Formation of the Moon Richard Urata Colin Wallace Monica Hoke Bonnie Meinke ESA Older Formation Hypotheses http www nineplanets org gif earthrise jpg Fission Hypothesis http burro cwru edu Academics Astr221 SolarSys lunaform html Binder 1986 http www es ucl ac uk research planetary undergraduate bugiolacchi origins htm Capture Hypothesis http www es ucl ac uk research planetary undergraduate bugiolacchi origins htm Singer 1986 Co Accretion Hypothesis http www es ucl ac uk research planetary undergraduate bugiolacchi origins htm http burro cwru edu Academics Astr221 SolarSys lunaform html Impact Hypothesis ESA Size of the Moon Large satellites are an uncommon feature among terrestrial planets The ratio of the moon s mass to the Earth s mass is the largest of the eight planets 0 0122 Wood 1986 Suggests an unusual origin Large amount of angular momentum in the EarthMoon system 3 45 x 1041 rad g cm2 s Important factor in orbital studies and a fact that many lunar formation models such as the fission hypothesis have sought to explain from Wood 1986 Mean Density 5 52 g cm3 Mean Density 3 34 g cm3 The moon must have an underabundance compared to Earth of heavy elements specifically Fe Siderophiles Ni Fe and other elements that have a chemical affinity for Ni and Fe Abundance pattern of lunar siderophiles mimics the Earth s Depleted on Earth s crust due to differentiation from Wood 1986 Ratios of oxygen s three isotopes indicative of object s place of origin in solar system Earth and Moon have similar ratios from McSween 1999 Why should this be given their differences in Fe Lunar rocks found to be depleted in volatile elements and enhanced in refractory elements compared to the Earth Volatile readily evaporated Refractory high melting point Ex K U only 1 5th of terrestrial value from Wood 1986 Implies high temperatures during formation Impact Hypothesis Heat generated by impact explains volatile and refractory elements abundance on moon If impact occurred after differentiation then it explains lunar abundances of Fe and siderophile elements Moon and Earth formed in same area of solar system so oxygen ratios make sense Impact may also explain moon s size and large angular momentum of Earth Moon system Dynamics of Impact http starchild gsfc nasa gov docs StarChild questions question38 html Impact Variables Mass size of the protoEarth Early vs late phase of accretion Changes amount of iron in the moon Changes the O isotope similarities Mass size of the impactor Affects angular momentum of the system Affects mass size of the resulting objects Speed of impactor Affects how much material is put into the orbiting disk Constrains the pre collision orbit of the impactor Angle of impact Affects amount of orbiting material Affects the angular momentum of the system Ballistic Trajectories Ejected material Falls back to the protoEarth Stays in orbit Escapes the protoEarth system For material to stay in orbit must consider Gravitational torques Mutual interactions among ejected material Interaction with non spherical Earth Pressure gradients vaporization Important when specific impact energy exceeds the latent heat of vaporization for rock The Roche Limit The distance from a planet within which orbiting material will tend to disperse and form rings and beyond which the material will tend to coalesce and form a satellite Due to tidal forces from the planet exceeding the orbiting material s gravitational self attraction NASA Modeling the Collision Smooth particle hydrodynamics SPH Each particle s position velocity internal energy and density are evolved due to Gravity Compressional heating and expansional cooling Shock dissipation Ignors material strength and radiative processes Canup 2004 N 10 4 10 5 particles 100 simulations Mass size and angular momentum of impactor impact angle impact velocity differentiation compositions temperature Canup 2004 Canup 2004 The Result of the Impact Angular momentum Planet disk system has angular momentum of 1 18 times today s value Planet mass 0 994 Mearth rotational day 4 6 hours Disk Exterior to Roche limit mass 0 92 M lunar 80 is from the impactor 24 is vapor 1 9 is iron Interior to Roche limit mass 0 70 M lunar 85 is from the impactor 22 is vapor 9 1 is iron Escaped mass 0 41 M L Distribution of Particles and Temperature a Impactor and target particles color coded as to their final state Blue ends up in the protoearth Yellow orbits in the disk forms the moon Red escape the system b Change in particle temperatures degrees K experienced during impact Canup 2004 Disk Evolution and Lunar Accretion The initial protolunar disk Gravitational instability of the disk causes spiralarm structure to form Seed grows by accretion outside the Roche limit A lunar seed forms outside the Roche limit The accreted moon http th nao ac jp kokubo moon kit movie html Subsequent Evolution Moon s Formation Resulting Physical Properties Evolving Orbital Parameters Moon s Formation Material lost in giant impact Mantle of impactor Crust and upper mantle of Earth Core of impactor merged with Earth s core Material melted by energy of impact Magma oceans on new satellite Volatiles vaporized Material cooled and condensed Large moon coalesced Composition Low mean density Moon 3 3 g cm 3 Earth 5 5 g cm 3 Similar to Earth s Composition Lack of large Fe core Lack of Volatiles Physical Properties Core 340 km 2 MMoon Tidally locked Center of Mass offset 2 km toward Earth Crust thinner on near side Orbital Parameters Inclination At formation 10 At present 5 Tidally locked Moving away 22 500 km then 405 000 km now Angular Momentum L Ix Ltotal Lspin Lorbit As Spin Orbital distance Effects on Earth Moderates Earth s obliquity Makes Earth habitable Tides References Binder A B The binary fission origin of the moon Origin of the moon Proceedings of the Conference Kona HI October 13 16 1984 A86 46974 22 91 Houston TX Lunar and Planetary Institute 1986 p 499 516 Canup R M Dynamics of Lunar Formation Annu Rev Astron Astrophys 2004 Drake M J Is Lunar Bulk Material Similar to Earth s Mantle Origin of the moon Proceedings of the Conference Kona HI October 13 16 1984 A86 46974 22 91 Houston TX Lunar and Planetary Institute 1986 p 471 485 Hartmann W K Moon Origin The Impact Trigger Hypothesis Origin of the moon Proceedings of the Conference Kona HI October 13 16 1984 A86 46974 22 91 Houston TX Lunar and Planetary Institute 1986 p 471 485 McSween H Y Meteorites The New Solar System 4th ed New York NY Cambridge