks are similar to volcanic rock on Earth• Plains produced by volcanic eruptions and the

resulting enormous lava flows• Major eruptions: from 3.9-3.1 billion years ago• Plains are more common on the near side of the

moon, probably due to the thinner crust

Section 17.1

Craters and BasinsBest known features of moon’s surface

• ~30,000 seen using an Earth-based telescope.• Range in size – microscopic to 100’s km• Shapes of craters change with diameter.

• 1 km – smooth bowl-shaped interiors• >1 km – flat interiors• >>1 km – flat floor with central peak

• Most craters formed 4.4-3.9 billion year ago- time of intense meteorite bombardment.

• 3.1 billion years ago: moon cooled down so that molten rock could no longer get to the surface.

• Moon appears to have been geologically quiet for the past 3.1 billion years or so.

Section 17.1

Craters on the Moon

Section 17.1

Rays and Rills

• Rays: streaks extending from some craters• Thought to be pulverized rock that was thrown out

when the crater formed• Rays are brighter than the crater

• Rills: long, narrow trenches or valleys• Some are straight and some are curved.• Thought to represent a separation (or crack) caused

by moonquakes• These are similar to the features known to form from

earthquakes.

Section 17.1

The Straight Wall

• A unique steep slope on the eastern side of Mare Nubium

• The wall is 113 km long by 244 m in height.

• Rima Birt 1 (upper left): irregular rill

Section 17.1

Mountain Ranges and Faults

• Mountain ranges: as high as 6100 m (>19,000 ft)• All mountain ranges appear to be formed in a

circular pattern, bordering the lunar plains.• Therefore, probably not formed by the same

processes as mountain ranges on Earth.• Fault: a break or fracture in the surface of the

moon, along which movement has occurred• Since there is little/no erosion on the surface of

the moon, the difference in elevation caused by these faults remain for millions of years

Section 17.1

Lunar Motion Effects

• The lunar orbital plane does not coincide with Earth’s orbital plane.• Approximately 5o with respect to Earth’s orbital plane.

• Due to this 5o tilt, it is possible for the moon to be directly overhead at any latitude between 28.5oN and 28.5oS.

• Both the rotation of the Earth on its axis and the moon’s revolution around the Earth are counterclockwise (from a N pole perspective).

Section 17.2

Relative Motions of the Moon and Earth

Section 17.2

Two Different Lunar Months

• Sidereal Month – 27.33 days, lunar cycle with respect to a star other than the Sun

• Synodic Month – 29.5 days, lunar cycle with respect to the Sun• From one’s perspective on Earth the synodic month

is one complete month of lunar phases.• In actuality the moon revolves more than 360o during

a synodic month.

• From Earth it appears that every day the moon rises in the east and sets in the west.

Section 17.2

Phases of the Moon

• The moon’s periodic change in appearance is its most outstanding visual feature.

• One-half of the moon is always reflecting light from the Sun, but only once during that cycle can an observer on Earth see the entire illuminated half, called a “full moon.”

• The starting point for the moon’s synodic month is arbitrarily taken a the “new moon” position.

Section 17.2

New Phase or New Moon

• Occurs when earth, Sun, and moon are all in the same plane, with the moon positioned between the Sun and Earth

• At this position, the dark side of the moon is fully toward the Earth (“dark of the moon”).

Section 17.2

Lunar Phase Vocabulary

• Waxing phase - the illuminated portion is getting larger

• Waning phase – the illuminated portion is getting smaller

• Crescent moon – less than ½ of the visible portion of the moon is illuminated

• Gibbous moon – more than ½ of the visible portion of the moon is illuminated

Section 17.2

Phases of the MoonAs observed from any latitude north of 28.5oN

The “observer” is looking south; therefore east is on the left.

Section 17.2

Waxing Phases of the Moon

• Waxing crescent phase – appears as a crescent moon, less than 90o east of sun (7.375 days)

• First-quarter phase – when the moon is exactly 90o east of sun

• Waxing gibbous phase – appears larger than ½ illuminated, but < a full moon (7.375 days)

• Full Moon – 180o

Section 17.2

Waning Phases of the Moon

• Waning gibbous phase – appears < a full moon, but larger than ½ moon (7.375 days)

• Last-quarter phase – when the moon is exactly 270o east of sun

• Waning crescent phase – appears smaller than ½ illuminated (7.375 days)

• New Moon – 360o

Section 17.2

Phases of the Moon

Section 17.2

Altitude of the Full Moon

• A full moon is always on the opposite side of the Earth from the Sun.

• Thus when the Sun is at its lowest position (winter solstice), the moon will be at its highest.

• Due to the 5o difference in the Earth’s and moon’s orbital planes, the moon will be situated directly over the 28.5o N latitude line on the winter solstice.

Section 17.2

Eclipses

• The Sun provides the light for our solar system• Planets and moons within the solar system cast

shadows that extend away from the Sun• The size and shape of the shadow cast depends on

the object’s size, shape, and distance from the Sun• The Earth and the Moon cast conical shadows, as

viewed from space

• Eclipse – the darkening of the light of one celestial body by another

Section 17.2

Eclipses

• The shadows cast by the Earth and the Moon cast two different degrees of darkness

• Umbra – the darkest and smallest region• A total eclipse occurs within the umbra region

• Penumbra – the semidark region• A partial eclipse occurs within the penumbra region

Section 17.2

Solar Eclipse

• Solar eclipse – occurs when the moon blocks some or all of the Sun’s rays from an observer on Earth

• A solar eclipse occurs when the moon is at or near new phase and is in or near the ecliptic plane

• When the moon lies between the Sun and Earth in nearly a straight line, the moon’s shadow will fall on the Earth

Section 17.2

Positions of the Sun, Moon, and Earth During a Total Solar Eclipse

The umbra and penumbra are, respectively, the dark and semidark shadows cast on the Earth by the Moon

Section 17.2

Types of Solar Eclipses

• The length of the Moon’s shadow varies with the moon’s distance from the Sun• In some cases the Moon’s umbra does not reach all

the way to Earth• If the umbra does not reach Earth, an observer on

Earth would see the Moon’s disk projected against the Sun and a bright ring or annulus outside the dark Moon – this is called an annular eclipse

• The maximum diameter of the umbra on Earth is about 270 km

• Total eclipse when the umbra reaches Earth

Section 17.2

Solar Eclipses

Section 17.2

An annular eclipse of the Sun.Umbra of the Moon’s shadow does not

reach all the way to the Earth

Solar corona during a total solar eclipse

Lunar Eclipse

• Lunar eclipse - occurs when the Earth blocks some or all of the Sun’s rays to the Moon

• A lunar eclipse occurs when the Moon is at or near full phase and is in or near the ecliptic plane

• When the Earth lies between the Sun and Moon in nearly a straight line, the Earth’s shadow will conceal the face of the Moon• A total lunar eclipse can last for more than 1.5 hours,

and partial eclipses may last for over 3 hours

Section 17.2

A Lunar Eclipse

Section 17.2

A Lunar Eclipse

Section 17.2

Ocean Tides

• Tides – the regular and alternating rise and fall of the ocean’s surface level

• Due to the gravitational attraction that the Sun and Moon exert on Earth.

• The Moon, because it is closer, has a greater effect on the tides.

• Actually it is the difference in the gravitational attraction at various parts of Earth that causes the tides.

Section 17.2

Tidal BulgesThe two tidal bulges result in

two high tides and two low tides daily

Section 17.2

Ocean Tides

• One of the tidal bulges is caused by the Moon’s gravitational attraction on the side of the Earth nearest to the Moon

• Another tidal bulge is located on the side of the Earth farthest away from the Moon

• This distant tidal bulge is caused by the force of gravity pulling the Earth away from the water on the distant side, forming a second and opposite bulge

Section 17.2

Spring Tide

• Spring tide occurs when the Sun, Moon, and Earth are all positioned in nearly a straight line

• In this situation, the gravitational forces of the Sun and Moon combine to produce higher high tides and lower low tides

• The variations between high and low tides are the greatest during a spring tide

• Spring tides occur twice during each lunar month; at new and full moons

Section 17.2

Neap Tide

• Neap tide occurs when the Sun and the Moon are at angles of 90o with respect to the Earth

• In this situation, the gravitational forces of the Sun and Moon tend to cancel and produce lower high tides and higher low tides

• The variations between high and low tides are at a minimum during the neap tide

• Neap tides occur twice during each lunar month; at first and last quarter

Section 17.2

Spring and Neap Tides

Section 17.2

Tidal Height Varies

• The height of the tide also varies with latitude• The two tidal bulges are the highest at the

latitude of the overhead position of the moon and on the other side of the Earth opposite the moon’s position

• The maximum height of the tidal bulge is at 23.5oN and 23.5oS, due to the overhead position of the moon and the Sun on the summer solstice

Section 17.2

Tidal Effects on the Earth/Moon System

• The regular tidal cycle retards the Earth’s rotation at a rate of about 0.002 s per century

• Since angular momentum must be conserved, this decrease in Earth’s angular momentum results in an increase in the moon’s angular momentum• The moon’s orbit is increasing about 1.3 cm/y

• If we went back 1 billion years, the solar day would have been 5.6 h shorter, and the moon would have been 13,000 km closer to Earth

Section 17.2

Small Solar System Bodies

• Asteroids• Meteoroids, Meteors, and Meteorites• Comets• Interplanetary Dust

Section 17.6

Asteroids

• Asteroids - over 20,000 named or numbered objects that orbit the Sun in a belt between Mars and Jupiter• This belt contains millions of asteroids• About 100,000 are bright enough to be photographed

by Earth-based telescopes

• Sometimes referred to as minor planets

Section 17.6

Asteroids or Minor Planets

Section 17.6

Origin of Asteroid Belt

• Sometimes attributed to the breakup of a former planet• Estimates of the total mass of the asteroids is much

less than the mass of an ordinary planet.

• A more plausible explanation is that the debris never formed into a planet• The enormous gravitational force Jupiter may have

stirred up nearby debris, preventing it from condensing into a planet.

Section 17.6

Meteoroids, Meteors, and Meteorites

• Meteoroid – interplanetary metallic and stony objects that range in size from sub-millimeter to 100’s of meters.

• Meteor – a meteoroid that enters the Earth’s atmosphere and becomes luminous – a “shooting star.”

• Meteorite – a meteor that survives the flight through the Earth’s atmosphere and strikes the Earth.

Section 17.6

Barringer Meteorite Crater, Arizona

• Meteorite impact is estimated to have been 50,000 years ago, when a large meteorite struck near Winslow, AZ.

Section 17.6

Comets

• Relatively small objects that are composed of dust and ice, and revolve around the Sun in a highly elliptical orbit

Section 17.6

Parts of a Comet

• A comet consists of four parts• Nucleus – typically a few kilometers in diameter and

composed of rocky or metallic material• Coma (head) – surrounds the nucleus. Several

hundred kilometers in diameter. Formed from the nucleus as it approaches 5AU of the Sun.

• Tail – long and voluminous composed of ionied molecules, dust, or a combination. Can be millions of kilometers in length.

• Hydrogen cloud – surrounds coma and thought to be composed of dissociation of water molecules from the nucleus.

Section 17.6

The Principal Parts of a Comet

Section 17.6

Comet

• Note how the comet’s tail is affected by and is directed away from the Sun, and does not indicate the comet’s direction of movement.

Section 17.6

Micrometeorids or Interplanetary Dust

• A tremendous amount of interplanetary dust can be found throughout the Solar System.

• Two celestial phenomena can be observed and photographed, showing that dust particles do exist:• Zodiacal light—a faint band of light along the zodiac.• Gegenshein “counterglow”—due to sunlight reflected

from dust particles

Section 17.6