The Io Torus
Surrounding Jupiter at the distance of Io is a donut-shaped volume, or torus, of plasma that originates at the satellite. At first, the atoms escaping from Io expand outward as a gas, but soon they are stripped of electrons and become electrically charged. Some of these gases, such as sulfur dioxide, apparently originate in the large volcanic eruptions; other, such as the sodium cloud being studied with Earth-based telescopes, result from sputtering of surface materials by energetic particles in the magnetosphere. After they are ionized by the loss of one or more electrons, the atoms are caught by the spinning magnetic field of Jupiter and become a part of what is called a co-rotating plasma, spinning at 74 kilometers per second with the same ten-hour period as Jupiter itself.
The Io torus was easily detected on Voyager by the ultraviolet spectrometer, even from a distance of 150 million kilometers. The strongest ultraviolet radiation comes from twice-ionized sulfur (atoms that have lost two electrons) (S III), emitting a wavelength of 69 nanometers or about one-eighth the wavelength of visible light. The spectrometer also detected glows from atoms of triply ionized sulfur (S IV) and twice-ionized oxygen (O III).
At the time of the Voyager 1 encounter, the most abundant heavy ions in the Jovian magnetosphere were sulfur and oxygen. Multiply ionized sulfur and oxygen both emit strongly in the ultraviolet, where they could be observed by the ultraviolet spectrometer. This spectrum of the Io torus registers the tremendous amount of ultraviolet energy (about a million million watts) being radiated. To emit so strongly, the temperatures in the torus must be near 100 000 K.
Direct measurement of the heavy ions associated with the Io torus were made by the Voyager 1 LECP instrument. Here the amounts of various elements are shown for two cases: the Jovian inner magnetosphere (solid line) and a typical solar event. Both the Jovian and the solar particles have been scaled to show similar amounts of oxygen, but the solar particles are also rich in carbon and iron, whereas Jupiter has a great deal of sulfur. Evidence such as this demonstrates that the sulfur does not come from the Sun; rather, the sulfur and most of the oxygen appear to be the product of the Io sulfur dioxide volcanic eruptions.
Scans across the torus showed that it had a thickness of 1.0 RJ and was centered at a distance of 5.9 RJ from Jupiter. The torus is centered on the magnetic, rather than the rotational, equator of the planet. To produce the intense glow observed, the electron temperatures in the torus must be 100 000 K, with an electron density of about 1000 per cubic centimeter. The brightness in the ultraviolet corresponds to a radiated power of more than a million million (10¹²) watts. This enormous amount of energy must be continuously supplied by the magnetosphere.
The ultraviolet emissions from the Io torus seen by Voyager were dramatically different from those seen in 1973 and 1974 with the simpler ultraviolet instrument on board Pioneers 10 and 11. These changes correspond to more than a factor of 10 in brightness. As noted by the Voyager Team, “Because of the remarkable differences we conclude that the Jupiter-Io environment has changed significantly since December 1973. The observed differences are so spectacularly large that this conclusion does not depend on a detailed comparison of the two instruments, their calibrations, or the observing geometry.” The reason for this change, or the degree to which it reflects a large-scale variation in the volcanic activity of Io, is one of the major questions arising from the Voyager mission.
The Io torus can also be observed from the ground. In 1976, spectra showed the glow of singly ionized sulfur (S II), and in 1979 singly ionized oxygen (O II) was detected. The observations of S II and O II are particularly interesting because they provide a measure of the density and temperature of the plasma. In April 1979, between the two Voyager flybys, Carl Pilcher of the University of Hawaii succeeded in obtaining a telescopic image of the torus in the light of S II. He measured nearly the same ring diameter (5.3-5.7 RJ) as had Voyager; interestingly, both agree that the sulfur torus is centered slightly inside the orbit of Io (6.0 RJ).
The emission of light from sulfur ions in the Io torus is so strong it can be measured from the Earth. In these pictures, University of Hawaii astronomer Carl Pilcher photographed the torus in the light of ionized sulfur on the night of April 9, 1979. As the planet rotates, the torus is seen first partly opened, then edge-on, and again opened in the opposite direction. The dark band on the right of each image is due to light from Jupiter scattered in the telescope, as shown in the bottom picture, which contains the scattered light only.
Direct measurements of the torus were made from Voyager 1 as the spacecraft passed twice through this region, once inbound and once outbound. The low energy charged particle instrument and the cosmic ray instrument both determined that the composition of the ions in the Io torus was primarily sulfur and oxygen. Ionized sodium was also observed. For several years, ground-based telescope observations had revealed a cloud of neutral sodium around Io; the Voyager instruments picked up these atoms after they had each lost an electron and become trapped in the magnetosphere. These instruments also derived the electron and ion density (about 1000 per cubic centimeter) and confirmed that the ions were co-rotating with the inner magnetosphere.
Another Io-associated phenomenon searched for by Voyager was the Io flux tube. As a conductor moving through the Jovian magnetic field, Io generates an electric current, estimated to have a strength of about 10 million (10⁷) amperes and a power of the order of a million million (10¹²) watts. The region of space through which this current flows from the satellite to Jupiter is called the flux tube.
Voyager 1 was targeted to fly through the Io flux tube. This was an important decision, since this option precluded the possibility of obtaining occultations by either Io or Ganymede. The event was to take place on March 5, just after closest approach to Io. The effects of the flux tube were clearly observed by the magnetometer, the LECP instrument, and other particle and field instruments; however, subsequent analysis indicated that the spacecraft had not penetrated the region of maximum current flow; it probably missed the center of the flux tube between 5000 and 10 000 kilometers.
The flux tube is not the only connection between Io and Jupiter. Radio emissions from the atmosphere are triggered by Io’s orbital position, and the aurorae that illuminate Jupiter’s polar regions are the result of charged particles falling into the planet from the Io torus. Other charged particles can occasionally escape outward and be detected as far away as Earth.
Io is unquestionably a remarkable world. The only planetary body known to be geologically more active than the Earth, it provides many extreme examples to test the theories of geoscientists. Its intimate interconnections with the Jovian magnetosphere and the planet itself provide a unifying theme to the complex processes taking place in the inner parts of the Jovian system.
The Galileo Probe will make a fiery entry into the Jovian atmosphere, carrying a payload of scientific instruments for the first direct sampling of the atmosphere of a giant planet. Shown here is the moment, at a pressure level of about 0.1 bar, when the parachute is deployed and the still-glowing heatshield drops free from the Probe.