CLICK ON EACH PICTURE TO GET A LARGER VERSIONReading: Chapter 8
TP= Terrestrial Planet
Classes 15 and 16 together cover the topic of interiors of the terrestrial planets, particularly their thermal evolution - the ultimate source of geological evolution.
Overview of Planetary Formation - started 4.5 billion years ago (evidence for starting date?). Accretion (once started - the tricky bit is getting from 2cm to few meters i.e. from golf balls to wrecking balls). Energy of accretion. Melting -> core formation (iron settling to center). Cooling, bombardment..... present geology.
CLICK ON EACH PICTURE TO GET A LARGER VERSION
Simplistic anatomy of a TP. Figure 8-13 of Hartmann is the grown-ups version for the Earth.
The family of TPs - note range in core sizes and lithosphere thicknesses. (Wish Hartmann had grown-ups version of this).
Densities and sizes - These are the simplest to measure (how?) and give very useful basic information.
The measured densities are COMPRESSED densities - where the pressure inside increases the density over the value the material would have at the surface. Remember the calculation of central pressure from the interiors of the Gas Giant planets?
Figure 8-2 of Hartmann shows how pressure affects density. Here's a useful table:
Compare these compressed and uncompressed densities with the size of the planet - Does the difference between the densities correlate with the size of the planet and the expected pressures inside? Compare the uncompressed densities with the materials above.
PUZZLER: A planet has a core density twice the mantle density and the radius of the core is half the radius of the planet. (a) What is the average density of the planet? (b) What is the core volume as a fraction of the total? (c) What is the core mass fraction? ANSWER.
Another important source of information about the interiors comes from the moment of inertia - see Table 8-1 of Hartmann. Note there are not values for Venus and Mercury - I do not believe they have been accurately measured yet.
TPs evolve through stages - the smaller planets do not reach the later stages - their development is arrested in adolescence. Again, thermal evolution is the key!
Thermal evolution of interior is responsible for evolution of crust - surface geology. The triangle below shows the current relative importance of conduction, volcanism and plate tectonics on the TPs.
Both composition and thermal evolution are important. Differentiation of the core and recycling of the upper layers leads to compositional layering. The interior processes are largely driven by thermal processes. See Hartmann Figure 8-3 for how the Earth's crust's composition differs from cosmic abundance (as typified by carbonaceous chondrite meteorites).
HEATING AND COOLING - first qualitative, then quantitative
Heat sources: The Astro 101 version
And cooling processes...
.
.. and the cooking version.
Hartmann has some neat - though tricky to work out at first - diagrams of the thermal evolution of planets. Figures 8-20, 23, 24 and 26. These have depth in the y-axis and time in the x-axis. Note how the isotherms evolve with time depending on the size of the planet.
Time to get QUANTITATIVE!
First, a calculation of HEAT OF FORMATION - that's accretion. Note the Mass squared dependance!
If you take the HEAT OF FORMATION ( 5.31 above ) and divide by the heat capacity of rock, you get the expected increase in temperature associated with this heating - as shown below:
NOTE - these are huge temperatures for the larger TPs.
There is no doubt that this sort of energy was generated in forming the planets - the question is this "Did this energy dissipate at about the same rate as it was generated or did it build up quicker than it could be dissipated?" For Earth we think there is still come remnant heat of formation. But for the smaller planets, the heat of formation may well have been dissipated as quickly as the planet formed.
But there is no doubt that the HEAT OF CORE FORMATION was generated quickly and before it could be dissipated.
More about this in the next class.....
