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June 2, 2011

GRAIL: Exploring the Moon from Crust to Core

Presented by: Dr. Sami Asmar and Don Boonstra

Exploring the Lunar Interior Structure

Crust To Core Via GRAIL Gravitational Field

Measurements

June 2011

Copyright 2011 California Institute of TechnologyGovernment sponsorship acknowledged

Presenter:

Sami AsmarCo-Investigator &

GRAIL Deputy Project ScientistJet 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

Wieczorek et al. [2006]

Notional view of the lunar interior

Measurement Technique: Differential One-Way Ranging

1

4 3

2

x

x

x

Let’s pause for questions from the audience

crustal densitycrustal thicknesselastic thickness

load densitysurface-subsurface load ratiophase relationship of loads

gravity

topography

geophysical model

internal structure & thermal evolution

geochemistrypetrologygeology

Gravity and topography

-300 +300 mGal

-9 +10 km

Comparing Gravity & TopographyAdmittance & coherence

Loading model predicts admittance and correlation spectra, which depend upon:

crustal densitymantle densitycrustal thicknesselastic thicknessmagnitude of surface and

subsurface loadsphase relationship of loads

Lithospheric crust, ρc

Lithospheric mantle, ρm

Subsurface load, offset in phase

Surface load SELENE

Kaguya

GRAIL

Kaguya

GRAIL

Let’s pause for questions from the audience

Thermal evolution: heat sources and sinks

External

HeatingImpacts

CoolingRadiation

Internal

HeatingRadioactivity

CoolingConvection

Reconstruct source/sink contributions throughout geologic time

Deep interior: Structure and state

inner core

McDonald Observatory, TexasRanging to the Moon

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

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:• Twin spacecraft launched on a

Delta 2920H-10• Launch in Sep. 2011• Short duration of 9-months• 82 day mapping mission• Low altitude, polar orbit• Single payload mission• Imaging for education programMeasurements:▪ Ka-band ranging measures

relative velocity of the centers of mass of two spacecraft

▪ Links to Deep Space Network for navigation, absolute position, and timing calibration

Institutions:▪ MIT:

▪ Principal Investigator & Deputy▪ Science analysis & interpretation

▪ JPL:▪ Project management, system

engineering, mission assurance, mission operations, payload development, science simulations & modeling, data processing

▪ Contract with Lockheed Martin for spacecraft system

▪ LM:▪ Spacecraft development & flight

operations

▪ GSFC:

▪ Data processing & analysis

▪ Sally Ride Science:

▪ Education & outreach cameras

GRAIL Concept

S-band link Telecom/Nav

S-band Timing Code

Ka-band ranging

X-band Radio Science Beacon

Let’s pause for questions from the audience

Education Resourceshttp://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-Dhttp://climate.nasa.gov/Eyes/

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