PX282 - J5 - interiors

earth

image: British Geological Survey

• earth is differentiated into layers
• it has an iron core, silicate mantle (rocks), and a crust of lighter minerals
• heavier materials sank when planet was molten
• the mantle is solid, but flows and convects on geological timescales

image: British Geological Survey

• seismic activities cause different waves:

  • pressure/primary/p-waves are compressional, that propagate in solids and liquids
  • shear/secondary/s-waves are transverse, that propagate only in solids

• these are probed with seismology

• the shadow zones of these waves allow the interior to be mapped

• bulk of the earth is dominated by Fe, O, Si, Mg in refractory compounds (solids at high temperatures)

• it is strongly deficient in common volatile* elements, eg: H, C, N

• this implies that it was formed in warm environment where H${}_2$O, CH${}_4$, CO${}_2$, NH${}_3$ did not form solids

• the interior remains hot due to radioactivity, with heat flux $\sim 0.1$ Wm${}^{-2}$

• a strong magnetic field, generated by a magnetic dynamo

• earth seems to be unique in exhibiting plate tectonics

• it has a fractured lithosphere (crust and upper mantle) that moves on convecting athenosphere

• new crust is created at mid ocean ridges that reveals the history of the magnetic field of earth

• this shows polarity reversal in a $\sim 0.5$ Myr cycle

• this is strong evidence of a dynamo operating in earth

• vocalic island chains (eg. canary islands) above mantle plumes (upwelling convection cells)

• it is expected other terrestrial planets also differentiated, and heated by radioactivity

• looking at the cooling timescales:

• so, the interior of smaller objects tend to be cooler

moon

• lunar seismology shows thicker lithosphere and small molten core
• there is no global magnetic field, so the dynamo is not active
• but there is evidence of global magnetic field in the past from rock samples

mars

• recent detections of marsquakes show that mars also has a thick lithosphere, and a molten core
• there is no global magnetic field
• there is evidence of dynamo in the past

venus

• no seismology, but volcanically active
• lacks plate tectonics and magnetic field, which is not well understood

mercury

• high density implies an oversized iron core
• that together with tidal heated may explain its weak magnetic field
• it may have lost most of its mantle in a giant collision
• this left behind the relatively large core, similar to the formation of earth's moon