Planetary Geology Continued


Planetary interiors show a range of properties, due to differences in temperature and pressure:

How do interiors get hot?

Important: Interior temperature is not determined by distance from the Sun!

  • Early accretion : planet-wide melting. (gravitational potential energy --> kinetic energy of impact --> heat)
  • Differentiation : heavy material sinking to core. (gravitational potential energy --> heat)
  • Radioactivity: radioactive elements in the core (uranium, etc) give off energy (mass-energy --> heat)


How do interiors cool off?

  • Conduction : heat transfer by contact. (think: holding a coffee cup)
  • Convection : hot material rises, cooler material sinks. 
  • Eruption : volcanos release heat!


Eventually, all the heat ends up on the surface of the planet and is radiated away into space. The planet cools off.

Convective currents in planetary mantles.


Bigger planets take longer to cool.

Heat content: more stuff, more heat. related to the volume of a planet: Volume = (4/3) pi R3
Cooling rate: planets cool from their surface: more surface area, more cooling: Area = 4 pi R2

So as a planet gets big, its ability to cool does not rise as fast as its heat content. So it takes longer to cool.

Small worlds (Moon, Mercury) cooled quickly, and are now solid. Big worlds like Venus and the Earth still have hot interiors. Mars is somewhere inbetween.


Magnetic Fields: what they tell us about the interior of a planet


Think of magnets:



Types of magnets
 Magnetic field lines


The Earth acts like an electromagnet:
  • liquid metallic core (lots of electrons)
  • rotating planet
  • electrons in the Earth move like the electric field of an electromagnet --> magnetic field

Earth has the strongest magnetic field of the terrestrial planets. The others:
  • Moon, Mars: none (why? )
  • Venus: none (why? )
  • Mercury: small, but present ( why is this confusing?)

the Earth's magnetic field


Shaping Planetary Surfaces


Major processes involved:
  • Impact cratering
  • Volcanic activity
  • Tectonics
  • Erosion

 Crucial concepts:
  • How do the properties of the planets drive these processes?
  • How do these processes change the properties of the planets?
Visualize via planetary flowcharts (eg Fig 9.10).



Impact Cratering

We see impact craters on all the terrestrial worlds.

Even on the Earth:

(Barringer Crater, Arizona, 1 km across)

Large impacts can form impact basins:


(Mare Orientale on the moon, 600 miles across)

The Moon's surface:

Early major impacts cracked through the Moon's lithosphere, allowing lava to seep out. When this lava cooled and solidified, it formed a smooth surface (the lunar maria).

Questions:
  • could this happen today?
  • why aren't the maria heavily cratered?

Most meteors are tiny, sand-sized. These micrometeorites slowly pulverize the surface rocks, making a powdery "soil".



Why doesn't this happen on the Earth?








Vulcanism


 Hot liquified rock, or magma, wells up and breaks through the surface, often violently.

Different compositions produce different types of magma, with different viscosity, or "flowability". We can get a variety of volcanic features:
 Volcanos not only spew out magma, but also hot gas , and are the main source of the atmospheres of the terrestrial planets.


Tectonics

Stresses on the planets crust caused by interior activity. On the Earth, this acts to break the crust up into plates , which ride on top of the mantle below.





Where plates ram into one another we have subduction zones, where ocean plates are pulled back into the mantle. Volcanoes are common here. On land, mountain ranges can form.
In the sea floors, we have active rifts, where new magma rises up from the mantle reforming the sea floor.
 

Plate tectonics is essentially a continual recycling of material between the Earth's crust and mantle:




Tectonics can be happening on other worlds, like Venus, where the Guinevere Plains may be evidence of spreading of the surface due to tectonic activity (planetary stretch marks!).


Why is there no tectonic activity on Mercury or the Moon?
What about Mars?
Guinevere Planes on Venus

Erosion

A process which breaks down surface rock.
  • Water : rain, rivers, oceans
  • Ice : glacier activity
  • Wind : reshaping surfaces

 How would erosion depend on
  • the presence or absence of an atmosphere?
  • the rotation rate of a planet?
  • the surface temperature of a planet?
  • the composition of a planet?