Class 6: Formation of the Moon and the nature of modern impactors • Finish off talking about the Moon • Recap possible formation theories. • Digression into scientific method. • The success of the giant impact theory. • The nature of modern impactors • Comets and asteroids.
First, let’s finish the material from last class on formation of the Moon…
III : Formation of the Moon • Possible theories… • Simultaneous formation theory • Predicts that composition should be same. • NO! • Fission theory • Not possible dynamically… • Capture theory • Capture requires simultaneous interaction of 3 objects (Earth, Moon and something else)… too unlikely.
The giant impact theory Canup (SWRI)
The giant impact theory, cont’d • Mars sized object hits early Earth. • Ejects mantle material into space… forms a disk/ring orbiting around Earth • Material in ring then forms Moon. • Probably takes only a year or so! • Moon initially formed very hot, and gravity is not large. • Thus volatile elements and compounds are lost into space. • Explains age, composition, lack of iron; and is very plausible given conditions in early solar system.
I : Asteroids Gaspra (20 km)
Asteroid properties • We have already mentioned Earth- crossing asteroids (ECAs)… • Over 1000 known. • Over 300 are “potentially dangerous”… more than 150 m diameter and on orbits that bring them “close”. • Largest is 15 km. • Typically, they would have… • Density of about 2000-3000 kg/m3 (rock). • Speed of about 20-30 km/s near Earth.
Asteroids, cont’d • Using these numbers… • 150 m diameter asteroid hits with energy of 600 1-Mton H-bombs (or up to 22,000,000 Mtons for a 5 km sized asteroid). • Later we will discuss the likelihood of such events, and what we might be able to do to prevent them. • Do you think it makes a difference if asteroids are solid or made or rubble (with the same bulk density)?
II : Comets Hale-Bopp
Comet properties • A “typical” comet has… • Diameter = few km. • Speed = 40-50 km/s near Earth. • Density = 500 kg/m3 (dirty snow). • Let’s take a very small comet (150 m)… Energy of collision is 300 Mton. More typical is 5 km comet… 10,000,000 Mton! • So, both asteroids and comets can be very dangerous!
Questions to think about… • How much energy would it take to… • Deflect an object from its path? • What factors would it depend on? • Blow it up? • What’s keeping it together?