ASTRONOMY 202 Spring 2007: Solar System Exploratio n

Instructor: Dr. David Alexander Web-site: w ww.ruf.rice.edu/~dalex/ASTR202_S07

Class 38: Review for Test 2 [4/23/07]

Announcements

Test

Chapters 7-14 & 24 (based class notes only not whole textbook)

Study Guide online

Review on Monday – send topics, requests, or suggestions via email

YOU WILL NEED A CALCULATOR!!!!

Chapters 7 & 8: Solar System

KEY POINTS:

Patterns in the Solar System

- rotation of planet, direction and plane of orbit- terrestrial vs. Jovian planets- exceptions

Nebular Hypothesis

- abundance of various elements- importance of condensation and frost line

Patterns in the Planets� All planetary orbits are nearly circular and lie in nearly the same plane.

� All planets orbit the Sun in the same direction, counterclockwise as viewed from above.

� Most planets rotate in the same direction in which they orbit, with fairly small axis tilts.

� Most of the Solar System’s large moons exhibit similar properties in their orbits around their planets.

Terrestrial and Jovian Planets

Hydrogen compounds on gas giants include water (H2O), ammonia (NH3), and methane (CH4)

JupiterSaturnUranusNeptune

MercuryVenusEarthMars

Pluto??

Summary of the Solar System

PATTERNS OF MOTION TWO TYPES OF PLANETS

ASTEROIDS AND COMETS EXCEPTIONS

The Collapse of the Solar NebulaThe formation of the solar system as we know it is a result of the conservation of energy(gravitational to kinetic to thermal) and the conservation of angular momentum.

• Heating

• Spinning

• Flattening

• Emptying the Nebula

As the nebula collapses, the density and temperature rapidly increases.

The collapsing sphere rotated faster and faster as it collapsed.

Collisions in the spinning, collapsing nebula result in the sphere flattening to form a disk.

A plasma wind from the newly formed stars sweeps the bulk of the hydrogen and helium gas from the solar system.

The three process of heating, spinning, and flattening explain the tidy layout of the solar system. The clearing out of the gas early in the history

of the solar system was crucial to determining the final nature of the solar system.

The Two Types of Planets

Condensation: The importance of the frost line.

In the outer parts of the collapsing nebula gravity needed a hand.

The cool temperatures in the outer reaches of the nebula allowed solid particles to condenseout of the gas.

• Hydrogen and Helium gas

• Hydrogen compounds

• Rock

• Metals

98% by mass, do not condense at nebular temperatures

Form ices of methane (CH4), ammonia (NH3) and water (H2O) below 150 K. Made up about 1.4% of mass.

Mostly silicon-based minerals making up about 0.4% nebular mass. Rock is gaseous above 500 – 1300 K.

E.g. iron, nickel, aluminium making up ~0.2% of the mass. Gaseous metals present above 1600 K.

The Two Types of Planets

Condensation: The importance of the frost line.

Different ‘seeds’ for condensation form at different parts of the collapsing nebula.

The Frost Line is defined by the temperature at which the H, He and H compounds could condense out, i.e ~150 K.

In the Solar System, the frost line lies between the orbit of Mars and Jupiter.

The Age of the Solar System

We can determine the age of the solar system by measuring the ages of the rocks within it using a process known as radiometric dating.

Radioactive isotopes in rock undergo spontaneous change (radioactive decay) from one isotope to another or one element to another. By measuring the amounts of the different isotopes and elements we can determine how long it has been since the rock solidified.

The oldest Earth rocks solidified 4 billion years ago.

Lunar rocks have yielded an age of ~4.5 billion yrs.

Radiometric dating

Example of a problem using radiometric dating:

Element 1 has a half-life of 1 billion years and decays into element 2. A rock is found on Mars which has 75% of element 2 and 25% of element 1. How old is the rock?

After 1 billion years there would be 50% of element 1 and 50% of element 2

After 2 billion years there would be 25% of element 1 and 75% of element 2

)2log(log

2

1

0

/

0

×−=

==

half

t

tt

t

t

t

N

N

amountoriginal

ttimeatamount

N

N half

Chapter 14: The Sun

KEY POINTS:

Nuclear Fusion

- Role of E=mc 2 and mass deficit

Layers of the Sun

- temperature

Source of solar energy: nuclear fusion

Hydrogen “burns” to Helium: E=mc2 does the rest

The Solar Thermostat: Fusion

Hot, dense core makes protons overcome electromagnetic repulsion causing them to ‘stick’ together via the strong force.

The Proton-Proton Chain

Fission– split nucleus –nuclear power plantsFusion – combine nuclei –cores of stars

1: p + p →→→→ pn + e+ + νννν

2: pn + p →→→→ He3 + γγγγ

3: He3 + He3 →→→→ He4 + p + p

pn = Deuterium – isotope of H e+ = Positron – anti-electronν = Neutrino – almost massless

He3 = rare isotope of Heliumγ = gamma-ray photon

He4 = regular Helium

Total: 4p →→→→ He4 + 2e+ + 2νννν + 2γγγγ

Mass Deficit

He4+2e++2ν – 4p = -0.7% (x 4mp)

= -0.007x4x1.67x10-27kg

= 4.7 x 10-29 kg

= 4.21 x 10-12 Joules(E=mc2)

15 Million degrees

1-5 Million degrees

1-3 Million degrees

0.1 – 1 Million degrees

6,000 degrees

6,000 – 100,000 degrees

20 – 1 Million degrees

1 Million – 6,000 degrees

Core: nuclear fusion

Low Corona

Convection Zone

Large -scale Corona

Transition Region

Radiative Zone

Photosphere

Magnetic Field

Chromosphere

Chapter 9: Planetary GeologyKEY POINTS:

Structure of Interior- layering by density- layering by strength

Internal heating- accretion, differentiation (early formation)- radioactive decay (throughout lifetime)- mantle convection

Heating vs. Cooling- surface to volume ratio

Generation of Magnetic Field

Main geological processes- factors affecting geology

Interior structure

Layering by density: three basic layers

• CORE Nickel and Iron at high density

• MANTLE Rocky material (e.g. minerals containing Silicon and Oxygen) surrounds core

• CRUST Lowest density rock (e.g. granite, basalt) forms thin crust

Interior structure

Layering by strength: the Lithosphere

The strength of the rock making up a planet’s interior plays an important part in its geology.

The Lithosphere encompasses the crust and part of the mantle and is defined by the strength of the rock rather than the density.

Internal HeatThe different geology of the terrestrial worlds is strongly governed by their differences in internal heat.

Main ways to heat a planet:

• Accretion

Heat generated at formation

• Differentiation

Re-distributes heat within planet

• Radioactive Decay

Conversion of mass-energy to heat

Radioactive decay is the only source of heat acting at the present time.

Planets cool by emitting thermal radiation (as infrared radiation) with the rate of cooling being determined by their size (surface area to volume ratio).

Mantle Convection

Rock strength (lithosphere) governs convection versus conduction

• Convection

Hot solid material expands and risesCool material contracts and falls

• Conduction

Transfer of heat through particles

• Radiation

Thermal energy of surface radiates into space

Mantle convection is closely tied to lithosphere thickness.

Relationship between internal heat and geological activity is the ability of rock to move within the mantle.

Main ways to move energy in a planet:

Magnetic Fields

Molten metals in the outer core of a planet can generate a magnetic field.

• An interior region of electrically conducting fluid such as molten metal

• Convection in that layer of fluid

• At least moderately rapid rotation

Three basic requirements for a planet to have a magnetic field

Earth is the only terrestrial planet with a strong magnetic field.

The presence or lack of a magnetic field provides important clues to a planet’s interior structure.

Moon: no metals or solid core Mars: solidification of core Venus: slow rotation or little convectionMercury: has weak field despite being small and slow!

Shaping the surface of a planet

The Four Basic Geological Processes

� Impact Cratering

Formed by collisions of asteroids or comets with planet

� Volcanism

Eruption of molten rock (lava) from planet’s interior

� Tectonics

Disruption of surface by internal stresses

� Erosion

Wind, water, ice deformations of surface features

Planetary parameters affecting geology

Heating vs Cooling

Heating is distributed throughout planet so total heating is proportional to volume:

Heating ∝ (4/3)πR3

Cooling is a result of radiation from the surface and is therefore only dependent on surface area of planet:

Cooling ∝ 4πR2

Therefore, for a planet of radius R the cooling to heating ratio is 3/R

Chapter 10: Planetary AtmospheresKEY POINTS:

Role of atmosphere

Greenhouse Effect- warming effect

Structure of Atmosphere- different layers- different sources of heating

Global wind patterns- Coriolis force

Factors affecting long-term climate change

Processes to create and remove an atmosphere- thermal velocity/escape velocity

Why is Earth’s atmosphere different from Venus and Mars- role of CO 2 cycle

Terrestrial Atmospheres

The atmospheres of the terrestrial worlds vary in their composition, density and pressure. It is the atmospheric pressure that generally defines the main characteristic of an atmosphere.

Unit of pressure is the bar : 1 bar is equivalent to 14.7 pounds per square inch at sea level on Earth

Role of an Atmosphere

Get better picture of Earth’s atmosphere

2/3 of Earth’s atmosphere lies within 10 km

but can have an impact on

satellites as high as several

hundreds of kilometres.

Atmospheres provide a crucial function in the development of a planet’s geology and more importantly on its ability to sustain life.

• Atmospheres make planet surfaces warmer(Greenhouse Effect)

• Atmospheres absorb and scatter light (including solar UV and X-rays)

• Atmospheres create pressure (allowing liquid water to form )

• Atmospheres create wind and weather (controlling long-term climate changes)

• Atmospheres can interact with planetarymagnetic fieldscreating magnetospheres

The Greenhouse Effect

The most important effect of an atmosphere is to regulate the surface temperature of the planet. It does this via the Greenhouse Effect.

Not all gases absorb infrared radiation.

The main Greenhouse gases are:

Water vapour (H2O)

Carbon Dioxide (CO2)

Methane (CH4)

Molecules comprised of different elements are more efficient absorbers of infrared radiation

Greenhouse Effect on Terrestrial Planets

Without a Greenhouse Effect, the balance of energy input and output of a planet would result in much colder surface temperatures since the radiated energy escapes completely.

No atmosphere: Temperature regulated by distance from Sun and reflectivity of the planet (albedo).

Earth’s Atmospheric Structure Pressure and density in the Earth’s atmosphere drop rapidly with increasing altitude and so have little effect on atmospheric layering. The temperature behaviour is more complex, creating four major layers.

• Troposphere

Temperature drops with altitude until about 10km

• Stratosphere

Temperature begins to rise until a height of ~50km before falling again through the next 30 km

•Thermosphere

Once again the temperature begins to rise above 80km

•Exosphere

Upper most region which gradually fades of into space

Global Wind Patterns on Earth

Weather and climate are important components to the geological and physical development of a planet.

Planet-scale patterns can give a glimpse of the conditions prevalent on a planet.

Coriolis Effect

The rotation of the Earth sets up a force, the Coriolis Effect, which breaks up the large hemispherical convection cells and helps create the global wind patterns observed.

Long-term Climate Change

Over long time-scales planets can undergo major climatic changes.

SOLAR BRIGHTENING CHANGES IN AXIS TILT

CHANGES IN PLANETARY REFLECTIVITY CHANGES IN GREENHOUSE GAS ABUNDANCE

Creating an AtmosphereChanges in atmospheric gas levels (especially of the greenhouse gases) can radically affect the long-term climate of a planet.

The Earth, unlike the other planets, have an additional means of adding gases to the atmosphere.

Houston: Smoggy and Clear

Losing and AtmosphereChanges in atmospheric gas levels (especially of the greenhouse gases) can radically affect the long-term climate of a planet.

Thermal EscapeThe thermal velocity of a gas particle in a planetary atmosphere can be calculated from the formula:

pth m

Txv 121025.5 −=

where T is the temperature and mp is the mass of the gas particle.

To escape, kinetic energy should exceed gravitational potential energy:

½mv2 > GMm/r

⇒r

GMescv 2=

where M is the mass of the planet and r is the radius.

What makes Earth’s atmosphere special?

� Earth is the only planet with appreciable atmospheric oxygen

� Earth is the only planet with conditions suitable to maintain liquid water on its surface

� Earth is the only terrestrial world with a stable climate

Why is the atmosphere of Earth so different from Ma rs and Venus?

Why did Earth retain most of its outgassed water?

Why does Earth have so little CO2?

Why does Earth have so much Oxygen (O2)?

Why does Earth have a UV absorbing stratosphere?

On all three planets outgassing released the same gases – mostly water, carbon dioxide and nitrogen.

95% CO2

96% CO2

77% N2

21% O2

The Water-covered Earth

4 billion years ago, Venus, Mars and the Earth may all have had plentiful rainfall and surface water.

The key to why Earth still has plentiful water in liquid form, whereas Venus and Mars do not, is due almost entirely due to the different strengths of the greenhouse effect on the three planets.

On Venus, the runaway greenhouse effect sent water vapour high into atmosphere where solar UV broke it down allowing the hydrogen to escape.

On Mars, the weakening greenhouse effect caused the water to freeze out of the atmosphere at the polar caps.

The missing CO 2

The Earth has just the right level of greenhouse effect primarily because of the lack of Carbon Dioxide (CO2) in the atmosphere.

The CO2 is not missing but bound up in the oceans and rocks of the Earth. The total amount of CO2 trapped in the oceans and rocks is about 170,000 times that in the air!

The absorption of Carbon Dioxide into rocks can only occur via a chemical reaction in the presence of liquid water.

So, the presence of oceans is due in part to the amount of CO2 in the atmosphere, the amount of which in turn is due to the presence of the oceans.

The balance of Nature

CO2 ↔↔↔↔ H2O Life ↔↔↔↔ O2

Liquid water + rock removes CO2 from atmosphere

CO2 in atmosphere allows liquid water through greenhouse effect

The Earth’s Thermostat

Rate at which carbonate minerals form in the ocean is very sensitive to temperature which is strongly affected by amount of CO2 in the atmosphere.

This has kept the Earth’s climate stable despite changes in the rate of volcanism, changes in the Sun’s brightness and other climate effects.

Chapter 11: Jovian Planets

KEY POINTS:

Interior structure

Structure of atmosphere

- Reason for bands on Jupiter

Jovian moons

- tidal heating on Io

Planetary rings

- gap moons and divisions (orbital resonances)

Jovian Planet Interiors

Most of our information about the interiors of the gas giants comes from limited observations and lots of theoretical calculations and modeling.

� Jupiter’s core is about 10 times as massive as the Earth but the same size

� The rocky core is very different from the terrestrial worlds because of the huge pressures and high temperatures there

� Metallic Hydrogen is an important component of Jupiter’s interior structure as it is responsible for Jupiter’s strong magnetic field

� Jupiter generates a lot of internal heat (it emits twice as much energy as it receives from the Sun)

Interior Structure

The composition of the cores of all four Jovian planets is expected to be very similar despite their large range of size and density.

Jupiter and Saturn are large enough to have metallic hydrogen and to have liquid cores of rock, metal and H compounds.

The cores of Uranus and Neptune are relatively large because they are less compressed by the surrounding gas.

Jovian Planetary AtmospheresLike Earth, the Jovian planets have a complicated atmospheric structure, featuring a troposphere, stratosphere and thermosphere.

The tropospheres are particularly interesting giving a range of dynamic weather features such as clouds, storms and global wind patterns.

Jupiter’s Cloud Cover

Io : the most volcanically active world in the solar s ystem

Volcanic eruptions on Io’s dark side

Io has so much volcanic activity that no impact craters are evident.

The outgassing from the volcanoes are the source of the large amounts of ionized gas (plasma) in Jupiter’s magnetosphere.

Io loses atmospheric gas faster than any other world in the solar system.

Tidal heatingMoons like Io are too small and too old to be generating significant amounts of internal heat so the source of energy for the volcanism on Io has to be due to something else.

The size and shape of Jupiter together with the closeness and eccentricity of Io’s orbit provides for internal heating due to tidal forces.

Planetary RingsThe Jovian planets all display a system of rings comprised of millions of icy particles ranging in size from dust to boulders.

Rings and Gaps Gap Moons Spokes

Pan

Cassini Division

Ring particles are made mostly of water ice and are bright where there are enough particles to scatter sunlight back to us.

Each particle in the rings orbit according to Kepler’s laws.

The rings of Saturn show a large number of features

Chapters 12: Asteroids and Comets

KEY POINTS:

Differences in composition

Orbital structure of asteroid belt- resonances with Jupiter

Meteorite types

Origins of comets- Kuiper Belt, Oort cloud

Comet tails

Meteor impacts- kinetic energy calculations- mass extinctions

What are they called and where are they?

WHAT

•Asteroid

Rocky leftover planetesimal

• Comet

Icy leftover planetesimal

• Meteor

Particle entering atmosphere

•Meteorite

Any piece of rock from space that reaches the ground

WHERE

•Asteroids

Most are found in asteroid beltwhich lies between the orbits ofMars and Jupiter

• Comets

a) Kuiper Belt

Orbit Sun in same direction andnearly same plane as planets at adistance ranging from Neptune to twice as far as Pluto

b) Oort Cloud

Orbits randomly inclined to ecliptic plane and much furtheraway than Kuiper Belt

The Asteroid Belt

The Asteroid Belt, is a result of a gravitational interaction known as orbital resonance.

Objects will line up periodically whenever the periods of their orbits have a simple relationship.

The asteroids in the Asteroid Belt are organized due to the interaction with Jupiter.

Meteorites

Meteorites can be identified by their very different isotope ratios and the presence of rare elements.

More than 20,000 meteorites have been catalogued by scientists and are found to fall into two categories:

Primitive meteorites: 4.6 billion years old, unchanged since formed

stony – composed mostly of rocky minerals with some metallic flakescarbon-rich – significant amounts of carbon compounds and some water

Processed Meteorites: younger and once part of a larger object

core-like – high density iron/nickel mixture with traces of other metalscrust/mantle-like – lower density rock, some similar to volcanic basalts.

CometsComets are icy planetesimals formed in the outer reaches of the Solar System and congregated in two distinct regions.

The Kuiper Belt:

This region begins at a distance of about Neptune’s orbit and extends to about three times the distance of Neptune’s orbit (30-100 AU). Like the asteroid belt the Kuiper belt rotates in the same direction as the planets and is roughly in the ecliptic plane.

The Oort Cloud

The Oort cloud is a spherical cloud ofcomets extending a about a light year away from the Sun. The velocities ofthese comets tend to be larger and more random than the Kuiper belt comets.

Cometary Tails

Comets are completely frozen when far away from the Sun and are only a few kms across.

When they approach the Sun we see the rich structure which distinguishes a comet from an asteroid.

• Nucleusis a “dirty snowball”

• Coma is large dusty “atmosphere” surrounding nucleus (mostly sublimated gas)

• Tail gas and dust extending hundreds of millions of kms.

Most comets have two tails:

a plasma tail (ionized gas)and

a dust tail (small solid particles)

Mass Extinctions

Energy in a Collision

E = ½mv2

Small meteor mass 1012 kgTypical velocity 30 km/s

Kinetic energy of meteor:4.5 x 1020 Joules

Some of these impacts have had apparently catastrophic consequences.

Mass ExtinctionsWhile still contentious, it is becoming more and more accepted that the impact of a large meteor with the Earth some 65 million years ago was responsible for killing off 99% of all living organisms.

EVIDENCE:

� Thin layer of dark sediments rich in iridium found around the world at a depth aged at 65 million years.

� High abundances of other rare metals, evidence for shocked quartz, spherical rock droplets, and soot also found in sedimentary layer.

� 200km crater of correct age found in Yucatan peninsula, Chicxulub crater .

K-T Boundary Layer

Mass Extinctions

shower of hot molten rockHuge tidal wave

IMPACT

Forest firesToxic chemicals

Acid rainLong global winter

Decades of global warming

MASS EXTINCTION

There appear to have been at least four other mass extinctions during the past 500 million years

Chapter 13: ExoplanetsKEY POINTS:

Comparison to our own solar system

Methods of detection

Doppler shift

Kepler’s 3 rd law

What we can learn from spectra of atmospheres

Properties of Other Planetary Systems

• planets appear to be Jovian• more massive than our system• planets are close to their stars• many more highly eccentric orbits than in our Solar System

Great source for all things extrasolar

planetquest.jpl.nasa.gov

Cool 3D map of all known extrasolar planets:

planetquest1.jpl.nasa.gov/atlas/atlas_index.cfm

Total Planets discovered: 223# of planetary systems: 185

Detecting Extrasolar Planets

Gravitational wobble of star Transit of star by planet

Astrometry

Doppler effect

An object moving away from us has the waves ‘stretched out’ to a longer wavelength and is said to be redshifted .

An object moving towards us has the waves ‘bunched up’ to a smaller wavelength and is said to be blueshifted .

wavelengthrest

shiftwavelength

lightofspeed

sightoflinealongvelocity =0

0

λλλ −=

c

v

e.g. a star moving away from us at 230 km/s has its Hydrogen-alpha line (656.3 nm) shifted by ~0.5 nm to 656.8 nm.

KeplerKepler ’’ss Three Laws of Planetary MotionThree Laws of Planetary Motion

Kepler’s Third Law:

More distant planets orbit the Sun at slower average

speed, obeying the following precise

mathematical relationship:

p2 = a3

p = planet’s orbital period in yearsa = planet’s average distance from Sun in AU

A major consequence of this law is that:

The more distant a planet from the Sun, the slower its average orbital velocity.

2/1

22

ap

avavg

ππ ==

Habitable exoplanets?

• In the near future, NASA plans to launch Terrestrial Planet Finder.• an interferometer in space

• take spectra and make crude images of Earth-sized extrasolar planets

• Spectrum of a planet can tell us if it is habitable.• look for absorption lines of ozone

and water

Chapter 24: Life in the Universe

KEY POINTS:

Evidence for common ancestor

Lifeline for life on Earth

A common ancestor

• All known organisms:• build proteins from same subset of

amino acids• use ATP to store energy in cells• use DNA molecules to transmit

genes• All organisms share same genetic

code…sequence of chemical bases• Organisms have similar genes.

• Indicates that all living organisms share a common ancestor.

• Life on Earth is:• divided into three major

groupings• plants & animals are just two

tiny branches

The Life-line

>3.5 billion yrs ago 3.5 – 2 billion yrs ago 2 billion yrs ago 540 million yrs ago

Beginning of Life Early Life in the Ocean The Rise of Oxygen Explosion of Diversity

Bacterial colonies of - Single-celled organisms cyanobacteria + Over 40 million yrs thestromatolites - no ozone to protect photosynthesis → full diversity of life as we

surface oxygen + animals know it occurred.Cambrian Explosion

Theory of Evolution: Naturally occurring mutations plus mechanism of natural selection pave the way for ‘better’ organisms.

Which Stars make Good Suns?• Which stars are most likely to have planets harboring life?

• they must be old enough so that life could arise in a few x 108

years• this rules out the massive O & B main sequence stars

• they must allow for stable planetary orbits• this rules out binary and multiple star systems

• they must have relatively large habitable zones• region where large terrestrial planets could have surface temperature

that allow water to exist as a liquid