IV.

Jupiter’s Atmosphere

As light travels outward from the Sun it spreads equally in all directions, decreasing in intensity. Because Jupiter is five times more distant from the Sun than Earth is, the light that falls on Jupiter is 25 times less intense than the light that strikes Earth, and the intensity of solar energy reaching Jupiter is therefore only about 4 percent of that reaching Earth. Studies of infrared radiation (energy radiated as heat) from Jupiter reveal that the planet gives off 1.67 times as much energy as it receives from the Sun. The source of the excess radiated energy is apparently stored heat that was created by the energy of impacts that occurred during Jupiter’s formation and the subsequent gravitational compression of the planet’s material. The difference in temperature between the top of Jupiter’s atmosphere and its deepest layers drives the circulation that transports heat from deep within the planet outward.

A.

Banded Appearance

From a distance Jupiter appears to have horizontal stripes, which result from winds that shear its cloud layers into sharply defined bands. These bands circle the planet, with winds along the edges of adjacent bands blowing in opposite directions. Earth’s trade winds form a similar pattern, but Jupiter’s winds are much stronger and more stable. The strongest winds, at low latitudes near Jupiter’s equator, drive individual cloud systems 11° eastward every 24 hours. At higher latitudes the clouds alternately shift westward and eastward corresponding to the banded structure of the atmosphere, which is sculpted by these wind jets. This cloud motion indicates winds of 600 km/h (370 mph) at low latitudes with winds decreasing to tens of kilometers per hour at high latitudes.

Some of the cloud bands appear whitish, while others are orangey or brown. Scientists believe that the colors result from the presence of trace gases in Jupiter’s atmosphere. In the upper reaches of the atmosphere, the temperature drops below the freezing point of ammonia, one of the trace gases. In regions where warmer gases are carried up from below, the fresh ammonia freezes to form highly reflective white ice crystals. The ice crystals are swept horizontally by prevailing winds, causing the formation of bands that appear bright from reflected sunlight. Ultraviolet radiation from the Sun interacts with molecules of other trace gases in the upper atmosphere and generates yellow-brown smog. This smog settles down on the clouds causing those that are deeper in the atmosphere to appear darker brown. Within the darker bands, the atmosphere tends to sink and the ammonia ice crystals melt, exposing more brown smog particles and causing further darkening.

B.

Storms

Major storms often appear suddenly on Jupiter. Evidence suggests that, unlike storms on Earth, which are driven by solar heating of the atmosphere, Jupiter’s storms are caused by bubbles of warmer gas rising through the atmosphere from deep within the planet. These bubbles, carrying varying amounts of heat, create cloud systems that are constrained on the north and south by bands of strong wind blowing in opposite directions. Unable to move north or south, and with no solid landmasses to create friction, the storms roll in the winds and feed off smaller storm systems for weeks or longer.

Jupiter’s most famous storm, the Great Red Spot, has persisted for centuries. The Great Red Spot is so enormous that if three Earths were placed side by side in front of it, they would scarcely span it. The earliest report of a red spot was by Robert Hooke in 1664, although scientists are not sure if the current spot has existed continuously since that time. The cause of the Great Red Spot is not yet known, but its motion is such that it must sustain itself on energy gained from the upper atmosphere, perhaps by absorbing the energy of smaller atmospheric disturbances. It cannot be linked to a heat source deep in the atmosphere, because it moves slowly westward at an irregular rate. The red color of the spot appears to be caused by impurities such as sulfur or phosphorus compounds that absorb ultraviolet, violet, and blue light.

In 1938, three smaller, separate storms formed in a belt near 30° south latitude. Because of their color and shape, these storms were called white ovals. In 1998 astronomers observed that two of these white ovals had merged to form a slightly larger storm system, visible as a single white oval. In 2000 the remaining two storm systems combined into a single storm with a diameter half that of the Great Red Spot. The storm was rotating in a counterclockwise direction as seen from above. Weather systems on Earth that behave in this manner have air masses rising near their centers.

In late 2005, the storm turned brown and in early 2006 it turned red. Scientists dubbed it the Little Red Spot, although the official name is Oval BA. It was the first time scientists were able to witness the birth of a red spot on Jupiter. The color change probably resulted from chemicals being pulled into the upper atmosphere by the storm and exposed to ultraviolet radiation from the Sun. By late 2006 the winds in the Little Red Spot were blowing at about 650 km/h (400 mph), similar to wind speeds in the Great Red Spot. The Little Red Spot has about the same diameter as Earth.

C.

Comet Shoemaker-Levy

In 1994 the comet Shoemaker-Levy 9 provided a unique opportunity to study Jupiter’s atmosphere. The comet was torn apart by Jupiter’s gravitational field as it approached the planet. The resulting fragments collided with Jupiter’s upper atmosphere at speeds of up to 216,000 km/h (134,000 mph). The collisions generated huge explosions in Jupiter’s stratosphere. About a minute after the fragments entered Jupiter’s upper atmosphere, an explosion ejected a rapidly expanding cloud of material about 3,000 km (1,900 mi) above Jupiter’s cloud layer. When this material fell back into Jupiter’s stratosphere, it generated shock waves and discharged enough energy to heat an area several thousand kilometers in diameter from its normally frigid -100°C (-150°F) to more than 700°C (1,300°F). The resulting debris cooled and formed a dark layer in Jupiter’s stratosphere that slowly settled into the deeper atmosphere. Winds then swept the debris around the planet and removed all trace of the event within months.