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Astronomy 251 Life on Other Worlds Mondays 4-6, MP102 Prof. Ray Jayawardhana MP 1408 416 946 7291 [email protected] Office hours: Tuesdays 11-12 or by appt. Course website and syllabus: http://www.astro.utoronto.ca/~rayjay/ast251 Course Textbook Goldsmith & Owen The Search for Life in the Universe READING 1st WEEK: Ch. 1, 2 & 7 (p. 163-166) READING 2nd WEEK: Ch. 6 & 3 READING 3rd WEEK: Ch. 4 & 5 READING THIS WEEK: Ch. 11 & 17 Astronomy 251 Who is my TA? Parandis Khavari Duy Nguyen Tim Rothwell Marija Stankovic khavari@astro nguyen@astro rothwell@astro stankovic@astro A-C D-La Le-R S-Z Upcoming Dates Sample paper topics available on website TA consultation sessions (for paper topics): Tue Jan 25 3-4pm McLennan 15th floor conference room Fri Jan 28 12-1pm McLennan 15th floor conference room Paper topics due in class: Mon Jan 31 ***NEXT CLASS*** Topics approved by Feb 7; Modifications finalized by Feb 14 Paper outlines due in class: Mon Feb 21 Midterm test: Mon Feb 7 4pm Sidney Smith Hall 2102 Sidney Smith Hall 2118 A-La Le-Z Midterm review sessions by TAs: Tue Feb 1 3-4pm McLennan 15th floor conference room Fri Feb 4 12-1pm McLennan 15th floor conference room Instructions for Paper Topics Turn in topic on paper (typed) in class on (or before) Jan 31 DO NOT SUBMIT PAPER TOPIC BY EMAIL Must include 1. Your name and student ID 2. Email address 3. Proposed paper topic 4. Thesis statement (e.g., Past life on Mars) (e.g., There is good evidence that bacterial life existed on Mars in the past.) 5. Two or three sample references (e.g., book title & author, title & author of an article in Scientific American, a web site) Response from TAs by Feb 7 (at midterm); any final modifications must be done by Feb 14. 10% penalty for each day’s delay after Jan 31; zero after one week’s delay One letter grade deduction for not handing in paper topic Another letter grade deduction for handing in paper outline (Feb 21) Major Solar Flare Last Thursday Huygens on Titan Mosaic of river channel and ridge area Evidence of water ice and methane springs Huygens on Titan Titan’s varied terrain: (lighter-colored uplifted areas, marked with what appear to be drainage channels, and darker lower areas) Saturn now visible (on clear nights!) http://SkyandTelescope.com 25 Msun 1 Msun Points to Remember 1. Stars are in a state of hydrostatic balance between pressure and gravity 2. Without a source of energy, they would contract. The Sun would contract significantly in about 15 Myr. 3. Nuclear fusion of H to He, then on to heavier elements, creates new heat to sustain a star in energetic balance. 4. Elements with more protons require higher core temperatures to undergo fusion 5. This only works up to 56Fe – heavier elements require energy to create and give off energy only in fission Points to Remember 6. Stars are created in clusters. There are more stars like the Sun (G and M stars) than massive (O and B) stars. 7. More massive stars are much more luminous, so they live much shorter lives. 8. Stars more massive than 8 solar masses explode as supernovae. 9. Supernovae draw energy from gravity to create the elements heavier than 56Fe. 10. Stars get slightly brighter on the main sequence and then much brighter and bigger as red giant stars. AST 251 Life on Other Worlds Lecture 4 Formation of Stars and Planets Basic properties of the solar system Historical ideas on the origin of the solar System Modern theories and observations Planet fact sheet – 1 Orbits and rotation •All planets orbit in the same plane and in the same direction as the Sun’s rotation (prograde). •6 of the planets also rotate in that plane and direction (prograde). The larger, close-in moons all do, too. •In our Solar System, the orbits of eight (!) planets are nearly circular. So are the orbits of the large, close-in moons. •The mass of the Solar System is concentrated in the Sun (99.95%), but the angular momentum is mainly (98%) in the orbits of Jupiter and Saturn (mainly Jupiter). Planet fact sheet – 2 Solar System Inhabitants Inner (Terrestrial) Planets: Mercury, Venus, Earth, Mars Composition: Dense, refractory (highly depleted of volatiles); otherwise, similar to Solar composition. Density: about 5-10 times that of water Asteroid Belt: small rocky bodies between Mars & Jupiter Gas Giant Planets: Jupiter, Saturn Composition: almost the same as the Sun’s; somewhat depleted of H, He Density: about that of water (same as the Sun). Mass: 1/1000 of the Sun’s. Ice Giant Planets: Uranus, Neptune Composition: Significantly depleted of H, He (just 5-20% by mass) Density: a couple times water Kuiper Belt: A flattened disk of asteroids, of which Pluto is considered the largest Comets: a trillion of ‘em, in the (spherical) Oort cloud, extending 1/20 parsec away Why are the planets in such a flat disk? This question spawned two competing theories… Where did the planets come from? The Encounter Hypothesis (Georges Buffon, 1745) The Sun was pulled apart by a nearby star, and the stream of gas condensed to form the planets Implications for Drake’s equation: Planetary systems are very rare (fp<<1), because stellar collisions essentially never happen (outside of globular clusters and the centre of the Galaxy). The Nebular Hypothesis (Immanuel Kant 1755; Laplace, 1796) The Sun and planets formed together out of a swirling cloud of gas (the “Solar Nebula”) Implications for Drake’s equation: Planetary systems can be quite common (fp~1). Sun Other star Sketch of the Encounter Hypothesis Tidal stream Condensation into planets Where did the planets come from? The Encounter Hypothesis (Georges Buffon, 1745) The Sun was pulled apart by a nearby star, and the stream of gas condensed to form the planets Implications for Drake’s equation: Planetary systems are very rare (fp<<1), because stellar collisions essentially never happen (outside of globular clusters and the centre of the Galaxy). The Nebular Hypothesis (Immanuel Kant 1755; Pierre-Simon Laplace,1796) The Sun and planets formed together out of a swirling cloud of gas (the “Solar Nebula”) Implications for Drake’s equation: Planetary systems can be quite common (fp~1). Sketch of the Nebular Hypothesis Gas cloud Formation of planets Condensation to disk The Nebular Hypothesis – Why flattened disks? As gas clouds evolve, their fate is determined by the conservation of angular momentum. 1. The cloud begins in a state of slow (even imperceptible) rotation. 2. It contracts, and it radiates away heat (recall Kelvin’s theory for the contraction of the Sun). 3. However, it tends to keep its angular momentum, because this changes only if there is a torque on the cloud from outside. 4. For the same angular momentum, the cloud must spin faster as it contracts 5. Flattening is the result of angular momentum conservation Two major views of the Nebular Hypothesis Geologists have drawn evidence primarily from the mineral and isotope ratios in the Earth and in meteorites. They posit a hot primordial nebula from which the minerals condensed as it cooled. Astronomers have only had a good picture of star formation since the advent of infrared astronomy, in the past few decades. They posit the accretion of cold gas onto the star and its disk; so that the gas remains cold unless it is close to the forming Sun. In their model, minerals and ices are inherited from the interstellar material and get chemically processed depending on location in the Nebula. In the astronomical model, the protoplanetary disk (Solar Nebula) is the remnant of the protostellar disk through which the Sun accreted much of its material. Important concept: the Minimum Solar Nebula The Minimum Solar Nebula is what you get if you take the material that’s currently in the planets, and then add enough volatile materials (mainly H & He) to “reconstitute” a gas disk with the same composition as the Sun. This is considered to be a lower limit to how much material could have been in the Solar Nebula (but this assumes that the planets did not move much after they formed). The Minimum Solar Nebula has about 1% the mass of the Sun. At Earth’s location, it has about 1 kilogram per square centimeter – so it is roughly as “thick” as the Earth’s current atmosphere. Exercise 1 In what ways are massive stars (in contrast to low mass stars) important for life in the universe? Exercise 2 Please explain why most stars are seen in the strip of the Hertzprung-Russell Diagram known as the “Main Sequence”. Establishing the life history of a meteorite using radioactive decays time What can be learned about the life history of a meteorite – Remnants of the Solar System’s formative epoch – 1a. When it solidified from a liquid 1b. When it cooled to a temperature comparable to the Earth’s mantle … from light, noble-gas daughters that leak away while it’s hot The cooling rate indicates how big the parent body was. 2. The magnetic field there was in when it cooled … is recorded in magnetic minerals 3. How long it spent floating around in space … from radioactive isotopes created by cosmic-ray impact 4. When it fell to Earth … from the decline of cosmic-ray isotopes after it’s protected by the Earth’s atmosphere Types of Meteorites Differentiated Meteorites are broken out of the interiors of larger bodies (planetesimals) whose insides melted because they were large enough to retain heat (from radioactivity). Examples: Stony and iron-nickel meteorites Primitive Meteorites have not been inside a molten body. Many primitive meteorites are carbonaceous chondrites – consisting of chondrules within an organic substrate. •Chondrules are 1-mm spheres that were briefly melted to about 2000 K and cooled (in a region of high magnetic field). They are depleted of volatiles. •The organic substrate is material that never got heated to the temperatures that made the chondrules. Ages of Meteorites Differentiated meteorites 4.4 – 4.56 Gyr Primitive meteorites 4.56±0.02 Gyr The Condensation Sequence in the Geological Nebular Theory Substance 1760 K Al203 Corundum 1510 K Temp. Substance 550-330 K Hydrated Minerals MgAl204 Spinel 173 K H2O Water ice 1470 K Ni-Fe Metal 120 K NH3H2O Ammonia ice 1440 K (Mg,Fe)2SiO4 Olivine 70 K CH46H2O Methane Ice 1000 K (Na,K) AlSi3O8 Alkali Feldspars 70 K N26H2O Nitrogen ice The Condensation Sequence indicates which substances can exist (or might form) below each temperature. Volatile Refractory Temp. The astronomical view of the Nebular Hypothesis 1. Stars are born in relatively dense “molecular clouds” The astronomical view of the Nebular Hypothesis 2. The cloud clumps collapse (contract) to form stars and disks Orion Disks in Silhouette Edge-on disks in Taurus The astronomical view of the Nebular Hypothesis 3. Within the dark, dense regions of molecular clouds, the shieldingout of ultraviolet radiation (by dust) allows relatively complex molecules – including organics like sugars and amino acids – to be created. These fall into the Solar nebula and, since it remains relatively cold, are not destroyed (if they stay a few AU out). How do planets form out of the disk? 1. Direct gravitational collapse This occurs if the disk gets dense enough that its own gravity pulls it together faster than its orbit can shear it apart --> portions of the disk collapse under their own gravity to make giant planets directly Pro: Con: Giant planets can form quickly (thousands of years) Most observed disks don’t seem to be dense enough How do planets form out of the disk? 2. Planetesimal accretion Model In this theory, dust grains combine to form pebble-size objects, which combine to form rocks, then boulders, then up to planets like the Earth. Pro: Both terrestrial and gas giant planets form the same way Con: Takes a while to make Jupiter (~5-10 million years): will there be enough gas in the disk for that long? © William K. Hartmann Runaway growth of kilometer-sized planetesimals Gravitational Focusing: Once a planetesimal grows to mountain sizes (1 km), it begins to pull other objects toward it as they pass by – increasing its “feeding zone” beyond its own size. Smaller than 1 km Larger than 1 km This leads to a runaway growth in which the largest body eats all its neighbors. This continues until its feeding zone is cleared of other objects (when the object is about the size of the Moon)… at which point growth slows down a lot, and relies on objects getting “kicked” into the feeding zone. The next big step occurs if it exceeds 15 Earth masses and gas is still around Forming Giant Planets When the planet mass exceeds about 15 Earth masses, it can begin to accrete gas as well as rocks. (Why? Then, its escape velocity exceeds the speed of gas particles.) This triggers another stage of runaway growth until the planet has opened a gap in the disk. Open questions: 1. Does the gas disk last long enough for the planets to get this big? 2. Once the planet gets big enough to affect the gas disk, it is rapidly shoved inward by the disk. How can giant planets ever stay put rather than being eaten by their stars? Planetesimals to Planets: Consequences Location Matters: Since planets are built up from solids in the disk, it matters a lot what solids exist at which location. Inside Mercury’s orbit, even refractory grains are evaporated. From Jupiter’s orbit outward, ices exist in the nebula. Planetesimals to Planets: Consequences The last stages of planet growth involve some very serious collisions… •The Moon is made of the same material as Earth’s mantle, and has no core. A popular explanation is that the Moon formed from material ripped out of the Earth by a collision with a planet somewhat smaller than Mars. •Mercury is a lot like the iron-nickel core of a planet that has no silicate mantle. Does this mean its mantle was stripped away in a collision? •The collisions that built the planets are still with us, but much rarer now than in the early Solar System. [The current impact rate is not sufficient to create the cratering seen on the Moon in the age of the Solar System.] There was a phase of heavy bombardment in our first 80 Myr. Observational clues to planet formation 0.5 Myr old star 2 Myr old star Gemini North Telescope, Hawaii Disk around 10-Myr-old star Cerro Tololo Inter-American Observatory, Chile James Clerk Maxwell Telescope, Hawaii Disk around 500-Myr-old star Solar System Epsilon Eridani Atacama Large Millimeter Array (now being built in Chile) 64 12-meter antennae with 10-km baseline