The Ghost Nebula Credit Adam Block Mount Lemmon SkyCenter U Arizona Course Announcements Thanksgiving Week planning No classes Wednesday 23 Nov Exam 2 grading underway Expected return date Friday 4 Nov Assignments Reading Assignments No new reading Parallel Lectures CC Astronomy Episode 13 Mercury Watch for Wed 2 Nov Mastering Astronomy Chapter 8 Homework Due Wed 2 Nov at 11 59 PM EDT Origins of the Moon Evidence to help us discriminate between hypotheses The overall composition of the Moon is very similar to the composition of Earth s mantle and crust Similar rocky material and metal content Unlike the Earth the Moon lacks a significant iron core Overall deficiency of amount of iron compared with Earth Relative to the size of the planet it orbits the Moon is very large largest relative to planet size in the Solar System Angular momentum of the the Earth Moon system must be accounted for Given a formation hypothesis does the physics work Finer detail Oxygen isotopes volatile content etc Interested in these details See these Science articles 1 2 Origin of the Moon Need an explanation that accounts for chemical similarity to Earth s mantle and overall iron deficiency Composition and density Old Ideas Coformation Sister Hypothesis Earth and Moon separately but at the same time and out of the same disk material Explains chemical similarity between the Moon and Earth s crust and mantle but if true the Moon should have a roughly equivalent amount of iron as the Earth Origin of the Moon Need an explanation that accounts for chemical similarity to Earth s mantle and overall iron deficiency Composition and density Old Ideas Coformation Sister Hypothesis Earth and Moon separately but at the same time and out of the same disk material e p Explains chemical between the Moon o Nsimilarity and Earth s crust and mantle but if true the Moon should have a roughly equivalent amount of iron as the Earth Origin of the Moon Need an explanation that accounts for chemical similarity to Earth s mantle and overall iron deficiency Composition and density Old Ideas Capture Theory Earth and Moon formed separately in different locations in the disk and then the Moon was captured Explains overall composition and density dissimilarities BUT if we consider the physics involved for Earth to capture such a large object is it possible Very Very Difficult to nearly impossible Origin of the Moon Need an explanation that accounts for chemical similarity to Earth s mantle and overall iron deficiency Composition and density Old Ideas Capture Theory Earth and Moon formed separately in different locations in the disk and then the Moon was captured e p No Explains overall composition and density dissimilarities BUT if we consider the physics involved for Earth to capture such a large object is it possible Very Very Difficult to nearly impossible Origin of the Moon Need an explanation that accounts for chemical similarity to Earth s mantle and overall iron deficiency Composition and density Old Ideas Fission Daughter Hypothesis Earth and Moon formed as a single object and due to a very high rotation rate the Earth spun off rotational fission the Moon Explains composition similarity between Earth s mantle and the Moon But can this really happen based on physics NOPE No reason for Earth to spin that fast and even if it did the spun off object wouldn t have a stable orbit Origin of the Moon Need an explanation that accounts for chemical similarity to Earth s mantle and overall iron deficiency Composition and density Old Ideas Fission Daughter Hypothesis Earth and Moon formed as a single object and due to a very high rotation rate the Earth spun off rotational fission the Moon Explains composition similarity between Earth s mantle and the Moon But can this really happen based on physics NOPE No reason for Earth to spin that fast and even if it did the spun off object wouldn t have a stable orbit Wise et al 1963 Origin of the Moon Need an explanation that accounts for chemical similarity to Earth s mantle and overall iron deficiency Composition and density Old Ideas Fission Daugher Hypothesis Earth and Moon formed as a single object and due to a very high rotation rate the Earth spun off rotational fission the Moon e p o N Explains composition similarity between Earth s mantle and the Moon But can this really happen based on physics NOPE No reason for Earth to spin that fast and even if it did the spun off object wouldn t have a stable orbit Origin of the Moon Need an explanation that accounts for chemical similarity to Earth s mantle and overall iron deficiency Composition and density Current Idea Giant Impact Hypothesis When the Earth was still young and molten it experienced a grazing impact with a Mars sized protoplanet dubbed Theia The result was the current Earth and a Moon composed almost entirely of Earth and Theia mantle material Explains composition similarity between Earth s mantle and the Moon also can really happen based on physics Origin of the Moon Giant Impact Hypothesis Snap shots of a Giant Impact Hypothesis simulation Colors indicate composition Blue Silicates Rock Red Iron End of simulation is disk rock rich iron poor disk of material 3x mass of Moon in this sim out of which the Moon will form Cool Video of a sim 1 SwRI Simulation 2 From Canup et al 2012 Science Origin of the Moon Giant Impact Hypothesis Simulation of Proto Earth and Theia a Mars sized protoplanet Iron core of Theia incorporated into Earth Mantle material of Earth largely goes into making the newly formed Moon Credit Robin Canup SwRI Origin of the Moon Summary We gauge the validity of Moon formation hypotheses based on the following criteria Moon composition being very similar to Earth s Mantle Lack of Iron in the Moon Physics Do the rotation rates angular momentum make sense Finer Detail Volatile composition isotopic ratios 1 2 Alternative Ideas Not currently favored Coformation Sister Hypothesis Capture Hypothesis Fission Daughter Hypothesis Currently favored Idea Giant Impact Hypothesis Evolution of the Moon The maria really were seas seas of lava Formation date 4 5 Gya slightly younger than Earth but not by much Oldest rocks dated 4 4 Gya age of highlands Indicates that the crust must have already formed by this time Energy from formation and meteoritic bombardment heated and melted at least the surface layers depths to 100s of km but perhaps shortly after formation entire Moon molten hotly debated topic in lunar science Main Point At
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