Part 4

The existence of a D-Ring between the C-Ring and the planet has been confirmed by observations during Voyager 1’s passage through Saturn’s shadow. The tenuous E-Ring, previously observed from Earth only when Saturn’s rings could be viewed edge-on (every 15 years), has also been observed during shadow passage. At least one other ring has been found between the E- and F-Rings in Voyager images.

Long, radial, spoke-like features in the B-Ring were dark when viewed upon approach and bright when observed after encounter when the spacecraft looked back toward the planet and the Sun.

NEW SATELLITES

Voyager 1 photographed six tiny moons, some that had never been seen before. Satellites 10 and 11, dubbed the “co-orbitals,” share an orbit 91,000 kilometers (57,000 miles) above Saturn’s cloudtops. The leading satellite has a diameter of about 160 kilometers (100 miles), while the trailing satellite has an irregular shape, approximately 105 by 65 kilometers (65 by 40 miles).

Little is known about satellites 12, 13, 14, and 15 aside from their orbits and periods. Satellite 12 orbits at the same distance from Saturn as Dione, at a point about 60 degrees ahead of Dione. Satellites 13 and 14, outside and inside the F-Ring (respectively), appear to “herd” this thin ring between them. Satellite 15 appears to limit the outer edge of the A-Ring in a similar manner.

INNER SATELLITES

Mimas, Enceladus, Tethys, Dione, and Rhea represent a body size not previously explored by spacecraft. They are larger than Jupiter’s Amalthea and Mars’ Phobos and Deimos, yet smaller than Mercury, our Moon, or Jupiter’s large satellites. Their diameters range from 390 kilometers (240 miles) for Mimas to 1530 kilometers (950 miles) for Rhea, and they are probably composed primarily of water ice.

With the exception of Enceladus, all of these moons have heavily cratered surfaces, looking much like the Moon and Mercury. Mimas displays an impact crater whose diameter is one-fourth that of the satellite—such an impact must have nearly shattered the icy satellite. Tethys has a valley 70 kilometers (40 miles) wide that stretches 800 kilometers (500 miles) across the satellite, an apparent crustal fracture resulting from seismic activity. Several sinuous valleys, some of which appear to branch, are visible on Dione’s surface. Both Dione and Rhea have bright, wispy streaks on their already highly reflective surfaces, perhaps caused by ice thrown out of craters by meteorite impacts.

Of the five inner moons, Enceladus appears the smoothest, but we will have to wait for Voyager 2 to photograph the satellite at greater resolution in 1981. Since the maximum intensity of the E-Ring occurs near Enceladus’ orbit, Enceladus may be a source of E-Ring particles.

TITAN

Titan is now known to be smaller than Jupiter’s Ganymede. Its diameter is less than 5120 kilometers (3180 miles), which implies a density twice that of water ice. A dense, hazy atmosphere at least 400 kilometers (250 miles) thick obscures the surface. Voyager 1 determined that Titan has a nitrogen-rich atmosphere (as does Earth), but with concentrations of hydrocarbons such as methane (natural gas), ethane, acetylene, ethylene, and deadly hydrogen cyanide. The haze layers merge into a darkened hood over the north pole. At the poles, liquid nitrogen lakes may form. The surface temperature is probably near 100 kelvins (-280° Fahrenheit), only slightly warmer than the boiling point of liquid nitrogen.

Titan has no appreciable magnetic field and therefore possesses no large liquid conducting core. It does, however, supply a small amount of charged particles to Saturn’s magnetosphere.

The southern hemisphere is somewhat brighter than the northern, perhaps as a result of seasonal effects.

OUTER SATELLITES

Of the three known outer satellites, Voyager 1 studied from a distance only Hyperion and Iapetus. Tiny Phoebe, in its retrograde (clockwise) orbit, will be studied by Voyager 2 in the summer of 1981. Hyperion and Iapetus are most likely composed of water ice, although their masses and densities are uncertain. Iapetus has one bright and one dark hemisphere. The dark side, which faces forward as Iapetus circles Saturn, reflects about one-fifth as much light as the trailing, bright side.

MAGNETOSPHERE

Although it is only about one-third the size of Jupiter’s magnetosphere, Saturn’s magnetosphere is still an enormous structure, extending nearly two million kilometers from the planet toward the Sun. The size of the magnetosphere fluctuates rhythmically as the flow of charged particles in the solar wind increases or decreases in intensity. The magnetosphere can be pushed inside Titan’s orbit, so that at times the satellite finds itself outside of the magnetosphere altogether.

Charged particles in the planet’s magnetosphere are dragged along by the magnetic field, circling the planet at Saturn’s rotation rate of 10 hours, 39 minutes. These charged particles whiz by Titan at a dizzying rate of more than 200 kilometers (120 miles) per second. Titan leaves a motorboat-like wake in its orbital path.

Extending from the orbit of Titan inward to the orbit of Rhea, an enormous cloud of uncharged hydrogen atoms forms a doughnut-shaped torus of ultraviolet-emitting particles. Because of their neutrality, these atoms are not towed around by Saturn’s magnetic field.

Close to the planet, Saturn’s rings act as an effective shield or absorber of charged particles. The rings themselves are apparently substantially affected in this process, however, as evidenced by their “spokes” of fine particles and the lightning-like electrical discharges attributed to the rings.

Scientific Investigations

INVESTIGATION SATURN ENCOUNTER OBJECTIVES

Imaging science Planetary meteorology; satellite geology; ring structure and dynamics Infrared Atmospheric composition, thermal structure interferometry and dynamics; satellite surface composition and thermal properties; ring composition Radio science Atmospheric and ionospheric structure, constituents, and dynamics at Saturn and Titan; ring particle size Ultraviolet Upper atmospheric composition and structure; spectroscopy auroral processes; distribution of ions and neutral atoms in the Saturn system Magnetic fields Planetary magnetic field; magnetospheric structure Plasma particles Magnetospheric ion and electron distribution; solar wind interaction with Saturn; ions from satellites Plasma waves Plasma electron densities; wave-particle interactions; low-frequency wave emissions Planetary radio Polarization and spectra of radio-frequency astronomy emissions; plasma densities Low-energy charged Distribution, composition, and flow of particles energetic ions and electrons; satellite-energetic particle interactions Cosmic ray particles Distribution, composition, and flow of high-energy trapped nuclei; energetic electron spectra

“Notre voyageur connaissait merveilleusement les lois de la gravitation, et toutes les forces attractives et répulsives. Il s’en servait si à propos, que tantôt à l’aide d’un rayon de soleil, tantôt par la commodité d’une comète, il allait de globe en globe, lui et les siens, comme un oiseau voltige de branche en branche.”

“Our voyager knew marvelously the laws of gravitation, and all attractive and repulsive forces. He used them in such a timely way that, once with the help of a ray of sunshine, another time thanks to a cooperative comet, he went from globe to globe, he and his kin, as a bird flutters from branch to branch.”

VOLTAIRE—Micromégas, Histoire Philosophique, 1752.

National Aeronautics and Space Administration

Jet Propulsion Laboratory California Institute of Technology Pasadena, California