GE 11a, 2014, Lecture 5 Spherical structure of the earth The earth, ca. 1800 Nevil Maskelyne and the Schiehallion experiment (1774) Schiehallion (‘Sidh Chailleann’) Scotland Nevil Maskelyne doing his impression of Ben Franklin d F Ms m.g . 2 2 2 . 24 F = m g tan( ) = G m Ms/d ME = (RE /d ) (Ms/tan( )) ~ 6 10 kg . 2 R = 6.37.106 m; V = 1.1.1021 m3 m g = G m ME/RE E E ~ 5.5 g/cm2 (initially found ~ 4.5) Densities of common substances (all in g/cc) Ice 0.917 Water 1.000 Seawater 1.025 Graphite 2.200 Granite ~2.70 Titanium 4.507 Iron 7.870 Copper 8.960 Mercury 13.58 Gas: proportional to P/RT Two options: sub-equal mix of metal and rock or… an ideal gas, w/ high density at high P (B. Franklin) Mass distribution in earth’s interior Period of precession Moment of inertia Period of spin Torque (sun and moon trying to pull earth’s tidal bulge into plane of ecliptic) . 2 ri mi I = i mi ri Higher Earth has I much less than expected for homogeneous sphere Lower Kraemer, 1902 View combining known density, moment of inertia, oblateness, rigidity of surface rocks, and topography Note bad for a bunch of turn-of-the-century quacks! Focus “sample” outer ca. 200 km, but most energy in upper 10 km Mantle Core Seismograph S P A mechanical seismograph Anatomy of a seismic signal Minutes 0 10 20 30 40 50 Surface waves P S ‘Primary’ (first to arrive) ‘Secondary’ (second to arrive) measure the amplitude of the P S largest seismic Amplitude =23 mm wave… P-wave S-wave interval = 24 seconds …and the time interval between the P- and S-waves (I.e., the distance from the epicenter. Distance (km) Distance Connect the points S and P waves (s) waves P and S Interval between Interval to determine the Richter magnitude. Richter Amplitude magnitude (mm) Real data is more complicated… Look at your notes, John! Look at your notes, John! Look at your notes, John! Mg2SiO4 in upper mantle Mg2SiO4 in lower mantle The core’s density is less than that of pure Fe. Requires a low-mass Alloying agent. S? O? H? ??? The events in early earth history that controlled core/mantle segregation Robin Canup’s movie Animation of Theia forming in Earth's L5 point and then drifting into impact. The animation progresses in one-year steps making Earth appear not to move. The view is of the south pole. Evidence for the ‘magma ocean’ stage of the earth is essentially erased by later convection, plate formation, subduction, etc. But the moon’s crust preserves evidence planets are largely liquid early in their history. And today Io, a moon of Jupiter, seems to possess a partially crystallized magma ocean, supported by tidal heating .