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Chapter 8

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John Lester

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 M Earth - large diameters - 3.88 to 11.2 D Earth - large volumes - 58 to 1400 V Earth - 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: H , H O,2 2 H 3, H C4 ... - 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 - 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, H + 2ther 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 M Earth - 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 H 2 - 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 H molecules 2 - 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 - small satellites - irregular shapes and orbits, probably captured Large Satellites - The masses are great enough for gravity to pull them into a spherical shape - like Earth and our Moon - Orbits are: - nearly circular - in the same direction as the planet’s rotation - located above the planet’s equator ▯ → These satellites formed with their pla
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