Sunday, January 31, 2010

Jupiter's Layers of Gas

On July 13, 1995, Galileo released an atmospheric probe, which plunged into Jupiter’s atmosphere and transmitted data for almost an hour before it was destroyed by intense atmospheric heat and pressure. After analysis of this data (and earlier data from Voyager), astronomers concluded that Jupiter’s atmosphere is arranged in distinct layers. Since there is no solid surface to call sea level, the troposphere (the region containing the clouds we see) is considered zero altitude, and the atmosphere is mapped in positive and negative distances from this. Just above the troposphere is a haze layer, and just below it are white clouds of ammonia ice. Temperatures in this region are 125–150 K. Starting at about –40 miles (60 km) below the ammonia ice level is a cloud layer of ammonium hydrosulfide ice, in which temperatures climb to 200 K. Below this level are clouds of water ice and water vapor, down to about –60 miles (100 km). Further down are the substances that make up the interior of the planet: hydrogen, helium, methane, ammonia, and water, with temperatures steadily rising the deeper we go.

Bands of Atmosphere

The atmospheric bands that are Jupiter’s most striking feature are the result of convective motion and zonal wind patterns. Warm gases rise, while cooler gases sink. The location of particular bands appear to be associated with the wind speed on Jupiter at various latitudes.
Anyone who watches an earthly television weather forecast is familiar with high-pressure and lowpressure areas. Air masses move from high pressure regions to low pressure regions. But we never see these regions on the earth as regular zones or bands that circle the planet. That’s because the earth doesn’t rotate nearly as fast as Jupiter. The rapid rotation of the giant planet spreads the regions of high and low pressure out over the entire planet.

The Great Red Spot

The Great Red Spot was first reported by the British scientist Robert Hooke (1635–1703). It is a storm, a swirling hurricane or whirlpool, of gigantic dimensions (twice the size of the earth), at least 300 years old. It rotates once every six days and is accompanied by other smaller storms. Neptune has a similar storm called the Great Dark Spot.
How could a storm last for three centuries or more?
We know from our experience on the earth that hurricanes form over the ocean and may remain active there for days. Once they move over land, however, they are soon spent (albeit often destructively); the land mass disrupts the flow pattern and removes the source of energy. On Jupiter, however, there is no land. Once a storm starts, it continues indefinitely, until a larger storm disrupts it. The Great Red Spot is the biggest storm on the planet.