The Great Red Spot

The great red spot of Jupiter is it's most prominent feature, and it along with the colored rotating atomosperic bands make up the characteristic view of the planet. First reported by British scientist Robert Hooke in the mid-seventeenth century, the great red spot has been observed for over 300 years. It is a pattern of swirling winds, like a hurricane twice the diameter of the Earth. Presently it is an oval some 25,000 by 15,000 kilometers, compared to the Earth's diameter of 12,756 km. Within the pattern of colored bands which move in alternating directions, the red spot moves around the planet with a period similar to the interior of the planet. This suggests that its roots reach down into the interior of the planet.

The flow around the red spot is counterclockwise, with a period of about 6 days. Wind speeds associated with the flow reach about 200 km/h. While the center of the red spot appears rather calm, the edges are turbulent, spinning off smaller eddies. The spot appears to be driven by the zonal flow which is westward at the top of the spot and eastward at the bottom, but many mysteries remain.

Besides the great red spot, there are numerous other ovals, some red and some white, which represent smaller storms on the planet. There are also features called "brown ovals" which appear to be holes in the atmoshpere which allow us to see deeper into the planet.

Accustomed as we are to the pattern of hurricanes on the Earth, which are thankfully short-lived, it is surprising to see the long-term persistence of these storm features on Jupiter. One comment by Chaisson and McMillan is that hurricanes tend to break up as they reach land masses, and there are no such obstructions on Jupiter. Another factor is the rapid rotation of Jupiter (just under 10 hours for this huge planet's period), which wraps the high and low pressure systems all the way around the planet, making conditions more stable.

The Atmospheric Bands of Jupiter

The surface of Jupiter is divided into rotating bands which alternate between bright zones and darker, reddish belts. These bands change with time, even though there are no seasons on Jupiter, so they appear to be dynamic changes in the atmosphere. Relative to the rotation rate of the interior of the planet, these bands can reach velocities over 300 km/h. The bright zones are modeled as upward-moving convective currents in the atmosphere. The darker belts represent cooler downflow zones, the other part of the convective cycle. The bright zones lie slightly higher in the atmosphere than the belts.

Jupiter: One of the Gas Giants

Jupiter, along with Saturn and Uranus are sometimes called the "gas giants". They are massive enough to have held on to most of their original atmospheres. Jupiters atmosphere is almost entirely hydrogen and helium. Molecular hydrogen makes up 86.1% of the atmosphere by number, and helium 13.8%. There are small amounts of methane, ammonia and water vapor in the atmosphere.

Hydrogen and helium are thought to make up the bulk of the interior of the planet as well, although the density of just over 1300 kg/m^3 suggests a small solid core.

Jupiter: Evidence for a Solid Core

Being one of the "gas giants", Jupiter is composed mostly of hydrogen and helium, but an overall density of 1300 kg/m^3 suggests that there is something more there. Another line of evidence is the modeling of the oblateness of the planet.