Lecture 19Part 2: Climates of the Past1) The geologic timescale:• the age of the Earth/ Solar System• the history of the Earth2) The evolution of Earth’s atmosphere - fromits origin to present-day3) The co-evolution of lifeTextbook: Chapters 10, 11, 12 (and some of 8)Dating rocks, part 1Basics of Chemistry/Physics: A reminder:Atoms have protons (positively charged) and neutrons (neutral)in a central nucleus. The number ofelectrons (negatively charged)surrounding the nucleus is equalto the number of protons for an atomwith no overall charge.Atomic number of atom = number of protons Mass number of atom = number of protons + neutronse.g. Rubidium-87 is written Atomic number = 37, Mass number (“atomic weight”) = 87i.e., the Rb atom has 37 protons and 50 neutrons in the nucleus† 3787RbIsotopesisotope = atoms with the same number of protons butdifferent number of neutrons (p.97 Kump)e.g. carbon has two stable isotopes, † 612C, 613Cand various radioisotopes, including† 614CVital for dating materials and biogeochemistry (i.e.,understanding how biology affects the chemistry of the solid Earthand atmosphere)with a half-life of 5570 years† 615Cwith a half-life of 2.3 secondsa radioisotope is unstable, undergoes radioactive decay, andchanges into another more stable elementhalf-life = the time for half the atoms in a sample to decayDating rocks, part 2Look at a radioactive element and measure the amount ofits daughter (decay product) in the rocke.g. Rubidium-87, which decays to Strontium-87 with a half-life of 49 billion years† 3787Rb Æ 3887Sr +ba “Beta ray” = a high-energy electron that isejectedThe amount of Sr-87 present depends on 3 things:1) Amount of Rb-87 originally in the rock2) Amount of Sr-87 originally in the rock3) Time (which decreases Rb-87 atoms and adds Sr-87 atoms)Dating rocks, part 3We can look at different minerals in the rock that startedoff with different Rb contents. (Abundance is given relativeto the unchanging abundance of a stable isotope, in this case Sr-86).original abundancesfinal abundancesmineral A mineral B† number of 87Sr atoms1000 86Sr atomsolderyoungerAge is proportional to the slope. Slopes on such plots are “isochrons”† number of 87Rb atoms1000 86Sr atomsAge of the Solar System & EarthMeteorites called “chondrites” have not been altered byheating or melting and are bits of rock that were not incorporatedinto a planet. They are leftovers from the formation of the planets.87Rb-87Sr gives an age for chondrites of 4.56±0.01 billion years oldWe can also use other radioisotope pairs and we get the SAME age(e.g. p.189 Kump describes uranium-lead isotopes in chondrites).Oldest Moon rocks (from Apollo astronaut collection) = 4.44 b.y.Oldest Mars rock (a Martian meteorite) = 4.5 b.y.Oldest Earth rock fragment = 4.4 b.y.Age of Earth (from deducing same original lead isotopes in theEarth’s mantle as originally in meteorites) = 4.56 b.y.Beta Pictoris (55.4 light years away) is surrounded bya “debris disk” of dust 1-30 micron size. Transientspectral features may be due to infalling comets.There is little dust within 20 AU of the star, probablybecause it has coalesced into planetesimals /grains:A solar system in the making.The Solar System formedby gravitational collapse ofa large, rotating cloud ofmatter. The central regiongrew denser and becamethe Sun. The remainderbecame a disk of gas anddust, the solar nebula.We can see circumstellardiscs that support thistheory>20,000 asteroidsbetween Jupiter & MarsIce-rich Kuiper BeltObjects (KBOs)at 35-1000 AUOort Cloud: a swarm of1012-1013 comets largerthan 1 km in size, orbits at >104 AUThe Earth-Moon systemThe Moon formed as a result of alarge collision with a Mars-sizedobject. The large impactorcoalesced with the proto-Earth,causing much of the planet tomelt. Under such conditions,iron (being dense) separated intothe Earth’s core. However, theimpact caused considerabledebris to be ejected into orbitaround the Earth from theEarth’s mantle. The debrisaccreted to form the Moon.A violent youthful neighborhoodWhen an asteroid or comet hitsa planet, it makes an impactcrater. Such cratersaccumulate with time.Moon Fig. Crater size distribution in 3.4 Galunar lava plains. The line turns up at smallsizes because of many small impacts due to"secondary" fragments blown out of craters.The solid line is the crater distribution in veryheavily cratered uplands called saturationequilibrium; if you added a new crater atthis crater density, the new one wouldobliterate older craters and on average theline would stay about the same.Qu.) (a) Why are there so fewer craters on the Earth?(b) At what diameter does the Earth have similar orsomewhat greater crater density than the
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