Formation of our Moon: The Giant Impact Hypothesis Michelle Kirchoff Southwest Research Institute Center for Lunar Origin and Evolution

Southwest Research Institute in Boulder

Center for Lunar Origin and Evolution (CLOE) History of Impacts on Our Moon  Late heavy bombardment (many large basins forming in a short time) -> “Nice” model or end of accretion? Goal: Learn more about how our Moon formed and changed in its early history

Center for Lunar Origin and Evolution (CLOE) History of Impacts on Our Moon  Late heavy bombardment (many large basins forming in a short time) -> “Nice” model or end of accretion? Goal: Learn more about how our Moon formed and changed in its early history

Center for Lunar Origin and Evolution (CLOE) 1)History of Impacts on Our Moon  explore different rates of impacts  Computer models constrained by new information about evolution of comet and asteroid populations  Analyze chemistries and ages of early Earth and Moon rocks  Analyze new images of Moon's surface to create a timeline of impact craters 2)Formation of Our Moon: Giant Impact Theory  How the disk evolved into the Moon we see today  Study physics of the disk -> motion, temperature  Powerful computer models constrained by information about chemistry of early Earth and Moon rocks Goal: Learn more about how our Moon formed and changed in its early history

Properties of the Moon  Mass ratio of Earth to Moon -> large moon  Moon formed near a rapidly-rotating Earth -> 5 vs. 24 hours  Moon is depleted in iron

Giant Impact Model of Moon Formation Mars-sized body hits Earth obliquely & Moon forms from debris disk Iron core / stony mantle Animation from Robin Canup Impactor Trajectory Early Earth This model explains: Mass ratio Earth-Moon Mass ratio Earth-Moon Earth fast rotation speed Earth fast rotation speed Lack of iron in Moon Lack of iron in Moon

> 11,000° 9100  10,000° 8200  9100° 7100  8200° 6200  7100° 5100  6200° 4200  5100° 3100  4200° 10 4  11,000° Temperature Animation from Robin Canup

Sea of bodies: Moon to Mars-sized smaller planetesimals ⇒ MANY COLLISIONS Distance From Sun (Further)(Closer) Elongation of Orbit Very Circular Very Elongated Location of Asteroid Belt Animation from Alessandro Morbidelli Large impacts are common! Jupiter

Lunar Accretion Simulations Models allow us to track disk particles forming into Moon The Moon could form in as short as a few years or as long as 10,000 years

Issues  Moon forms too fast and hot => completely molten – oxide, siderophile, and volatile ratios different than expected (e.g., water!) – diversity of basalts – crust too thin – global cracks from cooling New computer model Late veneer  ~ 80% of material from impactor, but Earth-Moon oxygen isotope ratios identical