Jupiter

Jupiter

Jupiter is the largest of all the planets. The distance across its equator is 88,760 miles or more than 11 times the corresponding distance on the Earth. Although feebly lit because of its great distance from the sun (about 483 million miles of the average), it can be surpassed in brightness among the planets only ve Venus, and sometimes by Mars at its brightest. Like Mars and the other so-called “superior planets” whose orbits lie outside the orbit of the Earth, Jupiter can be seen at all hours of the night.

Cloud Belts

In a good telescope Jupiter presents a bright yellowish disk crossed by bands or belts of various shades of gray and brown, arranged parallel to the planet’s equator. The disk is slightly flattened, the distance from pole to pole being smaller than the diameter at the equator by about 6,000 miles. After about an hour’s observation, it is apparent that the planet has turned slightly, for Jupiter rotates with remarkable rapidity. The time of one rotation at the equator is only nine hours, 50 minutes, 30 seconds, representing a speed of about 27,000 miles an hour. Away from the equator the rate of rotation is different in different latitudes.

Jupiter and its satellites

Changes in the appearance and positions of spots and other features in the belts show quite clearly that we are looking at the tops of clouds in a highly unsettled atmosphere. The latter is rich in gaseous ammonia and methane, and since these are compounds of hydrogen, it is reasonable to conclude that hydrogen gas is abundant. According to one theory, Jupiter consists largely of hydrogen, which, under great pressure in the planet’s interior, exists in the liquid and solid states. This assumes that Jupiter has little or no internal heat, and requires that its atmosphere should be comparatively shallow. The average temperature of the clouds is as low as – 145 degrees centigrade, but it does not follow that the extreme degree of cold extends to the central regions.

The Great Red Spot

One remarkable feature of the cloud belts is a large marking known as the Great Red Spot. During the century or so over which is has been regularly observed the spot has changed considerably in colour and position, sometimes appearing as a prominent red-brick oval patch, and at other times disappearing completely. It is about 30,000 miles long and 7,000 miles wide, but little is known about its structure.

Jupiter emits energy in the form of radio waves and can therefore be studied with radio telescopes. Part of this energy comes in sudden bursts whose occurrence seems to be influenced by the position of Io, one of Jupiter`s satellites. Another part is steadier in intensity but nevertheless varies as the planet rotates. The precise origin of these emissions is a subject for much speculation but they are thought to come from a strong and complex magnetic field associated with Jupiter itself. The field, like that of the Earth, deflects (and is deflected by) the solar wind, and also acts as a kind of reservoir for charged electrified particles.

Satellites

Jupiter has twelve satellites, the four largest of which were discovered by Galileo in 1610 with the newly-invented telescope. These four revolve about their parent in periods ranging from 1 ¾ to 16 ¾ days, and in doing so frequently travel in front of and behind the disk. When they pass in front they cast round, black shadows on the cloud belts. When they pass through the shadow of Jupiter they disappear and are said to be eclipsed. From variations in the predicted times of these eclipses, the Danish astronomer Olaus Roemer in 1675 discovered that light takes time to travel from Jupiter to the Earth. One of the four major satellites, Ganymede, is about the size of Mercury and slightly more than twice as massive as our moon; the other three are about equal in size to the moon. The other eight satellites range in diameter from 100 miles to about 10 miles and can therefore be seen only in large telescopes.