Europa vs. Earth’s WaterCourse AnnouncementsAssignmentsRadioactivityRadioactivityRadioactive DatingRadioactive DatingDifferentiationDifferentiationDifferentiationOutgassingOutgassingBack to DifferentiationTerrestrial PlanetsTerrestrial Planet CoresCreation of Planetary Magnetic FieldsTerrestrial Magnetic FieldsTerrestrial PlanetsTerrestrial PlanetsTerrestrial PlanetsTerrestrial PlanetsTerrestrial PlanetsGas Giant PlanetsGas Giant Planets - StructureGas Giant Planets - CompositionGas Giant PlanetsChapter 7Earth’s General StuctureOrigin of Earth’s AtmosphereOrigin of Earth’s AtmosphereOrigin of Earth’s AtmosphereOrigin of Earth’s AtmosphereEarth’s Atmosphere - CompositionEarth’s Atmosphere – Lower LayersEarth’s Atmosphere – Lower LayersEuropa vs. Earth’s WaterImage Credit and Copyright:Kevin Hand, J. Cook, H. PerlmanCourse Announcements•Quiz 3 returned Wednesday•Reduced Extra Credit SessionsUpcoming Class Schedule:•Quiz 4 on Monday 24, October•Exam 2 on Friday, 28 OctoberAssignmentsReading Assignments•Chapter 7: Sections 7.4 – 7.6[Read by Wednesday, 19 Oct.]Parallel Lectures•CC Astronomy – Episode 11: The EarthMastering Astronomy•Chapter 7 Homework[Due Monday, 14 October]Radioactivity•The original unstable isotope is the Parent Nuclide•The resulting, more stable, isotope (of a different element) is the Daughter Nuclide•Process is truly random and can therefore has a characteristic time (the describes the decay rate (the rate at which parent becomes daughter)–The time period to convert half of the parent material to daughter material is called the half-life.Process (It’s stochastic!) and terminologyRadioactivity•The time period to convert half of the parent material to daughter material is called the half-life.Process (It’s stochastic!) and terminology1 Half-life2 Half-lives3 Half-lives1/211/22 = 1/41/23 = 1/8Radioactive Dating•If we know the half-life, the original parent-daughter starting ratio, and the current parent-daughter ratio, we can accurately date materials.–This is how we know the age of the solar system from meteorites.•Original ratio is hard to know, but we can leverage multiple radioactive elements and use isotopes that have unique daughter elements (radioactivity is only way to make that isotope)–Or exploit how mineral crystals form that only incorporate parent nuclides into their structure (E.g., zircons)•All daughter nuclides can therefore be assumed to be radiogenicHalf-lives of radioactive isotopes provides a natural dating techniqueRadioactive DatingCommon long half-life isotopes useful for geologic datingHalf-life =Age of Solar SystemDifferentiationBack to planetary differentiation•The process by which a molten, or partially molten, protoplanet/planet will become layered according to density. The densest material will sink to the center, and the lightest will rise to the surface due to gravity.DifferentiationEvolving a ProtoplanetDenser Silicate MaterialSilicate MaterialMetallic MaterialRadioactive Elements[ 26Al , 60Fe ]Un-differentiated ProtoplanetRadioactive elements heat and melt the planet from inside outTIMEDense materials sink to centerLighter materials float to topDifferentiationEvolving to a planetLeast dense material forms in melt and floats to the top to form a planetary crustTIMEEventually cools from outside-in leaving a: thin planetary crustThick planetary mantleIron/nickle coreMost Dense material sinks to center to form a planetary corecrustmantlecoreOutgassingBuilding an atmosphereHowever, the terrestrial planets are not large enough to hold onto hydrogen and helium via gravity.=> Primary atmosphere rapidly lost and left with essentially no atmosphere at allOriginally the planet will have a primary atmosphere composed hydrogen and helium inherited from the Solar NebulaHHeHHHHHHHHeTIMEOutgassingBuilding an atmosphereThe volcanic outgassed secondary atmosphere is rich in water vapor, carbon dioxide, sulfur dioxide, methane, and nitrogen compounds (N2, NH3, NO, etc.).This secondary atmosphere will be what evolves into the currently observed atmospheres.The young planet is still very hot and interior still mostly molten=> Lots of volcanic activity that will spew gasses out of the interior to form a secondary atmosphereTIMEH2O, CO2, SO2, CH4, N2, Nitrogen compounds Secondary AtmosphereBack to Differentiation•In order to supply a large amount of energy (heat) through radioactive decay of unstable isotopes we need:–Abundant elements with short half-life radioactive isotopes•Two isotopes fit this requirement and meteoritic evidence indicates they were abundant in the Early Solar System.1. Aluminum-26: (26Al -> 26Mg) Half-life = 730,000 years2. Iron-60: (60Fe -> 60Co) Half-life = 2.6 million years–Supplies evidence that the shockwave of a supernova caused the initial collapse of the nebula that turned into our solar systemIsotopes responsibleTerrestrial PlanetsThe generalities of Earth-like worldsExpectations•Early after formation, volcanic activity will outgas a secondary atmosphere rich in carbon dioxide, sulfur dioxide, and nitrogen compoundsSummary•We expect all terrestrial planets to have a layered internal structure and to have had an atmosphere at some point in their history. •These generic factors of terrestrial planets plus the overall size will determine what how the planetary surfaces change over time (tectonic processes, erosion, volcanism, etc.).Terrestrial Planet CoresTo be molten, or not to be moltenThe “Baked Potato” AnalogyA big baked potato takes a lot longer to cool down compared to a small baked potato.•The terrestrial planets in our solar system have had 4.5 billion years cool down and let that heat involved in planetary differentiation escape to outer space.•How hot the interiors are, and therefore whether or not part of the metallic cores are still molten or have solidified, depends mostly on how large the planet is.•We care about this because the creation of a planetary magnetic field depends on TWO characteristics:1. Needs a electrically conducting fluid (molten metal will do) that is convecting2. Needs to rotate fast enough to act like a giant electro-magnet.Creation of Planetary Magnetic Fields1. Convection of conducting liquid in interior2. Sufficiently fast rotation rate (on the order of 10s of days at most).–Faster spin => more likely to have magnetic field if (1) is also satisfiedThe combination of these creates a dynamo
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