Upload
maxwell-hill
View
19
Download
2
Tags:
Embed Size (px)
DESCRIPTION
The Near Side Megabasin of the Moon. Presented to Star Astronomy May 1, 2008 By Charles J. Byrne Image Again [email protected] www.imageagain.com With topographic views from Nick Lordi. The Early Moon. - PowerPoint PPT Presentation
Citation preview
• Presented to Star Astronomy
• May 1, 2008
• By Charles J. Byrne– Image Again– [email protected]– 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
0
1
2
3
4
5
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
-8000
-6000
-4000
-2000
0
2000
4000
6000
8000
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.