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AST101H5 Lecture Notes - Binoculars, Galilean Moons

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John Lester

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Jovian Planet Systems
Review of Physical Properties
The Jovian planets share these physical properties (from chapter 6)
Large masses – 14.5 to 318 x Mearth
Large diameters – 3.88 to 11.2 x Dearth
Large volumes (D3) – 58 to 1400 x Vearth
Small average densities = mass/volume
From 0.71 to 1.67 g/cm3 similar to water = 1 g/cm3
Very different from the rock and metal of Earth
Composition of the Jovian Planets
Low densities require the lightest elements
Lightest element is hydrogen – nucleus has only 1 proton
Second lightest element helium – nucleus has only 2 protons + 2 neutrons
Conclusion: the Jovian planets are 99% (by count) hydrogen and helium
This is like the Sun’s composition
Observed Compositions
Sunlight reflects from the Jovian planets and come to Earth
Gas in the atmosphere absorbs specific wavelengths = Kirchhoff’s 3rd law (pg
The absorption wavelengths contain the patterns of many hydrogen molecules:
H2, H2O, H3N, H4C…
We do not see helium’s absorption pattern because helium does not form
molecules and helium atoms absorb light only at very high temperatures
Why is the Composition H and He?
The Jovian planets are very cold because they are all far from the sun
Intensity of sunlight decreases with distance from the sun as 1/(distance)2
Jupiter (5.2AU) gets 1/(5.2)2 = 4% of Earths heating -> temp = -148 C = 125 K
Recall (Chapter 4) that temperature represents energy of motion of atoms and molecules
At low temperatures the molecules and atoms move very slowly
Because of the slow speeds, not even the lightest elements (with the highest
speeds) can escape
Jovian planets hold onto H and He + all others
H & He are 99% of all matter -> large masses, diameter and gravities but low
Interiors of Jovian Planets**

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We cannot go inside these planets and we are just beginning to use “seismology” for
these planets ->only indirect methods:
Composition = hydrogen + helium
Jovian planets rotate rapidly -> causes the planets to bulge out at their equators
NO observed expansion or contraction
oOutward pressure = weight pulling in
Interiors of the Jovian Planets
Use a computer
Use the composition of hydrogen + helium
Solve the equation: outward pressure = weight
Apply the observed rotation
Predict the bulge at the equator
Compare prediction to the observed bulge
If it odes not match the observed bulge
oAdjust the interior by trial and error
oRepeat the calculation until the prediction matches the observed bulge
Results for Jupiter
Atmosphere of H, He, H2 + other molecules goes to a depth = 10% of radius
Fluid of compressed, liquid H + He depth
= Another 10% of radius =about 7000km
Liquid, metallic H (something we cannot create on Earth) – almost to Jupiter’s
Solid core of rock + metals = 10x Mearth
Results for Saturn
Similar to Jupiter but with different proportions:
oDeeper atmosphere and “ocean”
oSmaller H metallic zone but larger core
Results for Uranus and Neptune
More different:
o“Ocean” of H compounds, not H2
oNo metallic zone because the pressure is not large enough
Magnetic Fields
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