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Class 38: Review for Test 2 [4/23/07]dalex/ASTR202_S07/class38.pdfAll planetary orbits are nearly...
Transcript of Class 38: Review for Test 2 [4/23/07]dalex/ASTR202_S07/class38.pdfAll planetary orbits are nearly...
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