• Presented to Star Astronomy

• May 1, 2008

• By Charles J. Byrne– Image Again– charles.byrne@verizon.net– www.imageagain.com

• With topographic views from Nick Lordi

The Near Side Megabasin of the Moon

The Early Moon

• A Mars-sized body, its orbit perturbed, collided with Earth, 4.5 Billion years ago.

• This body, and part of Earth’s crust, were vaporized and formed a ring around Earth.

• Soon, this ring cooled, and the Moon accumulated.

• The heat released by gravity formed a magma ocean.

Crystallization of the Early Moon

Asymmetry of the Current Moon

• The “Man in the Moon” (maria) is mostly on the near side

• The near side is low, a bulge on the far side

• The crust is thin on the near side, thick on the far side

• Heavy element anomalies are mostly on the near side

• Uneven moments of inertia: offest C. G.

Near Side of the Moon

Far Side (Nozomi)

Eastern Limb (Lunar Orbiter)

Eastern Limb (Apollo 16)

Maps of the Current Moon

• Topography– Photography (photometry and stereo) – Laser and radar altimeters

• Gravity potential– Tracking of spacecraft

• Crustal thickness: inferred from topography and gravity

• Mineral concentrations

Clementine Elevation Map

-5000-5000 m m 50005000 m m0 0 m m

Why?

• Asymmetric impacts?

• Uneven crystalization of the magma ocean?

• Tidal effects in a complex early orbit?

Impact Dynamic Tutorial

• Incoming asteroids and comets

• Hypervelocity impacts: explosions

• Cavity formation

• Ejected target material

• Formation of rings

• Effects of target curvature

• Scaling laws

Lunar Basin (Orientale)

Orientale, LIDAR and ULCN (Lordi)

Noise Reduction for ULCN (Lordi)

Orientale, ULCN (Lordi)

Orientale, radial profile

Simulation of Impacts

Radial Profile of Selected Basins

Profile of Ejection Velocity

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1

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6

0.2 0.4 0.6 0.8 1

Normalized Internal Radius

Vel

oci

ty

Curvature of the Target

• A giant basin must consider the spherical nature of its target

• Ejecta is thrown into elliptical orbit

• There is less area for the ejecta to land near the antipode, so it piles up

Steps in Making the Model

• “Flat Moon” basin model

• Ejection velocity radial profile

• Orbital trajectory equations

• Focusing effect of the spherical Moon

• Final radial profile of ejecta

The Search

• The scaled model has these parameters:– Latitude and Longitude of center– Diameter– Depth

• Parameters were varied to make a best fit• Started with two large basins• They grew as the fit improved• They merged into one giant basin

Model of the Near Side Megabasin

600 km600 kmimpactorimpactor

EscapingEscapingejecta (hyperbola)ejecta (hyperbola)

EscapeEscapevelocityvelocity

Velocity lessVelocity lessthan escapethan escape

Ejecta Ejecta passes passes antipodeantipode

Transientcraterexpands

Velocity fallsVelocity fallsfurtherfurther

Ejecta is Ejecta is concentratedconcentratedat antipodeat antipode

Antipode

Velocity fallsVelocity fallsfurtherfurther

Ejecta falls betweenEjecta falls betweenbasin rim and antipodebasin rim and antipode

Radial Profile of the Near Side Megabasin

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4000

6000

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10000

0 1000 2000 3000 4000 5000 6000

Radius, center to antipode (km)

De

pth

(m

)

The NSM Floor is Refilled with Crust

Refilling of Plastic Crust

Model of the Moon with the NSM

The NSM and its Antipode

NSM and Titanium

NSM and Iron

NSM and Thorium

NSM and Maria

NSM Rim on the Far Side

NSM Rim at Tsiolkovskiy

Summary

• The history of the Moon from its origin to today has been reviewed

• Its original crust has been strongly modified by impacts, starting with the NSM and SPA.

• 4 billion years of bombardment followed• The major mineral anomalies on the surface

are associated with the impacts of the NSM and SPA.