Transcript NSM - IMAGE AGAIN

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
• 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 m
0 m
5000 m
Why?
• Asymmetric impacts?
• Uneven crystalization of the magma ocean?
• Tidal effects in a complex early orbit?
Impact Dynamic Tutorial
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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
6
5
Velocity
4
3
2
1
0
0.2
0.4
0.6
0.8
Normalized Internal Radius
1
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
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“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
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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
Escaping
ejecta (hyperbola)
Escape
velocity
600 km
impactor
Velocity less
than escape
Transient
crater
expands
Ejecta
passes
antipode
Velocity falls
further
Antipode
Ejecta is
concentrated
at antipode
Velocity falls
further
Ejecta falls between
basin rim and antipode
Radial Profile of the Near Side Megabasin
10000
8000
6000
Depth (m)
4000
2000
0
0
1000
2000
3000
4000
-2000
-4000
-6000
-8000
Radius, center to antipode (km)
5000
6000
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.