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AST101H5 Lecture Notes - Giant Planet, Equatorial Bulge, Galilean Moons

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John Lester

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Astronomy Ch. 8 - Jovian Planet Systems (Nov. 9)
Review of Physical Properties
- Recall that jovian planets share these physical properties:
- large masses - 14.5 to 318 MEarth
- large diameters - 3.88 to 11.2 DEarth
- large volumes - 58 to 1400 VEarth
- small average densities - 0.71 to 1.67 g/cm^3
- like water = 1 g/cm^3, not rock and metal
Composition of the Jovian Planets
- Low densities require the lightest elements
- Lightest element is hydrogen - nucleus has only 1 proton
- Second lightest element is helium - nucleus has 2 protons + 2 neutrons
- Conclusion: H + He must make up the 99% of these planets
- Note: Sun also is 99% H and He
Observed Compositions
- Sunlight reflecting from the jovian planets has absorption lines caused by their
atmospheric gases (Kirchhoff’s 3rd law, p. 118)
- ID the pattern of absorptions with the patterns of many hydrogen molecules: H2, H2O,
H3N, H4C, ...
- Note: we don’t see helium because it does not form molecules and its atoms absorb
light only at higher temperatures
Why is the Composition H and He?
- Key reason: the jovian planets are far from the Sun
- Intensity of sunlight decreases with distance from the Sun as 1/d^2
-Jupiter (5.2 AU) gets 1/(5.2)^2 = 4% of Earth’s heating temp = -148 C = 125 K
- Neptune (30.1 AU) gets just 1/(30.1)^2 = 0.1% of Earth’s heating
- Recall that temperature represents energy of motion
- At low temperatures the molecules and atoms move very slowly
- Even the lightest elements (with the highest speeds) cannot escape
- Hold onto H and He (99%) + all others
! large masses, diameters and gravities but low densities
Inside the Jovian Planets
- Only indirect methods are possible:
- composition = hydrogen + helium
-Jovian planets rotate rapidly causes the planets to bulge at their equators
- no observed expansion or contraction
! outward pressure = weight
- Use a computer:
- Solve the equation
- outward pressure = weight
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- for a composition of hydrogen + helium
- Apply the observed rotation
- Predict the equatorial bulge
- Compare to the observed bulge
- Adjust the interior by trial and error until a match is found
- Results for Jupiter:
- Atmosphere of H, He, H2 + other molecules - depth = 10% of radius
- “ocean” of compressed, liquid H + He depth = another 10% of radius
- Liquid, metallic H (a form of H we don’t have on Earth) - almost to Jupiter’s centre
- solid core of rock + metals = 10 x MEarth
-Results for Saturn: similar to Jupiter but with different proportions:
- Deeper atmosphere and “ocean”
- Thinner H metallic zone
-Results for Uranus and Neptune: more different
- “ocean” of H compounds, not H2
- Not enough pressure to create metallic H
Magnetic Fields
- Recall magnetic fields are created by metallic + fluid + rotation
-All the jovian planets rotate in less than 24 hours
-Jupiter and Saturn have fluid, metallic H
- Uranus and Neptune have no metallic H
- Measurements: Jupiter and Saturn - yes
- Uranus and Neptune - yes? off-centre??
Nov 11
Weather on the Jovian Planets
- Fundamentally different from our weather:
- atmospheres are much thicker
- composed of H, He and H2 molecules
- no land, just deep liquid H2 “oceans”
- much less solar heating
- but much more heat from within the planet
- much faster rotation
- Clouds are red, yellow, brown ... not white
- Rapid rotation wraps clouds into bands around the planet
- Storms never pass over land and die out, continue for years, decades, centuries ...
-Uranus and Neptune are blue because they are so cold some gases freeze out,
leaving methane that absorbs red light
Satellites of the Jovian Planets
- Each jovian planet has many satellites, ranging from 13 to 63+ (still being found)
- 3 families of satellites based on their properties:
- large satellites - formed with their planet
- medium satellites - formed with planet
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