Roots that displace denser mantle material gh 1 ρ 1

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roots that displace denser mantle material gH 1 ρ 1 + gr 1 ρ 1 = gr 1 ρ s Pratt Isostasy Compensation achieved by density variations in the lithosphere gD ρ u = gh 1 ρ 1 = gh 2 ρ 2 etc.. Simple view of mountains Supported by lithospheric strength Large negative free-air anomoly Bouguer correction should get rid of this Anomalies due to mountains are much weaker than expected though Due to compensation Effects of Compensation
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PYTS 411/511 – Planetary Gravity and Topography 18 Uncompensated Strong positive free-air anomaly Zero or weak negative Bouguer anomaly Compensated Weak positive free-air anomaly Strong negative Bouguer anomaly
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PYTS 411/511 – Planetary Gravity and Topography 19 Two ways to interpret Bouguer anomalies Mass excesses/deficits in the near surface Constant density crust that varies in thickness Interpretation of Anomalies
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PYTS 411/511 – Planetary Gravity and Topography 20 Craters <200km diameter Negative Bouguer anomalies Mass deficit due to excavated bowl and low density of fall-back rubble Mountains Positive free-air anomalies Support by a rigid lithosphere Mascons First extra-terrestrial gravity discovery Very strong positive anomalies Uplift of denser mantle material beneath large impact basins Later flooding with basalt Bulls eye pattern – multiring basins Only the center ring was flooded with mare lavas Flexure South pole Aitken Basin Appears fully compensated Older Lunar gravity Free-Air
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PYTS 411/511 – Planetary Gravity and Topography 21 Crustal thickness maps show lunar crustal dichotomy Zuber et al., 1994
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PYTS 411/511 – Planetary Gravity and Topography 22 Tharsis Large free-air anomaly indicates it is uncompensated But it’s too big and old to last like this Mantle plume provides dynamic compensation? Utopia Probably compensated initially Filled later when lithosphere was thicker Crustal thickness Assume Bouguer anomalies caused by thickness variations in a constant density crust Need to choose a mean crustal thickness Isidis basin sets a lower limit Mars Gravity Zuber et al., 2000 Free Air
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PYTS 411/511 – Planetary Gravity and Topography 23 Crustal thickness of different areas But many features are uncompensated…. And not supported by this Zuber et al., 2000
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PYTS 411/511 – Planetary Gravity and Topography 24 Smaller scale anomalies reveal buried flood channels Actual free-air anomaly Predicted free-air anomaly
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PYTS 411/511 – Planetary Gravity and Topography 25 Martian seasonal cap incorporate about 25% of the atmosphere About ~7x10 15 Kg Causes periodic flattening of the gravity field Observable with spacecraft Smith and Zuber, 2005
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PYTS 411/511 – Planetary Gravity and Topography 26 Large planets Slower cooling, thinner lithospheres Small free-air anomalies Topography supported isostatically
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PYTS 411/511 – Planetary Gravity and Topography 27 Small planets Faster cooling, thicker lithospheres Large free-air anomalies Topography supported by flexure of thick lithosphere
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