Lecture 7: Gas Giant = Jovian Planets

We are now transitioning to a study of gas giant planets, very different from the terrestrial planets in size, density, and composition.

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Jupiter's density is 1300 kg/m3. It is also exhibits differential rotation meaning that its apparent rotational period depends on latitude with the equator having a shorter rotational period than higher latitudes.

What can we make of these facts?

Differential rotation implies that Jupiter's visible surface is not solid but is fluid (e.g., either a liquid or a gas).

Suppose that Jupiter is made of hydrogen which is compressed -- what would the density of Jupiter be if it were composed of hydrogen atoms squeezed as tightly together into a cubic crystal lattice as the size of hydrogen atoms in the ground state would allow?

Later in Lecture 8, we will compute the radius of an electron's orbit in the hydrogen atom:

(where n tells which orbit the electron is in -- n=1 is the smallest or "ground " state)

For the ground state, n=1, and therefore

So the density of hydrogen with this radius in a cubic lattice is (cube root of 2 comes from the geometry of the crystal lattice):

Or in other words, Jupiter can be modeled as being purely hydrogen.

The actual situation is more complex with the outer layers being gaseous and the interior including a region of metallic hydrogen and a small core of heavier elements.

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Spectroscopy from the Earth and from the Galileo probe have confirmed that Jupiter is ~79% hydrogen, 20% helium and 1% of all other elements by mass (this is very similar to the composition of the Sun).

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Jupiter's Atmosphere

The atmosphere merges with the underlying liquid hydrogen layers with no solid surface being present.

The atmospheric features are the result of coloration by trace chemicals containing P and S and a complex pattern of circulation. Belts are darker appearing regions where material has cooled and is sinking while zones are comprised of warmer that is rising on a convective air flow. These regions get stretched all around Jupiter due to its rapid rotation rate. The velocities of gas in the belts and zones are similar to the jet stream velocities on Earth.

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Turbulence at the boundaries between belts and zones leads to the formation of large, whirlpool-like storms analogous to hurricanes on Earth. The Great Red Spot is an example of such a storm. Storms can persist for years on Jupiter because, unlike hurricanes on Earth, there are no continental land masses to disrupt the flow causing the storm.

Jupiter's Magnetic Field