Chapter 5

Earth and Its Moon

The images of the Earth and

Moon are to scale

Reading assignment: Chapter 5

Earth •Interior regions: Crust, mantle and core

•Solid inner core about 1300 km radius

• Liquid outer core around 3500 km radius

• Atmosphere, most located less than 50 km

from surface

•It has an hydrosphere

• Magnetosphere, area dominated by the

terrestrial magnetic field. Charged particles

from the Sun are caught in Earth’s magnetic

field

• No hydrosphere, atmosphere or

magnetosphere

• Same basic interior regions as Earth, crust,

mantle and core but their properties are

different

Moon

Tides: What are they and what causes them?

Let’s review about some basics concepts regarding gravity

• Every mass in the Universe has gravity associated with it.

•Gravity is an attractive force - it tries to pull things together.

RF 2g

1

Force of gravity between two objects (Fg) is inversely proportional

to the square of the distance between them (R).

The tides are caused by the differential gravitational force

A plot of the gravitational force (The force of gravity is proportional to 1/R²)

Inverse square

law

RF 2g

1

Example:

R= 1 F=1

R=2 F=1/4

R=3 F=1/9

Inverse square law How can we understand the differential gravitational

attraction?

The differential gravitational force and the tidal effect The differential gravitational force is the difference between the

gravitational forces on the two sides of the body, caused by another body.

Example are the Earth and the Moon

The force on the side facing the other body is stronger than the force of

the opposite side

How does gravity cause tides?

• Moon’s gravity pulls harder on the near side of Earth than on the far side.

• Difference in Moon’s gravitational pull stretches Earth. The mantle and the

crust are solid, not easy to stretch, but the oceans are more easy to be distorted.

• What causes the tidal effect is the differential gravitational attraction

Gravitational attraction

stronger here

Gravitational

attraction weaker here

In the case of the Earth and the Moon

• The Moon is exerting a greater

gravitational force on the near

side of the Earth as compared to

the far side causing the Earth to

be displaced slightly toward the

Moon.

• Oceans are easy to be distorted.

This causes the Earth to

experience a tidal bulge on the

near and farther side which is

manifested in the ocean tides that

occur twice a day.

• The combined effects of the Sun and the

Moon produce variations in high and low

tides

The Sun is about 375 times farter away

than the Moon but its mass is much larger,

2.0 x10^30 kg compared with the moon,

7.4x10^22 kg. A factor of 27 million in

mass

The Sun still have an effect in the tidal

force on the Earth. The tidal effect is

smaller due to the small differential

gravitational force

Variation in the Strength of the Tides

Tides and Phases Size of tides depends on the

phase of the Moon.

Spring tides occur at new

and full moon. The tidal

forces of the Sun and the

Moon reinforce each other

(add)

Neap tides occur at first

quarter and third quarter

moon when the Sun and the

Moon tidal forces act at

90degrees from each other

The Earth develop two

bulges. A consequence of

this is that every 24 hours

there are two high tides and

two low tides.

Spring tides

Neap tides

Over time, tides have the following effects on the Earth and the Moon:

1. Slowing the Earth’s rotation - the day

is increasing by 0.002 sec/century.

2. Increasing the size of the Moon’s orbit

- its distance from the Earth is increasing

by 4 cm/century (2 inch/century)

Effect of tides on the Earth rotation, bulge and the Moon orbit

Because the Earth rotates

faster (24 hours) than the

moon orbital period (27

days), the bulge is offset

from the point right below

the Moon. This creates

friction (lost of energy)

between the oceans and the

crust. This slow down the

Earth rotation

Tidal Locking

• The Earth rotation will slow down over a period of billions of years so

that the orbital period of the Moon and the rotational period of the

Earth will coincide. Both periods will be synchronized or tidally

locked.

• Under that condition, the Moon will always be above and visible in the

sky over one hemisphere of the Earth, the hemisphere facing the Moon

and under the horizon and not visible from the other side of the Earth

Does the Sun has a tidal effect on the Earth?

• The Sun has much more mass than the Moon.

• The Sun gravitational attraction on the Earth is larger than

the Moon gravitational attraction.

• But the Sun is much farther away than the Moon.

• The differential gravitational attraction of the Sun on the

near and far side of the Earth is smaller than the

differential gravitational effect of the Moon

• Both the Sun and the Moon cause tidal effect on Earth

• The moon has a larger differential gravitational attraction

and causes larger tides.

Earth’s Atmosphere Composition: • nitrogen (78%)

• oxygen (21%)

• argon (0.9%)

• carbon dioxide (0.03%)

• water vapor (0.1-3%)

Convection occurs here in the

troposphere, below 12 km

Convection cause weather

The ionosphere is the layer above

80 km. Part of the atoms are ionized

by the solar UV radiation

The Ozone is a molecule formed by

three oxygen atoms O₃. Ozone

absorbs UV light

• It shields from damaging solar

and cosmic radiation. Protects

the surface

• Regulates temperature

Curve of temperature

Convection

Warmer air travels up and cooler air comes down to take its place.

• The result is the development of convection cells which heat the atmosphere.

•Humidity close to the surface rises with the hot air, the water vapor condense

as the temperature decrease and form clouds

• Convection creates surface winds and is responsible for most types of

weather.

The Greenhouse Effect

• Sunlight not reflected by clouds or ices in the poles reaches the Earth’s surface

• Light reaching the Earth is absorbed and heat the surface

• Earth mean temperature about 300 K, (Remember black body radiation)

• The heated Earth re-radiates this light in the form of infrared (IR) radiation

The average Earth temperature is

about 40 K hotter because of the

greenhouse effect than it would be

without it!

• Infrared light is partially blocked by

the Earth’s carbon dioxide and water

vapor content.

• The three main greenhouse gasses

in the terrestrial atmosphere are CO₂ , H₂O vapor and Methane .

• Only part of the IR light goes into

space, part is retained in the

atmosphere

Global warming (Read 5.2 Discovery: The Greenhouse effect and Global Warming

The increase of the Earth global

temperature • The effect of the increase of abundance

of carbon dioxide (CO₂) and the increase

in global temperature.

• Burning fossil fuel increases the CO₂ of

the atmosphere

• Deforestation also decrease the

absorption of CO₂

An important effect:

• An increase in the temperature will

increase the melting of the polar ices.

•That will increase the absorption of

light (energy). Less light coming from

the Sun will be reflected back to space.

•That will increase even more the

temperature, melting more ice

This is a positive feedback that

may create a runaway process!

Why has Earth retained its atmosphere?

(and why the Moon has no atmosphere)

The answer: Gravity! •Gas molecules are in constant motion - hotter gas, faster motion •The gas molecular velocity depends on the temperature, higher temperatures, higher velocities • The fact that the atmosphere is heated keeps it from falling onto Earth. • Important concept: Escape velocity is the speed an object (or molecule) has to have to be able to escape the gravitational attraction (leave permanently) of an object’s surface ( Examples of objects: Planets, satellites, stars, black holes). •If a planet’s escape velocity is at least ≈6 times greater than the mean molecules’ velocity, the molecules of that type will not “escape” in significant quantities.

An example: The Earth

Earth’s escape velocity = 11.2 km/s

Oxygen and nitrogen molecular velocity (For a temperature of about 300 K, ~78 F)) = 0.6 km/s

The ratio of escape velocity/molecular velocity = 11.2/0.6 = 18.7 .

Oxygen may not escape the Earth atmosphere. The escape velocity is about 18.7 times the molecular

velocity

The Moon

Moon’s escape velocity = 2.4 km/s (1/5 of Earth escape velocity)

Ratio of escape velocity/oxygen molecular velocity is 4 (less than 6). Oxygen will escape the Moon.

The escape velocity and the mean molecular speed

Read: Section 5.1 More Precisely

“Why Air sticks around”

• The escape velocity and the molecular velocity formulas (in words)

Escape velocity = 11.2 {Mass of body (Earth masses) /radius of body(Earth radius)}½

Average molecular speed = 0.157 {Gas temperature (K) / molecular mass (Hydrogen atom mass)} ½

The equations for escape velocity and

molecular speed

Vesc = Escape velocity in km/s

M = Mass of body in Earth masses

R = Radius of body in Earth radius

Vmol = Speed of molecules in km/s

T= Temperature in K

W = Molecular mass in Hydrogen atom masses

RMVesc /2.11

WTVmol /157.0

An example: The curve of distribution of

velocities in a gas

This is the

distribution

of thermal

velocities in

a gas

Escape velocity

These atoms

have a velocity

larger than the

escape velocity

The ozone layer and the growing ozone hole (Read Section 5.1 Discovery: Earth’s Growing Ozone “Hole”)

• The ozone layer is located at about 20-30 km above the surface. It contains

about 10 part per million of ozone.

• A groups of chemicals known as CFCs, choroflourocarbons (commonly

named Freon) are very efficient to combine with the ozone molecule (O3)

destroying the ozone molecule. The molecule of CFC contains carbon,

chlorine and fluorine.

• How that works: The CFC molecule dissociate in the upper atmosphere by

solar radiation and releases chlorine which combine with ozone and forms

molecular oxygen (O₂)

• The chlorine acts as a catalyst. It participate in the reaction but it is

released at the end of the reaction.

• CFCs were widely used as refrigerants in A/C, refrigerators, car A/C,

propellant in aerosol cans and other uses.

• Substantial cuts in the use and production of CFC’s has improved the

situation.

• The ozone hole has been studied from around the1980’s. The hole is a

prominent feature of the ozone layer over the Antarctica.

The ozone hole over Antarctica on August 17, 2014

Antarctica

continent

Southern tip

of South

America

Ozone hole

How do we know anything about the interior of the Earth? Keep in mind that the Earth’s radius is about 6,400 km. Drilling of the Earth’s crust cannot go

deeper than about 10 km

• Earth’s interior structure is probed by studying

how seismic waves travel through it (we can only

drill so far! – 10 km).

• Earthquakes generate two types of seismic

waves: P and S waves.

•P waves (Pressure) can travel through the liquid

(outer) core but they are deflected by the core

•S waves (Shear) travel in the mantle but not

through the liquid core

• Waves are reflected and refracted when they

propagate in the interior of the Earth due to

variation in densities and temperature of different

materials in the layers. The waves travel through

these materials at different speeds (higher density

material, faster speed).

• Different densities force the waves to follow

curved patterns.

The answer: Seismology!

Liquid

core

• Crust - On average 15 km thick (8 km under ocean - 20-50 km under continents. • Mantle - 3000 km thick (80% of planet volume). • Core: Inner core is solid, outer core is fluid • Density and temperature increase with depth. • High central density suggests the core is mostly nickel and iron. • The “jump” in density between the mantle and the core is caused by different materials in the mantle and core. The core has denser material such as nickel and iron • No jump in density between inner and outer core because material is the same and just goes from liquid to solid. • The temperature in the core is about 5,000K and the density about 12,000 kg/m³ •(Density = Mass/volume. As a reference, the density of water is 1000 kg/m³)

The Earth interior Using seismic wave data and computer models it

is possible to model the interior of the Earth

Evolution of the Solid Earth

Accretion- The material that formed the Earth comes from the material

accreted that formed the protoplanetary disk around the Sun. •That material in the protoplanetary disk collapsed and formed protoplanets. This

process happened about 4.6 billion years ago (age of Sun).

•Earth was bombarded by interplanetary debris (Left over from the formation of the

Solar System).

•As we will see later, all the planets and satellites were bombarded by debris. The debris

created impact craters. The craters on the Moon are one example.

•The impacts contributes to an increase of the temperature.

•Radioactive decay. Unstable elements such as uranium and thorium will release heat

when their nuclei break up into simpler, lighter elements. This also contributed to the

increased temperature.

•Both effects (Impacts and radioactive decay) keep the Earth melted for about a billion

years

Once the melting occur, differentiation of the interior began

•Differentiation - Earth was molten, allowing higher-density material (nickel, iron) to

sink to the core. The material in the core is still at a high temperature, about 5,000K.

•Crustal Formation - cooling and thickening of crust about 3.7 billion years ago.

The Surface of the Earth is still active… • Seismic activity…earthquakes, volcanoes, plate tectonics…

• Sites of activity outline surface plates

• Continental drift – continents drift a few cm/year

•The technical term for continental drift is Plate Tectonics

• Plates collide head on (mountains) or shear past (earthquakes)

• Some plates are separating (under Atlantic) - new mantle material

wells up between them.

Sites of

earthquakes or

volcanoes in the

past 100 years

What causes the forces that shift the plates?

Convection!

• Each plate involves material from the crust and the upper mantle.

The plates are “floating” on top of the mantle

• Warm mantle rock (softened by high temps) moves up - cools - moves down:

Convection!

Lunar Surface ( large scale features)

Lack of atmosphere and water preserves surface features

Maria, singular Mare (younger areas) –

mantle material (maria means “seas’)

•Few impact craters

•Maria - darker areas resulting from

earlier lava flow

• Basaltic, relatively iron rich, high

density (3300 kg/m3).

Highlands (older areas) – crust material

•Many impact creaters

•Elevated many km above maria

• Aluminum rich, low density (2900

kg/m3).

Some of the features of the moon

New moon First quarter moon

Crater with

central peak

Rayed crater

(Tycho)

Mare

(Tranquillitatis)

Flat

bottom

crater

Mountain ranges

(Alps, Caucasus, Apennine )

The near and far side the Moon (Images taken by Clementine spacecraft)

Rayed crater

Maria

(Dark

regions,

richer in

iron)

• The rate of cratering on the moon is determined from the known ages of the

highland and maria regions.

• The Moon (and the solar system) experienced a sharp drop in the rate of

meteoritic bombardment ~ 3.9 billion years ago.

• The rate of cratering has been roughly constant since that time.

• Only a few craters appear on the maria

•The highlands have a large concentration of craters

•The mountains are the rim of the large craters (maria). No evidence of plate

tectonics on the Moon.

Apennine

mountain

range

Lunar Erosion

Lunar Craters - caused by meteoroid impacts

• The high speed, 30,00-40,00 km/s (and high energy) of

the impactor (meteoroid, nucleus of comet), cause an

explosion at the point of impact

•Pressure to the lunar surface heats the rock and deforms the

ground.

• Explosion pushes rock layers up and out.

• The ejecta blanket surrounds the crater

•It forms radial features around some craters. These are

called rayed craters

• In some cases, the compressed material bounces back and

form a crater with a central peak

•Because the craters are caused by an explosion, most of

them are round (circular shape)

•Craters can be up to 100 km in diameter

• A new 10 km crater is formed every 10 million years

• A new 1m crater is formed each month

• A new 1cm crater is formed every few minutes!

A recent impact crater

Formation of the Moon - theories

Earlier theories: • Both the Earth and the Moon formed together from the same material of the

proto planetary disk

But, the moon has much less iron

• The moon formed somewhere else and was captured by the Earth

But, the moon’s mantle is too similar to the Earth

Impact Theory (At the present the most accepted theory)

• Glancing collision between a Mars-sized body (Mars diameter is about ½ of Earth diameter) and the molten Earth. The Mars-size body was formed somewhere else. There was an exchange of material. • Parts of the Earth mantle were ejected during the collision and later condensed and formed the moon. • Earth had already gone through the process of differentiation so little iron from the core was ejected in the collision. Most of the material ejected came from the mantle. The moon has less iron that Earth.

The Earth’s Magnetosphere : The space around the Earth influenced by Earth’s magnetic field

• Magnetic field lines run from the south to north magnetic poles

• Magnetic poles are close to (but not the same as) the rotational axis poles

• The magnetic field is distorted by the solar wind. The composition of the

solar wind: high speed electrically charged particles (electrons, protons, ions)

• Solar flares eject high energy charged particles that compress the magnetic field.

The compression induces currents in power lines and may cause black out

• Van Allen belts (named after Dr. James Van Allen) are two doughnuts-shapes

zones where high energy particles are trapped on the magnetic field Iron filings

influenced by a

bar magnet

What causes the Earth’s magnetic field?

•The relatively fast rotation of the planet

•The electrically conducting metal core = dynamo effect

Aurora Borealis (“Northern Lights”)

Aurora Australis in southern hemisphere

• Charged particles are ejected from the Sun. In a

solar flare a large of them are ejected. They collide

with the Earth magnetosphere. The particles enter

the magnetic field and are directed by the Earth’s

magnetic field toward the poles.

• The particles precipitate near the poles colliding

with Earth’s upper atmosphere.

•The gases in the atmosphere are excited and emit

light (line emission).

•The colors of the auroras depend on the gasses

and the altitude (pressure) at which they are

excited. Oxygen emits green or brownish red.

Nitrogen emit blue or red.

•Auroras are common at high latitudes (Alaska,

Canada, northern Europe, Antartica) but a large

solar flare can produce auroras at low latitude.

Auroras at low latitudes (Gainesville, Florida) are

very unusual.

An unusual aurora photographed at low latitude at the UF Rosemary Hill

Observatory near Bronson, FL on April 6, 2000 (F. Reyes)