GRAIL: Exploring the Moon from Crust to Core

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Presented by: Dr. Sami Asmar and Don Boonstra

June 2, 2011 Exploring the Lunar Interior Structure

Crust To Core Via GRAIL Gravitational Field Measurements

June 2011

Sami Asmar Co-Investigator & GRAIL Deputy Project Scientist Jet Propulsion Laboratory California Institute of Technology The Earth/Moon System

• Moon’s mean radius: 1737 km • Ratio of Moon/Earth radii 1/3.67 • Mass ratio of Moon/Earth is 1/81.301 • Mean density of the Moon is 3.35 times the density of water • Saturn floats in water! • Moon’s highest point above mean: +11 km • Moon’s lowest point below mean: -9 km • Moon’s surface area is 3.79x107 km2 • Asia’s surface area: 4.46x107 km2 • Africa: 3.02x107 km2 • Moon’s age ~4.5 billion years What if the Moon Did Not Exist?

Contribute your thinking in the Chat What if the Moon Did Not Exist • The day would be eight hours long • The winds would be much stronger • Life, if any, might have a different biology • No honeymoons! • No lunatics! • Darker nights • No lunar calendar • No lecture today

The Moon is moving away (3.8 cm per year) Great coincidence for perfect solar eclipses How did Earth get Its Moon?

A. They formed together in this orbit (accretion)

B. Earth captured the moon

C.There was a giant impact The giant impact and the origin of the Moon Notional view of the lunar interior

Wieczorek et al. [2006] Measurement Technique: Differential One-Way Ranging

2 1

4 3 x x x Let’s pause for questions from the audience Gravity and topography

geochemistry petrology geology gravity

-300 +300 mGal crustal density crustal thickness geophysical model elastic thickness load density surface-subsurface load ratio phase relationship of loads

topography internal structure & thermal evolution -9 +10 km Comparing Gravity & Topography Admittance & coherence

Surface load SELENE GRAIL

Lithospheric crust, ρc Lithospheric mantle, ρm

Subsurface load, Kaguya offset in phase

Loading model predicts admittance and correlation spectra, which depend upon: GRAIL crustal density mantle density crustal thickness elastic thickness Kaguya magnitude of surface and subsurface loads phase relationship of loads Let’s pause for questions from the audience Thermal evolution: heat sources and sinks

External Internal

Heating Heating z Impacts z Radioactivity

Cooling Cooling z Convection z Radiation

Reconstruct source/sink contributions throughout geologic time Deep interior: Structure and state

Apollo 11 retro-reflector

• GRAIL gravity gives shape model (polar and equatorial flattening) • Lunar laser ranging gives physical librations • Since Apollo era • Combining the two allows us to deduce the existence of a lunar core

inner core

McDonald Observatory, Texas Ranging to the Moon If we had a good gravity field for the Moon, we could learn a lot. How do we get one?

• Isaac Newton: Motion of satellite around a planet is largely due to the gravity field • Tracking the motion of a satellite is a good way to measure the gravity field • Since the Moon always keeps the same face toward the Earth, we cannot track a satellite on the farside directly from the Earth • But we can track one satellite with another satellite • This technique was pioneered with the GRACE mission around the Earth in 2002 • GRAIL is the GRACE concept carried to the moon x Solar Radiation Pressure to <0.001 mGal x Spacecraft IR Radiation x Lunar albedo & Lunar IR Project Overview

Mission: Institutions: • Twin spacecraft launched on a ▪ MIT: Delta 2920H-10 ▪ Principal Investigator & Deputy • Launch in Sep. 2011 ▪ Science analysis & interpretation • Short duration of 9-months ▪ JPL: • 82 day mapping mission ▪ Project management, system • Low altitude, polar orbit engineering, mission assurance, mission operations, payload • Single payload mission development, science simulations • Imaging for education program & modeling, data processing Measurements: ▪ Contract with Lockheed Martin for spacecraft system ▪ Ka-band ranging measures relative velocity of the centers ▪ LM: of mass of two spacecraft ▪ Spacecraft development & flight ▪ Links to Deep Space Network for operations navigation, absolute position, and timing calibration ▪ GSFC: ▪ Data processing & analysis ▪ Sally Ride Science: ▪ Education & outreach cameras GRAIL Concept

X-band Radio Science Beacon

S-band link Telecom/Nav

S-band Timing Code

Ka-band ranging Let’s pause for questions from the audience Education Resources http://solarsystem.nasa.gov/yss How Long is a Year?

A. About 3 Earth Months

B. About 12 Earth months

C.About 23 Earth months

D.About 164.79 Earth years

E. All of the above Education Resources Education Resources Education Resources Education Resources Eyes on Earth 3-D http://climate.nasa.gov/Eyes/

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