Atmospheres of Jupiter and Saturn

Jupiter and Saturn are gaseous and so where their atmospheres begin as well as end depends upon arbitrary definitions. The atmospheres of Jupiter and Saturn contain tropospheres (the "weather" forming layer) and stratospheres. The brightly colored stripes seen on the surface of Jupiter and Saturn are due to different cloud layers in the planet. One sees to different depths (and thus to different temperature regions) when one looks at the stripes. The ordering of the colors is Red, White, Brown, and Blue as one moves inward toward higher temperatures. That is, the reddish regions are the coolest, they sit highest up in the atmospheres. Why the stripes are the particular colors they are is not clear however; the color depends upon chemistry which we do not quite have a handle on.



The light colored stripes are known as zones while the darker colored stripes are known as belts. The thin narrow dark stripes which separate the regions are known as bands. Also, we cannot forget the Great Red Spot, which is a large and stable (Robert Hooke discovered the Great Red Spot in the mid 1600s) atmospheric disturbance which exhibits cyclonic motion. In the following section, we address these features in a little more detail.

Atmospheric Motions

The interesting patterns and motions seen in the Jovian atmospheres are driven primarily by the large heat flow through the atmospheres (due to flow from the interior and heating by the Sun) and by the rapid rotations of Jupiter and Saturn.

Comment

Both Jupiter and Saturn rotate with periods on the order of 10 hours, the Earth has a rotational period of 24 hours. The difference is more extreme than these numbers imply however. Why?

Recall that a person standing on the equator of the Earth is carried along by the rotation of the Earth with a speed of

speed = circumference/day = 25,000 mi/24 hr = 1,000 mi per hr

On Jupiter, a person standing at the equator is carried along at a speed of

speed = circumference/"day" = 250,000 mi/10 hr = 25,000 mi per hr

Due to the large diameters of the Jovian planets, the centrifugal forces (and other "rotational" forces) are huge on the Jovian planets.

Let's consider some effects of the heat flow coupled to the rotation in the atmospheres of Jupiter and Saturn.
The heat flow drives motions in the atmosphere of Jupiter and Saturn. Material heated deep in the atmosphere heats ---> it becomes less dense and so rises (carrying heat upward). It cools as it rises ---> it becomes denser and eventually reaches a point where it start to will sink. It then returns to the deeper layers where it is heated and again starts to rise. As in the interior of the Earth, this leads to large scale motions in the atmospheres of Jupiter and Saturn. The rising stuff cools as it rises and so is light while the sinking stuff warms as it sinks and so is brownish.

Also, because different latitudes on Jupiter and Saturn are heated by different amounts, latitudinal motions are also driven (that is, motions from the equator to the poles).

If the effects of rotation were negligible, we would simply have this up and down (more or less vertical) motion and motion straight toward the poles. However, rotation is not negligible and the motion becomes more complicated. To understand what can happen, consider something known as angular momentum.

So, on Jupiter and Saturn, we find that the hottest material near the equator of the planets is forced to flow toward the poles. The hot flows moving poleward forces some of the cooler material to circulate downward toward the equator. Now recalling the Coriolis force, we note that the poleward moving material deflects in the eastward direction while the equatorward moving material deflects in the westward direction. The preceding effects produce the horizontal stripes which run parallel to the equator on Jupiter and Saturn. A similar thing happens on the Earth where we get eastward winds and westward winds at different latitudes.

Let's return to the Great Red Spot



Imagine that there is a low pressure region in the atmosphere. Because of the low pressure, some atmosphere is forced to flow into the low pressure region. What happens?



In the northern hemisphere, the air circulates in the CCW sense as it flows into a low pressure region. In the southern hemisphere the air circulate in the CW sense as it flows into the low pressure region. If we have a high pressure region, the opposite occurs. In the northern hemisphere, the air circulates in the CW sense and in the southern hemisphere it circulates in the CCW sense.

The Great Red Spot is in the southern hemisphere of Jupiter and the air circulates in it in the CCW sense with a period of ~ 6 days. The Great Red Spot may thus be similar to Terrestrial storms (hurricanes) although its nature is not clearly understood. Oh, there are also other Great Dark Spots on other planets, e.g., Neptune