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

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Published on 23 Nov 2011
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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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Document Summary

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. 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. Sunlight re ecting 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, Note: we don"t see helium because it does not form molecules and its atoms absorb light only at higher temperatures. Key reason: the jovian planets are far from the sun.

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