AST 301Introduction to AstronomyJohn LacyRLM [email protected] LiRLM [email protected] JeonRLM [email protected] site: www.as.utexas.eduGo to Department of Astronomy courses,AST 301 (Lacy), course websiteRearranged schedule for next few weeksToday: finish Ch 8, start atmospheresNext week: atmospheres, 2nd testReading:10/4 Ch 8 (and skim Ch 12)10/11 Ch 1010/18 Ch 13 (back on schedule)Topics from this weekHow terrestrial planet surface features are formedMass, volume, and density: a clue to a planet’s compositionAsteroids and comets: description and originWhat must a theory of planetary origin explain?Nebular theory of planetary originGravitational contractionRotational flatteningAccretion to form planetesimalsCollisions of planetesimals to form planetsFormation of giant planetsImportance of frost lineCore accretion vs. gravitational instabilityAge of the solar system: how it is determinedABCD questionsWhich of the following planets has the densestatmosphere?A. VenusB. EarthC. MarsD. JupiterE. ErisABCD questionsWhich of the following planets has the densestatmosphere?A. VenusB. EarthC. MarsD. JupiterE. ErisThe part of Jupiter’s atmosphere we can see (above theclouds) is about as dense as the Earth’s. But Jupiter isgaseous almost all of the way to its center, and itsaverage density is about the same as liquid water.Venus has a very dense atmosphere for a terrestrial planet.It is about 1/10 as dense as liquid water.A clue to compositionWhich of the following properties of a planet gives us agood clue about its interior composition?A. MassB. VolumeC. DensityD. Magnetic fieldA clue to compositionWhich of the following properties of a planet gives us agood clue about its interior composition?A. MassB. VolumeC. DensityD. Magnetic fieldComposition: what types of atoms or molecules a planet ismade ofDensity = Mass / VolumeTwo identical rocks glued together will have twice the massand twice the volume of one rock.So their density will be the same as that of one rock, andthey are still made of the same stuff.Small bodiesAsteroids and comet nuclei are two types of small bodies inthe solar system. Describe two ways in which they aresimilar and two ways in which they differ.CompositionSizesOrbitsOriginCan they have moons?Can you think of others?Nebular theoryWhat does the word nebular mean?Why is it an appropriate name for the theory?Describe the stages in the nebular theoryMolecular cloud coreGravitational contractionRotational flatteningAccretion to form planetesimalsFrom planetesimals to planetsLeftoversWhich explanation for the giant planets do youprefer?1. Beyond the frost line, dust grains had icy coatings. So planetesimals formed from both dust and ice. So they started out larger than planetesimals in the innersolar system, which were made of just dust. They grew large enough so that their gravities werestrong enough to pull in gasses.2. There was more mass available to make planets in theouter solar system and things out there weren’t orbitingas fast as near the Sun. So gravity could cause a clump of the nebula to pulltogether and form a planet made of the same stuff as thenebula was made of.The age of the Solar SystemHow can we know when the solar system formed?We think the asteroids formed during or soon after theformation of the solar system.We can determine the ages of pieces of asteroids that hitthe Earth (meteorites) by radioactive dating.The answer we get for the oldest asteroids we’ve found is4.5 billion years (4.5 Gyr).How does radioactive dating work?Potassium-Argon datingPotassium-40 has 19 protons and 21 neutrons in itsnucleus.Occasionally, one of the protons decays into a neutron anda positron, which flies out of the nucleus.The nucleus then has 18 protons and 22 neutrons, whichmakes it an argon-40 nucleus.Some rocks contain potassium, but none would containargon when they form, since argon is a gas.In 1,250,000,000 years (1.25 Gyr) one half of thepotassium-40 nuclei will become argon-40 nuclei, whichcan be trapped in the rock.By measuring how much potassium-40 and argon-40 arock has in it, we can determine its age.An exampleAssume a rock formed with 1,000 potassium-40 atoms in it.Here’s how many atoms it would have a various time later:40K40Ar 0 yr 1000 01.25 Gyr 500 5002.50 Gyr 250 7503.75 Gyr 125 875What if you found a rock with 1000 40K atoms and 300040Ar atoms in it?How old do you think it is?An example40K40Ar 0 yr 1000 01.25 Gyr 500 5002.50 Gyr 250 7503.75 Gyr 125 875What if you found a rock with 1000 40K atoms and 300040Ar atoms in it?It must have started with 4000 40K atoms in it, or 4 timesas many as in the table. If you multiply all of thenumbers in the table by 4, the 2.5 Gyr line fits.Or just ask at what time there are 3 times as many 40Aratoms as 40K atoms.Topics for next weekHow can we use the concept of thermal equilibrium tocalculate the temperature of the surface of a rock orbitingthe Sun?How does the result depend on the distance of the rock fromthe Sun?How does the Earth’s atmosphere affect the surfacetemperature of the Earth?Why do Venus and Mars have such different surfacetemperatures?How are we changing the Earth’s atmosphere, and how dowe think this will affect the surface temperature?The terrestrial planetsAll four terrestrial planets have interiors made mostly ofpartially molten metals (iron) and rocks. Mercury andMars have about ½ the diameter of Earth and Venus.Mercury has almost no atmosphere. Its surface temperatureranges from 100K to 700K (-300oF to 800oF).Venus has about 100 times denser atmosphere than theEarth’s. Its surface is a uniform 740K (880oF).Earth has an atmosphere about 1000 times less dense thanliquid water. Its average surface temperature is about290K (60oF).Mars has an atmosphere about 100 times less dense thanthe Earth’s. It surface temperature ranges from 130K to290K (-220oF to 70oF).Why are these atmospheres and temperatures so different?EquilibriumA balance between opposing influencesConsider a mass hanging from a spring.Gravity pulls it downward. The spring pulls it upward.If gravity and the spring pull equally hard in oppositedirections, the mass will not move.It is in equilibrium.If we increase the mass, gravity will pull harder.As a result, the spring will stretch.This will make the spring pull harder, until the spring forceand gravity balance
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