Chapter 6 Terrestrial Worlds in the Inner Solar System Personal E Book Notes comparative planetology comparing worlds with one another teaches us about their fu ture 6 1 Four Main Processes Shape Our Planet Earth s hydrosphere blue of oceans seas lakes and rivers covers most of the planet Earth s lithosphere brown of outer rocky shell of Earth the crust Earth s biosphere green of vegetation 4 geological processes Impact Cratering Tectonism Volcanism and Erosion all these create topographic relief 6 2 Impacts Help Shape the Evolution of the Planets large impacts cause the most concentrated and sudden release of energy when an object hits a planet its kinetic energy heats and compresses the surface and throws material far from the re sulting impact crater sometimes this ejecta falls back to the surface to cause secondary craters or it could lead to a central peak or a ring of mountains on the crater floor the impact could be great enough to melt or even vaporize rock the floors of some craters arre the cooled surfaces of rock melted by the impact impact can also form new minerals such as shock modified quartz forms only during an impact the rocks that cause these impacts are either meteoroids meteors or meteorites Meteroid objects while in space Meteor objects when passing through the atmosphere Meteorite objects when they hit the ground Meteor Crater in Arizona is one of the best preserved craters caused 50 000 yrs ago from a nickel iron asteroid fragment about 50 m across mass of 300 million kg traveled at 13 km s half of the original mass was vaporized in the atmosphere before hitting the ground 300 times as much energy as the first atomic bomb yet only 1 200 m in diameter Meteor Crater is tiny compared with those on the Moon or ancient scars on Earth on Earth and Venus most impact craters have been destroyed on Earth by tectonic move ment and erosion on Venus by lava flow lots of craters on Mercury Mars and the Moon also Earth and Venus are protected by their atmospheres meteors smaller than 100m in diameter are burned or broken up by friction before they reach Earth s surface pinhead craters found on the moon Venus is even better protected thicker atmosphere different craters have different traits Moon surrounded by strings of smaller secondary craters Mars surrounded by structures like if you a threw a rock in mud perhaps Mars had wa ter or ice at this time evidence of riverbeds now dry or frozen lunar surface has remained unchanged because its not geologically active no atmosphere water or active interior Mercury is similar extensive cratering signifies an older planetary surface and minimal geological activity between 1969 1976 Apollo astronauts and Soviet probes visited the Moon and brought back samples from 9 different locations by measuring radioactive elements and the elements into which they decay radiometric dating scientist could determine the ages of different lunar regions smooth areas younger than cratered ones but still very old almost all cratering in the Solar System took place within its first billion years 6 3 The Interiors of the Terrestrial Planets Tell Their Own Tales We ve only gone about 12km into the 6 350 km of Earth but the strength of Earth s grav ity tells us its mass average density of about 5 500 kg m 3 surface material about half that so since interior is denser it should contain large amounts of iron density of 8 000 kg m 3 since meteorites and Earth formed at the same time out of similar materials we can reason that the overall composition of Earth should have large amounts of iron like meteorites Earthquakes cause seismic waves surface waves travel like waves on the ocean across the surface of a planet primary waves are longitudinal waves which probe the interior of the planet compress and rarefy material like a spring secondary waves are transverse waves that move material sidewards like waves that move along the length of a string P waves can travel through solids or liquids but S waves only through solids all waves travel at different speeds depending on the density temp and composition of the rock if rock density shifts suddenly P waves can be refracted or reflected at the boundary just as light is refracted or reflected by a pane of glass the part where no waves hit is called a shadow area for S waves usually half of planet opposite the epicenter is a shadow area because they can t travel through liquid core seismometers measure seismic waves when building a model of Earth s interior scientists take into account the fact that the pressure at any point in Earth s interior must be just high enough that the outward forces bal ance the weight of all the material above that point they calculate how seismic waves would work in a structure like that and compare these predictions with actual seismic earthquake ob servations 2 component core a thick mantle and a crust Core iron nickel and other dense meatls Mantle medium density materials Crust Lower density materials Materials have been separated by density this is called differentiation rocks of different types mix melt and the denser sink to the bottom less dense float to the top this structure shows Earth was once much hotter interior was all liquid Moon has a tiny core of material similar to Earth s mantle probably created from a Mars sized protoplanet colliding with Earth blasting vaporizing debris which condense to form the moon during vaporization many gases were lost to space this accounts for the Moon s lack of water and other volatiles 6 4 The Evolution of Planetary Interiors Depends on Heating and Cooling geological activity within a planet is caused by energy received and produced which governs the heating and cooling of a planet part of this thermal energy is from when the Earth was formed the tremendous energy of collisions melted the planet the surface radiated energy into space cooled rapidly and formed a solid crust on top of a molten interior keeping in the remaining heat although some of it leaked through the crust and radiated into space so the mantle and inner core solidified if this was the only source of hearting Earth s outer core would have long ago solidified completely additional energy must be continuing to heat the interior one source is friction from tidal effects of the Moon and Sun the gravitational pull of the Moon causes Earth to stretch along a line in the direction of the moon these tides are called lunar tides the pull of the Sun causes solar tides about