Images of Jupiter in visible light (below) and five-micrometer infrared light show the planet’s characteristic belts and zones. The infrared image reveals areas that emit large amounts of thermal energy. The source of the energy is thought to be breaks in the Jovian cloud cover, which allow investigators a glimpse of the deep regions of the atmosphere. One of the mysteries of Jupiter concerns its heat balance: The planet appears to radiate more heat than it receives from the Sun. [P-20957]
At the same time that the internal heat source on Jupiter was being revealed with long-wave airborne infrared telescopes, a new discovery was being made from short-wave infrared observations. The clouds of Jupiter are too cold to emit any detectable thermal radiation at a wavelength of 5 micrometers (about ten times the wavelength of green light). Nevertheless, images of Jupiter at 5 micrometers revealed a few small spots where large amounts of thermal energy were being emitted. The sources of the energy appeared to be holes or breaks in the clouds, where it was possible to see deeper into hotter regions. The discovery of these hot spots opened the possibility of probing deep regions of the Jovian atmosphere that had previously been beyond the reach of direct investigation.
The Jovian Magnetosphere
The discovery of planetary magnetospheres began in 1959 when the first U.S. Explorer satellite detected the radiation belts around the Earth. Named for James Van Allen of the University of Iowa, whose geiger-counter instrument aboard Explorer 1 first measured them, these belts are regions in which charged atomic particles—primarily electrons and protons—are trapped by the magnetic field of the Earth. They are one manifestation of the terrestrial magnetosphere—a large, dynamic region around the Earth in which the magnetic field of our planet interacts with streams of charged particles emanating from the Sun.
At almost the same time that the terrestrial magnetosphere was being discovered by artificial satellites, astronomers were finding evidence to suggest similar phenomena around Jupiter. Radio astronomy is a branch of science that measures radiation from celestial bodies at radio frequencies, which correspond to wavelengths much longer than those of visible or infrared light. All planets emit weak thermal radio radiation, but in the late 1950s investigators found that Jupiter was a much stronger long-wave radio source than would be expected for a planet with its temperature. This radiation bore the signature of higher-energy processes. Physicists had seen similar emissions produced in synchrotron electron accelerators, huge machines in which electrons are whirled around at nearly the speed of light so that they can be used for experiments in nuclear physics. The Russian theorist I. S. Shklovsky identified the Jovian radio radiation as also resulting from the synchrotron process, due to spiraling electrons trapped in the planet’s magnetic field. From the intensity and spectrum of the observed synchrotron radiation, it was clear that both the magnetic field of the planet and the energy of charged particles in its Van Allen belts were much greater than was the case for Earth.
Using radio telescopes of high sensitivity, astronomers determined the approximate strength and orientation of the magnetic field of Jupiter. Although they were able to measure synchrotron radiation only from the innermost parts of the Jovian magnetosphere, they could infer that the total volume occupied by the magnetosphere was enormous. If our eyes were sensitive to magnetospheric emissions, Jupiter would look more than twice the diameter of the full moon in the sky.
All four Galilean satellites orbit within the magnetosphere of Jupiter; in contrast, our Moon lies well outside the terrestrial magnetosphere. Striking evidence of the interaction of the satellites and the magnetosphere was provided when it was found that the innermost large satellite—Io—actually affects the bursts of radio static produced by Jupiter. Only when Io is at certain places in its orbit are these strong bursts detected. Theorists suggested that electric currents flowing between the satellite and the planet might be responsible for this effect.
The Jovian Satellites
For nearly three centuries after their discovery in 1610, the only known moons of Jupiter were the four large Galilean satellites. In 1892 E. E. Barnard, an American astronomer, found a much smaller fifth satellite orbiting very close to the planet, and between 1904 and 1974 eight additional satellites were found far outside the orbits of the Galilean satellites. The outer satellites are quite faint and presumably no more than a few tens of kilometers in diameter, and all have orbits that are much less regular than those of the five inner satellites. Four of them revolve in a retrograde direction, opposite to that of the inner satellites and Jupiter itself.