Class 18 - Giant Planets 2

Reading - New Solar System - Chapter 15

Atmospheric Dynamics

First, let's look at the observations.....remembering that what we are seeing are the clouds which form in a very THIN layer - a ~hundred kilometers - a few scaleheights - a fraction of a percent of the radius. We will concentrate on JUPITER today - the planet where we have the most detailed information.

NOTE - below are telescopic views - upside down!!

 

 

Next - look at the Voyager movies (still the best collection for Jupiter).

Keep in mind what we learned from the terrestrial planet atmospheric dynamics about CORIOLIS force, CYCLONES, ANTI-CYCLONES....

E.g. the Great Red Spot

Circulation around the GRS takes 6-7 days - wind speeds of ~400MPH.

By tracking the motion of clouds we can measure wind speeds....

And then there are the Galileo and Cassini observations at Jupiter.....

Download powerpoint of Galileo observations

Now - having looked at lots of observations WHAT DOES IT ALL MEAN????


HEAT FLUXES - and Equilibrium Temperatures

The giant planets receive, absorb and reflect sunlight - that's how we see them (bottom picture). But they also emit heat - infrared light (top)

Integratingn over the whole disk, the spectrum of the whole disk has a "double hump" - visible reflected sunlight at short wavelengths, and thermal IR at longer wavelengths.

Knowing the total output of sunlight and that light decreases as 1/distance2, we can calculate the amount of sunlight that should be hitting a square meter of each planet. The ALBEDO (A) of a planet is the reflectivity of a planet. Therefore, the total amount of sunlight absorbed the by the planet per square meter is (1-A)x Solar Flux@Earth / distance2 (where distance from the Sun is in AU). In equilibrium, we expect

ENERGY IN = ENERGY OUT

The energy emitted per square meter is described by the Stefan-Boltzmann law for thermal emission: Power/Area = sigma x T4 where T = Temperature of the radiating surface.

Allowing for the fact that objects receive an area 2piR2 of sunlight but emit from all 4piR2, and "normalizing" to the Earth (at 1 AU), we get

Tequilibrium = 288K [(1-A)/a2]1/4

 

What happens when this equilibrium temperature is compared with the TRUE temperature? How do we measure the TRUE temperature?

This figure (from Hubbard's chapter in The New Solar System) shows that the giant planets tend to emit more energy than they receive - all except Uranus where the internal heat source (red arrow) is negligible. Hartmann quotes these ratios:

  Jupiter Saturn Uranus Neptune
Heat Emitted / Sunlight Absorbed 2.5 2.3 ~1.1 2.7

This table tells us that all of the giant planets except Uranus emits about 21/2 times the amount of solar energy absorbed. What's the story with Uranus? Why does it emit so much less energy? This is a MAJOR issue of planetary science.

OBLIQUITY & SEASONS

Which planet(s) do you expect to have strong seasonal effects? Which planets have weak seasonal effects?

 

LATITUDE

Starting in the deep interior (about which we know the least!).... Fluids that are dominated by rotation produce columnar layers, centered around the spin axis.

This is consistent with:

 

But does not explain:

Current ideas are:

 

 

Next week - How do Saturn, Uranus and Neptune compare with this picture at Jupiter?