This is probably the bigger force of the 2 o Role of hotspots (concentrated jets of hot material riding from ~2900 km down) is controversial (figure 2.15) 55
Chapter 9: • Early history of Earth and Moon: o Universe thought to be ~14 b.y. old. o Origin of solar system nebular hypothesis formed from nebular (figure 9.1), (mainly) gas (hydrogen/helium) and dust o Rotating cloud flattens into disk, gravity pulled most material into center— proto-sun formed (figure 9.2) o Temperature rose inside sun to millions of degrees, nuclear engine fired up. This continues today —process same as in hydrogen bomb (Einstein—some mass converted into energy) o Remainder of material in nebula, continued spinning. In some places, gravitation causes clumping of material. As these clumps grow, they pull in more material— accretion . Finally sweeps out most of the remaining material (figure 9.2) • Inner (terrestrial) planets—mercury, Venus, Earth, Mars (fig 9.3) o Small, rocky quite dense. Formed close to sun so most volatiles boiled away. Radiation from sun blows away light gases—hydrogen/helium Occurs about 4.56 b.y. ago • Outer (Jovian) planets—Jupiter, saturn, Uranus, Neptune (fig 9.3) o Much bigger, more gas, relatively low density. (Saturn would float in water) o Contain rock/metals but enough gravity to hold on to lots of gases (example: Hydrogen/helium) o Lots of small (<100 km size) bodies left over. Rocky ones are called asteroids . Gassy ones orbiting far out are called comets o Sometimes these bodies come into near Earth orbit and can lead to collisions. • Differentiation of earth: o Earth heats up rapidly after “birth”, caused by: Accretion process Compression Radioactive decay inside Very early, big collisions also cause extra heating March 30, 2007 o One at ~4.5 b.y. of Mars sized body causes: Formation of Moon Earth’s rotational axis to tilt (causing seasons Outer layers to melt (fig. 4.9) o Oldest rocks on Moon ~4.47 b.y. old— dated radiometrically o Heavy material (iron) falls to center creating molten core. 56
o Least dense material floats to surface forming the crust o Recent work shows some (very little) crust formed in presence of water at 4.3-4.4 b.y. o Between core and crust have mantle —left behind material consisting of more Fe and Mg than crustal rocks (fig. 9.5) • Earth’s ocean and atmosphere o Light material (gases) escape during heating process. Form early oceans and atmosphere o Some say a lot of water brought in by accreting comets o Others say most water released from earth’s interior. Latter process continues today from volcanoes, releasing gases (figure 9.6) o Composition of early atmosphere very different from today (example: little or no free oxygen) • Late heavy bombardment: o Around 3.9 m.y. ago, very heavy bombardment of inner planets by asteroids/comets. Accretion process wipes out most of earlier evidence (resets radiometric clocks) o (known again from direct dating of moon rocks) this is why few rocks on earth give radiometric age > 4b.y.