Lunar Asymmetry Monica Hoke Brian Morsony Phil Oakley Observational Evidence 1 Geographical 2 Geophysical 3 Geochemical Geographical Asymmetry Luna 3 Mission 1959 First ever image of the Moon s dark side Took 29 pictures over 40 minutes Photographs covered 70 of the far side s surface Clementine High Resolution Camera obtained 1 8 million images of the Moon s surface 360o View Geophysical Asymmetry Ranger program impacts Sjogren 1967 Earth based radar bounce data Shapiro et al 1968 Offset Center of Mass Lunar Orbiter 1 maps gravitational anomalies Apollo 15 Roberson and Kaula 1972 Clementine LIDAR Geochemical Asymmetry KREEP 1 Th anomaly 2 Radioactive material heavily concentrated around mare Metzger et al 1972 Theories must account for 1 Maria located almost exclusively on the nearside 2 COM located off center 3 Location of KREEP coincides with the lunar maria 4 Time constraints Possible Mechanism to face the Asymmetry towards Earth Tidal Force Mechanism First proposed by Smith et al 1970 Assume an asymmetric crust Dense mare basalt liquids drawn to the near surface Tilted Convection Review of Early Lunar Interior Figure 1 Initial lunar structure Figure 2 Solidification of a cumulate layer of olivine and or pyroxene at the base of the LMO Figure 3 Solidification of isolated buoyant anorthositic crystals D E Loper and C L Werner 2002 Forms of Convection Rayleigh Number 0 Ra 1 7E3 Static state 1 7E3 Ra 1E4 Steady convection with a cellular structure 1E4 Ra 5E5 Unsteady convection cells 5E5 Ra 2E6 Unsteady convection with no cellular structure 2E6 Ra Tilted convection Tilted Convection Rising and sinking plumes during tilted convection The large scale flow associated with tilted convection D E Loper and C L Werner 2002 Tilted Convection in the LMO LMO must be thicker than Lcrit Lcrit Racrit mu kappa delta rho g 1 3 With Racrit 2x106 for Tilted convection Results in Lcrit 3m Convection in the LMO almost certainly was sufficiently vigorous that tilted convection occurred D E Loper and C L Werner 2002 Orientation of the Tilt Can be controlled by small thermal variation in the convective system Horizontal current near upper surface is directed away from the hotter region Proximity of Earth results in uneven radiative cooling on the moon Produces a nearside temp increase of 2 5 21K at 2 Roche limits 1 3 11K at 3 Roche limits Wasson and Warren 1980 D E Loper and C L Werner 2002 Growth of the Farside Crust D E Loper and C L Werner 2002 Convection and the Coriolis Force Related to the density rotation rate and layer thickness Lcrit 1 m Convective pattern in the LMO would be sensitive to the lunar latitude May have produced the crustal thickening observed on the lunar equator Rayleigh Taylor Instability and Degree 1 Spherical Harmonics Spherical Harmonic Instabiliti es Sinking IC material carries U Th and K Becomes enriched in remaining liquid as LMO crystallized Creates a thicker denser mixed layer than underlying mantle This creates an instability at long wavelengths Zhong et al 2000 Velocity vectors show the harmonic degree 1 flow pattern Down welling to form the KREEP Asymmetry Gravitational instability of the mixed layer single down welling asymmetry If formed prior to complete solidification of LMO Remaining liquid in LMO will flow toward low pressure region above down welling KREEP layer which crystallizes from these last liquids would be enhanced in this region Up welling to form the Mare Asymmetry Previous down welled IC material initially stable Buoyant rise due to heating from U and Th Rising material undergoes decompressional melting Results in mare basalt volcanism Zhong et al 2000 Asymmetry due to Crustal Thickness Variation Locally thinner crust means a deeper magma ocean will form underneath Thinner ocean crystallizes faster removing KREEP layer from far side Later impacts can release magma under thin crust but not thick crust Large Impact Destroys local crust and adds local heat creating thin crust and thick magma ocean Mass imbalance can reorient moon so that impact in on equator and towards Earth No clear evidence for large impact although may have been covered up by subsequent impacts Equatorial bulge difficult to explain if moon reoriented Multiple Small Impacts Multiple impacts that happen to be near each other have similar effect to single large impact Why would impact sites be concentrated in one region South Aiken Basin is largest visible impact crater and older than Mare regions but is not filled with basaltic material Asymmetry on other Bodies Earth Mars Enceladus Earth Elevation asymmetry due to continents Basaltic ocean floor Granite continents Young ocean floors Continental drift multiple upwelling and downwelling plumes in mantel Earth Mars North South asymmetry Older highlands in south Thinner crust in north Due to single upwelling plume lifting south Single impact in north Multiple impacts in north Enceladus Warmer south pole Older north younger south Plume of ice from south pole Upwelling of lowdensity material due to tidal heating Location possibly due to reorientation Summary of Lunar Asymmetry Center of Mass offset towards Earth from geometric center near side at lower elevation Near side less heavily cratered therefore younger Mare regions mostly on near side KREEP material found mostly on near side surrounding mare regions Equatorial bulge not necessarily due to asymmetry but must be taken into account Summary of Formation Theories Model Pros Cons Upwelling Accounts for formation and composition of mare right timescale for mare formation Requires core smaller than observed may not explain KREEP concentration Downwelling Concentrates KREEP at single Requires high viscosity location heating can lead to mare downwelling material may not formation explain composition of mare Tilted Convection Explains equatorial bulge mare and KREEP concentration Moon must form near Earth giant impact Single Impact Independent of moon formation location capture No evidence for large impact must occur near equator Multiple Impact Independent of moon formation location capture Impacts must be concentrated in one region South Aiken Basin References Gillis J J Jolliff B L Korotev R L 2004 Geochimica et Cosmochimica Acta 68 3791 http ciclops org http denali gsfc nasa gov http www scotese com http www solarviews com http www space eduhttp www svengrahn pp se trackind luna3 Luna3story html Kaula et al 1972 Proc of Lunar Sci Conf 2 2189 Loper D E C L Werner On lunar asymmetries