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Lecture14

# Lecture14 - Interiors large bodies(e.g planets have all...

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Interiors large bodies (e.g. planets) have all undergone significant changes (in surfaces and interiors) since their formation. except for the Earth and Moon, all of the data we obtain on interiors are based completely on “remote” observations: ¾ Density ¾ magnetic field ¾ moment of inertia. ¾ Seismic data (Earth and moon only). The bulk density of an object is simply its mass divided by its volume ( ρ =M/V) the density of an element depends on the pressure of its environment. ¾ the pressures inside planets must mean that their bulk densities are greater than the densities of their components at 1 atmosphere. Rock is deformable and can move: even more so under partial melting conditions Internal structure obeys the hydrostatic equilibrium equation ¾ The equation of state (relating density, pressure and temperature) for solids is generally complex. But in practice T and density do not vary much.

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Moment of Inertia and Gravity Field The gravity field of a planet, i.e. the gravitational force it exerts on an external body, depends on the planet’s internal mass distribution the main factors affecting the gravity field are a planet’s shape, internal mass distribution and rotation. Measurements of the gravitational field of a planet, usually from space, can be mapped and deviations from a uniform, homogeneous sphere can be determined. ¾ This led to the discovery of mascons on the moon. These are large lava flows that filled ancient impact basins. The lava is denser than the surface rock, but has not been able to sink back into hydrostatic equilibrium ¾ Find that the centre of mass is offset from the geometric centre of the moon and Mars Shows the mass distribution is not symmetric The moment of inertia of a body is simply its resistance to rotation – analogous to mass which is a body’s resistance to “straight line” motion. The moment of inetria for a sphere can be written as: , where M is mass, R is radius, I is the moment of inertia and k is a constant. 2 kMR I = ¾ For a homogeneous sphere k=0.4 ¾ For a hollow shell k=2/3 ¾ For a point mass k=0 ¾ as central concentration in a body increases, k=I/MR 2 decreases.
Magnetic Fields Magnetic fields are lines of force , with both magnitude and direction. Record of Earth’s magnetic field can be determined from

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Lecture14 - Interiors large bodies(e.g planets have all...

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