• Geological evidence supports the hypothesis that the Earth's Moon formed as a result of a Giant Impact with a Mars-sized body late in formation. We don't have time to discuss the evidence and virtues of this theory relative to other possibilities.

• Know that atmospheres are very thin layers on the surface!
• Know which terrestrial planets have the thickest (most dense) atmospheres, and which have no atmospheres.
• Know that the atmospheres were originally due to gases baked out of the planet's rocks, and volatile materials from impacting icy planetesimals. This means the original atmospheres were mostly CO2, N2, water vapor, and ammonia.
• Notice that Venus and Mars have atmospheres with similar compositions (mostly carbon dioxide), but very different densities/pressures. Notice also that although Earth and Venus have similar sizes and masses, their atmospheres are very different. Earth's atmosphere is mostly Nitrogen and Oxygen! Venus' mostly CO2 atmosphere is 90 times thicker at the surface!
• Understand why (in general), the least massive planets have the least atmosphere. The gases escape more easily, because the escape velocity due to gravity is smaller. This explains why Mercury and the Moon have no atmospheres, and Mars has only a thin atmosphere.
• Venus, although slightly less massive than the Earth, has a much thicker atmosphere. This is an exception to the above rule!
• Understand why Venus and Earth are so different: liquid water and the Greenhouse Effect!
• Understand how the greenhouse effect works - CO2 and water vapor in the atmosphere allow visible light to come in, which warms the surface, but don't allow infrared radiation to escape, making the surface warmer (heat can't escape).

March 11-The cycles that shape planets

Giant Impacts

• We believe that the meteors seen in meteor showers are dust/debris from comets because we see meteor showers when the Earth passes near the orbit of a comet.
• Most meteorites (rocks that reach the ground) are believed to come from asteroids (icy comet debris is too weak to survive passage through the atmosphere).
• There's plenty of geological evidence supporting our theory that meteorites were once part of larger rocky bodies, and that they are debris from asteroids which have been broken apart in collisions. (No details required).

• We believe that asteroids formed between Mars and Jupiter, but couldn't coalesce into a single planet due to Jupiter's gravitational influence. This is supported by the fact that the density of asteroids is higher in the inner parts of the asteroid belt than it is in the outer parts (condensation sequence in the solar nebula).
• We think some asteroids were once part of larger bodies (geology/spectroscopy - No details required).
• Asteroids are heavily cratered, some are double asteroids (so there have been collisions); this supports our theory of asteroid evolution and meteorite origin.

• Dirty snowball model: lump of dirty ices that vaporize, releasing gas and dust, as the comet comes close to the sun. See only when close to the sun.
• The orbits of comets imply 2 populations - "Kuiper Belt" and "Oort Cloud"
• "Kuiper Belt" is a thick disk between Neptune (30 AU) and 100 AU from the Sun. These comets formed where they are now.
• Pluto/Charon are believed to be the largest of the "Kuiper Belt" objects.
• Evidence for KB: have seen ~60 of these objects in the KB, and also have seen dust disks around other stars that are believed to be produced by dust released from comets.
• The "Oort Cloud" is a spherical swarm of comets 10,000 to 100,000 AU from the Sun! Comets in the Oort Cloud formed between Saturn and Uranus - ejected from solar system by gravitational interactions with giant planets.

• Jupiter's rings are made out of tiny dust particles. We know this from the way they scatter light. These particles would have been blown out of their orbits by the solar wind and sunlight if they had been around since the beginning of the solar system. Instead, we think they are debris from impacts between the moons and asteroids.
• Saturn's spectacular rings are made of ice: dust-sized to house-sized particles. The ice would have melted in the heat of formation, and darkens with time due to micrometeorite impacts. They are icy and bright, so are very fresh. We believe they are debris from impacts between Saturn's moons (dust) and comets (ice).
• Uranus and Neptune have rings black as coal.

• Know why the orbits of Pluto/Charon and Neptune's moons, as well as the rotation of Uranus and its moons/rings, are evidence for giant impacts early in the solar system. In the previous lecture I also mentioned some evidence for such impacts from the inner solar system.
• Know that a comet hit Jupiter in 1994!
• Know that craters can form from much smaller objects.
• Know that the Earth has been hit in the past by giant asteroids/comets: We believe that climate changes due to this caused the death of the dinosaurs.
• EVIDENCE: Layer of Iridium (rare on Earth, common in asteroids) in sediments 65 million years ago. Soot layer (from forest fires?) also in sediments. Plentiful dino fossils under this layer (earlier), and no fossils above this layer (later). NOTE THAT WE CAN'T BE CERTAIN OF THE REASON FOR THE CORRELATION.
• There are many near-earth asteroids that could hit the Earth, but sizable ones aren't expected very often. On the other hand, we haven't identified them all, so we might not know it is coming.

Geologic and Atmospheric processes that shape the planets.

• Convection drives many of the processes that shape the surfaces of planets. You should understand the basics how convection works, and how it transports energy from one place to another. You should also understand the following convective processes: Plate Tectonics, Atmospheric Circulation, Storms, and Ocean currents.
• Plate Tectonics
  • Heat is generated in the cores of planets from radioactive decay from heavy elements. (Understand why the heavy elements are in the cores of planets!)
  • The mantle is "plastic." That means that even though it is solid, it can flow over long periods of time. (Think about how ice in a glacier flows).
  • The lower portion of the mantle is heated and starts to rise.
  • Cold mantle near the survace starts to sink.
  • The crust is carried along with the mantle.
  • Understand how this relates to earthquakes and volcanoes
• Volcanism
  • On Venus and Mars, the mantle is not plastic. Because of this heat must get out in another way.
  • Heat trapped in the planet can melt rocks.
  • On Venus, the trapped heat is released planet wide in a surge of vulcanism.
  • On Mars, there are only a few "hot spots" that result in volcanos. These may all be too cool to erupt in the future.
• Atmospheric Circulation (Winds)
  • Global wind patterns are driven by cold are descending at the poles and warm air rising at the equator.
  • Understand how rotation makes this pattern more complicated.
  • Be able to make diagrams showing which direction the surface and high altitude winds would be blowing. Understand why the surface winds blow from cold regions toward hot regions.
• Storms
  • Remember that evaporating water cools the air, condensing water into a cloud heats the air.
  • A storm is an example of "positive feedback." As warm moist air rises, it cools causing water droplets to condense. This heats the air causing it to rise further and more water to condense. The rising air draws more warm moist air in at the bottom of the storm.
  • A hurricane is a storm taken to extremes. A storm sitting in still air over warm water as a very large amount of energy available. (The warm water continually heats and humidifies the air, which feeds the storm.)
• Ocean Currents
  • Ocean currents are also driven by convection cold water in the arctic regions sinks, warm water in the tropics rises.
  • The overall effect of the currents is to transport warm water toward the poles, and cold water toward the equator. This is called the "global conveyor belt" It helps keep Europe and Australia warm.
  • Because fresh water is less dense than salt water, if the polar ice caps melt and dump a large amount of fresh water into the arctic, the conveyor could stop. This would trigger severely cold winters in the high latitude regions. Parts of Europe might be snow covered all year round.
• The Water Cycle The water cycle is just another way of thinking about how atmospheric circulation and storms transport energy and water. One thing to remember when thinking about storms, atmospheric circulation, ocean currents and the water cycle is that they are all powered by the sun.
• The Carbonate-Silicate cycle
  • This cycle is an example of negative feedback. It helps to maintain the Earth in a narrow range of temperatures.
  • It is related to both the water cycle and plate tectonics.
  • Carbon dioxide is a greenhouse gas. It is opaque to infrared rays which traps heat in the atmosphere and results in increased temperatures.
    • Rain falling on the land erodes silicate rocks releasing ions into the water.
    • The water ends up in the sea, increasing the concentration of ions in the sea (making it saltier).
    • Carbon dioxide in the atmosphere dissolves in the sea to make carbonic acid.
    • Carbonic acid and the ions combine to build the shells of sea creatures and other carbonate minerals.
    • These carbonates collect on the sea floor.
    • Plate tectonics causes some of the carbonate materials to be accumulated deep in the crust of the earth.
    • Volcanoes release some of the carbon dioxide from the carbonate minerals into the atmosphere.
  • The negative feedback arises because an increase in the amount of carbon dioxide in the atmosphere results in an increase in temperature. Increased temperature increases evaporation of water from the oceans which increases the amount of rainfall. Increased rainfall increases the amount of ions in the water, which increases the rate at which carbon dioxide is incorporated into shells which eventually reduced the amount of carbon dioxide in the atmosphere. This allows the Earth to keep a relatively stable temperature over geologic time even though the sun has gotten significantly brighter
  • There are limits to the feedback. Water vapor is also a greenhouse gas. If the amount of water vapor in the atmosphere gets too high, it will become warmer which will cause more water to evaporate. This is a positive feedback mechanism, called the "runaway greenhouse."
  • On Venus, this positive feedback occurred, water vapor in the atmosphere increase the temperature until the oceans had entirely evaporated. Water in the atmosphere was broken into hydrogen and oxygen by UV light. The light-weight hydrogen escaped. The oxygen combined with minerals in the crust. Without liquid water, there was no way to make carbonate minerals so all the carbon dioxide in the crust was eventually released into the atmosphere.
  • Positive feedback in the opposite direction is also possible. Ice is a good reflector of sunlight. If sunlight isn't absorbed by the surface, little infrared is radiated, so surface ice decreases the effectiveness of greenhouse gasses. That results in more cooling and more ice build up. If this isn't balanced by an increase in greenhouse gasses, eventually the planet could get cold enough from carbon dioxide to freeze out of the atmosphere. With no carbon dioxide in the atmosphere there would be little chance of warming up again.
  • Understand how and why more carbon dioxide in the Earth's atmosphere will affect our global climate.

Mar 18 -

Properties of life on Earth

• Composed of Cells
  • Not necessarily a universal property of life
  • Some non-living things have cell like structures
• Uses energy
  • Many things that are not life also use energy
• Self Replicating
  • Based upon a stored pattern or template (DNA)
  • Some non-living chemicals can be considered to be self replicating.
• Capable of responding to the environment.
  • There are non-living things that also respond to the environment.
• Maintains homeostasis (internal balance)
  • Also present in some non-living things (i.e. climate on the Earth).
• Capable of evolution due to imperfect replication.

Evolution

• DNA is the molecule that contains the information used to construct an organism.
• DNA consists of strings of base pairs of nucleic acids.
• A group of three base pairs (a triplet codon) codes for an amino acid.
• A gene is a string of triplet codons that is a list of the amino acids used to build a protein.
• Large portions of the DNA in an organism are unused.
• Mutations are the primary mechanism by which the DNA molecule changes.
  • Mutations can be beneficial, neutral, or harmful. Most are neutral or harmful.
  • You probably have 10 unique mutations in your DNA. Three are probably harmful. The rest are likely to be neutral.
  • What is neutral to you may be harmful or beneficial to your distant descendants.
• Selection is the means by which genes are removed from a population.
  • This occurs if an organism dies before reproducing, cannot produce viable offspring, or cannot attract a sexual partner.
• The combination of selection and mutation cause isolated populations of organisms to become dissimilar.

The history of life on earth

• The oldest definite fossils are bacterial fossils about 3.5 Gy old.
• There are chemical hints that life may be as old as 4.2 Gy.
• Abiogenesis: the formation of life from non-living things.
  • Organic molecules, including amino acids, are found in star forming regions, meteors, asteroids and comets and could have arrived on Earth during bombardment.
  • Amino acids could also have been created by UV light or electrial discharge in the atmosphere of the early earth.
  • Given enough time, and the right conditions, long chains of these molecules formed through chemical evolutions. Eventually one capable of self-replication formed and biological evolution took over.
  • Some astronomers theorize that bacterial life arrived on Earth from another solar system. This theory is called panspermia.
  • The first organisms probably got their energy through chemosynthesis rather than photosynthesis. You should understand the difference.
• Know the timeline for the formation of life.
  • Life took between 0 and 700 My to get started after the oceans formed.
  • For 3 Gy, life on earth was dominated by bacteria and other single celled organisms.
  • Complex multi-cellular organisms have only been around for 550 My.
• We can use the timeline of life on earth to help us understand how difficult it is for certain steps to take place.
  • Think of playing Yahtzee. Rolling a 6 is easy, it might happen on the first roll. Rolling a yahtzee (5 of a kind) is far more difficult. That makes it likely to happen later in the game. It might not happen at all. But then again, sometimes yahtzee happens on the first roll.
  • Based on the timeline we think making single celled life is fairly easy, but getting complex multi-cellular organisms is probably hard. Once you've got them though, getting intelligence seems fairly easy.

Life elsewhere in the solar system

• Earthlike life requires: A planet or moon, assorted elements like carbon, oxygen, nitrogen, and metals, a liquid solvent (water), and an energy source.
  • Venus and mars have no liquids
  • Mars probably had liquid water in the past. A meteorite believed to have come from Mars shows carbonate deposits, complex organic molecules, and tiny "fossil-like" structures that MAY have been produced by bacteria. HOWEVER, the water that deposited the carbonates was probably too hot for life, the organic molecules may not be from life, contamination may have occurred, and the "fossils" are quite probably natural mineral features.
  • Venus may have had liquid water for a short time.
  • Gas giants could conceivable have life floating in the atmosphere or oceans.
  • Jupiter's moon Europa my have a liquid water ocean under its icy crust (remember that tidal heating keeps this moon somewhat warm).
  • Titan's moon is this cold because of an "inverse Greenhouse Effect": the atmosphere lets infrared (heat) out, but doesn't let new light from the Sun in. (Dust/soot particles can have this effect, which is why large volcanic eruptions or a nuclear war might produce a cool climate on Earth).
  • Titan's atmosphere contains a smog of organic molecules (produced in reactions with sunlight) that settle to the ground in a tarry goo.
  • Saturn's moon Titan may have methane oceans on the surface, and has an organic tarry goo settling out of its atmosphere. But the surface is COLD.

Life in other solar systems

• We have detected planets in other solar systems by detecting motion of the stars caused by the gravitational pull of the planets
  • We can only detect Jupiter or bigger, can't detect earth sized planets
• Stars have a "habitable zone", a range of distances at which an earth sized planet could support earthlike life.
• How long the habitable zone lasts is a function of the type of star

Mar 25-The properties of stars

• Stars: Hot dense object producing a continuous spectrum, with a cooler outer atmosphere which produces absorption lines.
• The presence of particular absorption lines can tell us something about which elements are present (the absence of a line doesn't insure the absence of the element, however).
• The strength of particular absorption lines depends on how many electrons are in the relevant energy level. If the gas is too hot the electrons will be out of the atoms (which are therefore ionized), and you might not see the lines. If the gas is too cool, the electrons will all be in the lowest energy levels, and you won't see transitions involving the higher energy levels. In both cases, the lines will be weak. The strength of absorption lines therefore indicates the temperature of the gas. The colour (wavelength of spectrum peak) also gives a cruder indication of the temperature, but the peak wavelength is not always in visible light.
• Parallax is very important. All distances to stars and galaxies are based on these measurements! Understand the idea of parallax, and that the measurement is smaller for more distant stars. We can only measure this for stars nearer than about 500 pc. Know the definition of a parsec!
• The Doppler effect is also important, in that it is how we measure motions along the line of sight between us and a star. Understand how the effect works for sound waves, and how we use it to measure radial velocity.
• Notice that we also need the transverse velocity to completely know the star's motion. To do this we measure a star's proper motion (or apparent change in direction with time), and combine that with distance.
• The inverse square law is also very important. The luminosity or intrinsic brightness of a star is the energy per second coming from the star. How bright the star really is. The apparent brightness is how bright it looks to you, or the energy per second per unit area that you see. The apparent brightness of a star depends on the inverse square of the distance to the star. In other words it is proportional to 1/(distance)². For two identical objects, something twice as far away looks 1/4 as bright. Something 10 times as far away looks 1/100th as bright.
• Be very familiar with the HR diagram, as we will use it for the next couple of weeks! A plot of luminosity vs. temperature, it shows that stars fall into distinct groups. Be able to recognize these groups.
• Understand that we can use the orbits of binary stars (two stars moving around each other) to estimate the masses of the stars. But we need to know everything about the orbits before we can use gravity to determine the masses! Finding masses is hard!
• Know the 3 different ways we can see a binary star system (measuring motions on the sky, seeing changing Doppler shifts in the absorption lines, and seeing a periodic dip in the light from eclipsing binaries).
• For the first case, a visual binary, only a few nearby systems with widely separated stars can be easily measured. And it takes a long time to determine the orbits!
• Understand that for the second case, a spectroscopic binary, we can only measure motion towards or away from us, so can't be sure we are measuring the whole velocity! Only if an eclipsing binary (where the orbit must be edge-on) is also seen as a spectroscopic binary can we figure out the whole orbit.
• In these cases, the maximum velocity of the stars, combined with the orbital period, can give us the size of the orbit, and so we can use gravity to find the masses. In these special cases, as a bonus, the time that the star spends in eclipse, plus the speed of the stars, gives us an measure of the radius of stars!
• Understand that for a blackbody, the energy emitted per second per unit of surface area is proportional to (temperature)⁴. Something twice as hot emits 16 times as much energy in each square meter of its surface. So if two stars are the same size, but one has twice the temperature, that one has 16 times the luminosity (or intrinsic brightness).
• Understand that this fact allows us to estimate the sizes of stars. By measuring the temperature, we know how much energy is emitted in each square meter of the star. By measuring the luminosity, we know how much total energy the star is emitting. Comparing the two numbers tells us how much surface area the star must have. Bigger stars are brighter. If two stars of the same temperature, but one has twice the radius of the other, then that one emits four times as much total energy.
• The above methods are fairly "direct". But once we accept the HR diagram, we can estimate the luminosity of a star just by measuring its temperature and which 'luminosity class' it falls into on the HR diagram. (These are both possible through a detailed analysis of the spectrum). Then we can use the HR diagram to estimate the luminosity. Comparing this with a measurement of the apparent brightness also gives an estimate of the distance. This is how we determine the distances to stars that are more distant than about 500 pc! (Remember the Milky Way Galaxy is up to 30 000 pc across!
• Handout with summary of how we measure stellar properties here
• Temperature: color of star, and strength of absorption lines
• Distance: parallax is the only direct method
• Luminosity: measure apparent brightness and distance, calculate luminosity
• Which class on HR diagram (ie main sequence or giant or white dwarf): width of absorption lines
• Distance: Measure temperature, and whether the star is on the main sequence or a giant or a white dwarf. Assume that it would fall in the same place on the HR diagram as other stars of its type. This gives you the luminosity. Measure the apparent brightness. Comparing the two gives an estimate of the distance.
• Radius or Diameter: Measure the temperature, and the luminosity. The temperature tells you how much energy is being emitted per second per unit surface area of the star. The luminosity is the total energy per second. Comparing the two gives you the surface area, which is proportional to (Radius)²
• Masses: Measuring the orbit of binary stars and applying Newton's generalization of Kepler's Laws (which follows from the law of gravity) allows you to estimate the masses of the orbiting stars.
• Understand how we use our knowledge of blackbody radiation to estimate the radii (sizes) of stars.
• Know that intrinsically bright (luminous) stars are rare, but can be seen for a long ways. The most common stars are the dim red main-sequence stars. Most nearby stars are dim.
• Know that due to differences in size (radius), giants and supergiants are much less dense than main sequence stars, while white dwarfs are much more dense.
• Know what I mean by the Mass-Luminosity relation: looking ONLY at main-sequence stars, the most massive ones are the most luminous.
• Recognize that the Mass-Luminosity relation tells us something about how long a star will live. A star 10 times the mass of the sun has 10 times as much fuel. But it has 10,000 times the luminosity, so it's burning it's fuel 10,000 times faster. That means a 10 solar mass star will only live about 1/1000 as long as the sun.

April 1- Stellar Structure