PHY 1060 Lecture Notes - Lecture 4: Mantle Plume, Protoplanet, Stratovolcano
Chapter 8: The Terrestrial Planets and Earth’s Moon
• Earth’s Moon and the terrestrial planets (Mercury, Venus, Earth, and Mars) have similar
and dissimilar properties
o Must be able to explain the differences
• Comparative planetology: studying planets by comparing them to one another
• Four processes have shaped Earth:
o Impact cratering
o Tectonism
▪ Modifications of the crust
o Volcanism
▪ Igneous activity
▪ Magma/lava
o Erosion
▪ Wind & water
• Material falling from spaces onto a planet’s surface create impact craters
o Secondary craters can be caused by falling ejecta from the impact
o The moon and all terrestrial planets experienced this.
o Large impacts can melt and vaporize rock
• Venus and Earth have relatively few craters
o Mars has more
▪ Craters on mars suggest it was once wetter
o Mercury and the moon are covered with craters
▪ Some process must reverse them
• Erosion
• The number of craters indicates the surface’s age
o The craters means an older surface and minimal geologic activity
o Tectonism and erosion can erase craters
• Rocks returned from Moon missions give ages through radioactive dating
o Almost all cratering happened in the first billion years of the Solar System
• Rock layers are formed through sedimentation
o To find the ages of rock in these layers (or from Mars) scientists use radiometric
dating
o Parent particles decay into stable daughter particles at a steady rate
• A radioactive isotope decays to half its original amount in a half-life
• We model the Earth’s interior by studying earthquakes
o Seismic waves travel differently through different materials
o Some waves are surface waves; others travel through
o Primary waves: travel through solids and liquids, longitudinal waves
o Secondary waves: go through solids only, transverse waves
• Earth has a crust, mantle, and core (dense materials
o Produced by differentiation in the early Earth: dense materials sink, low-density
materials rise.
o Lithosphere: outer most layer, crust
• Current prevailing theory: moon formed in a large collision between Earth and Mars-
sized protoplanet
o The material collected to form the moon
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o The composition of the moon is like that of Earth’s crust
o (some theories are that the moon formed by accretion, but that is less likely)
o Sometimes multiple hypotheses must be considered though one might fit the data
better than others
• The moon and other terrestrial planets have interiors similar to earth
o Deeper in a planet means hotter and more pressure
o Formation energy and radioactive material help to heat the interior
o Smaller planets lose heat faster and produce less internal heat
• Earth’s magnetic field acts like a giant bar magnet
o It originates from processes deep in the interior that are not understood fully
o Iron-bearing minerals tell us the orientation changes over time
• Earth and mercury are the only terrestrial planets with a substantial magnetic field
o It is odd why Venus and Mars do not
o The moon once had one
• Tectonism is the deformation of Earth’s crust
o Earth’s crust is broken into lithospheric plates
o Continental drift and plate tectonics describe the movement of those plates
• Crustal plates are moved around by convection
o The rising and falling of hot/cold material
o Earth has seven major plates and six smaller ones
• Only earth has its crust broken into plates
o All terrestrial planets have seen some form of deformation
o Mercury’s surface shrank after it cooled, leaving cliffs
o Mars has experienced extensive tectonism, and boasts the massive chasm Valles
Marineris
o Venus may have a different form of tectonism, with melting and overturning of
the crust
• The mantle plume spreads out when it reaches the base of the lithosphere…
o Resulting magma
o Resulting volcanoes
• Volcanoes form at hot spots and plate boundaries
o Very fluid lava forms shield volcanoes
o Think lava forms composite volcanoes
• The moon does not have any volcanoes, but lava flows smoothed out parts of its surface
o Mercury also has smooth surfaces from past volcanism, and a few inactive
volcanoes have been identified
• The volcanoes on Mars are the largest mountains in the Solar System, and are shield
volcanoes (Olympus Mons)
o Venus has the largest amount of volcanoes in the Solar System
• Erosion includes processes that wear down the high spots and fill in the low
o On earth, wind and water strongly erode features
o Wind also modifies the surfaces of venus and mars, especially on mars
• While earth is the only planet with liquid water, there is evidence that mars was once
wetter.
o Canyons, dry riverbeds, layered rock,
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