Europa vs Earth s Water Image Credit and Copyright Kevin Hand J Cook H Perlman Course Announcements Quiz 3 returned Wednesday Reduced Extra Credit Sessions Upcoming Class Schedule Quiz 4 on Monday 24 October Exam 2 on Friday 28 October Assignments Reading Assignments Chapter 7 Sections 7 4 7 6 Read by Wednesday 19 Oct Parallel Lectures CC Astronomy Episode 11 The Earth Mastering Astronomy Chapter 7 Homework Due Monday 14 October Radioactivity Process It s stochastic and terminology 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 Radioactivity Process It s stochastic and terminology The time period to convert half of the parent material to daughter material is called the half life 1 Half life 2 Half lives 3 Half lives 1 21 1 22 1 4 1 23 1 8 Radioactive Dating Half lives of radioactive isotopes provides a natural dating technique 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 radiogenic Radioactive Dating Common long half life isotopes useful for geologic dating Half life Age of Solar System Differentiation Back 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 Silicate Material Differentiation Denser Silicate Material Evolving a Protoplanet Metallic Material Radioactive Elements 26Al 60Fe Un differentiated Protoplanet Radioactive elements heat and melt the planet from inside out TIME Dense materials sink to center Lighter materials float to top Differentiation Eventually cools from outsidein leaving a thin planetary crust Thick planetary mantle Iron nickle core Evolving to a planet Least dense material forms in melt and floats to the top to form a planetary crust crust mantle core TIME Most Dense material sinks to center to form a planetary core Outgassing Building an atmosphere Originally the planet will have a primary atmosphere composed hydrogen and helium inherited from the Solar Nebula H He H H He H TIME H H H H However 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 all Outgassing Building an atmosphere 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 atmosphere The 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 H2O CO2 SO2 CH4 N2 Nitrogen compounds TIME Secondary Atmosphere Back to Differentiation Isotopes responsible 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 years 2 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 system Terrestrial Planets The generalities of Earth like worlds Expectations Early after formation volcanic activity will outgas a secondary atmosphere rich in carbon dioxide sulfur dioxide and nitrogen compounds Summary 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 Cores To be molten or not to be molten The Baked Potato Analogy A 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 convecting 2 Needs to rotate fast enough to act like a giant electro magnet Creation of Planetary Magnetic Fields Requirements 1 Convection of conducting liquid in interior 2 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 satisfied The combination of these creates a dynamo electrical generator which produces a magnetic field This theory of how planetary magnetic fields are created is called Dynamo Theory Terrestrial Magnetic Fields Do we expect them We expect a dynamo to be in effect if the planet fulfills the two requirements In order to have liquid the planet must be sufficiently large so that it s interior hasn t completely solidified The Moon and Mars for example are much smaller than Earth and have likely cooled and solidified Mercury as we will see is a bit of an oddball here If rotation rate is less than 25 days perhaps up to 50 then we call that sufficient Details still not well understood Terrestrial Planets The generalities of Earth like worlds A good set of concepts to know hint hint Expectations Will be made of rocks and metal formed too close to the Sun to inherit much if any water and organics If sufficient amount of short half life
View Full Document
Unlocking...