Saturn

I

Introduction

Saturn, sixth planet from the Sun, and the second-largest in the solar system. Saturn's most distinctive feature is its ring system, which was first observed in 1610 by Galileo, using one of the first telescopes. He did not realize that the rings are separate from the body of the planet, and so he described them as handles (ansae). The first person to describe the rings correctly was the Dutch astronomer Christiaan Huygens. In 1655, desiring further time to verify his explanation without losing his claim to priority, Huygens wrote an anagram, the letters of which, when properly rearranged, formed a Latin sentence that reads in translation “It is girdled by a thin flat ring, nowhere touching, inclined to the ecliptic.” The rings are named in order of their discovery, and from the planet outwards they are the D, C, B, A, F, G, and E rings. They are now known to comprise more than 100,000 individual thin rings, each of which circles the planet.

II

Exploration of the Saturnian System

As seen from Earth, Saturn appears as a yellowish object—one of the brightest in the night sky. Observed through a telescope, the A and B rings are easily visible, whereas the D and E rings can be seen only under optimal conditions. Sensitive ground-based telescopes can detect the brightest of Saturn’s numerous satellites, and in the haze of its gaseous envelope, pale belts and zones parallel to the equator can be distinguished.

Three US spacecraft have enormously increased our knowledge of the Saturnian system. The Pioneer 11 probe flew past the planet in September 1979, followed by Voyager 1 in November 1980 and Voyager 2 in August 1981. These spacecraft carried cameras and instruments for analysing the intensity and polarization of radiation in the visible, ultraviolet, infrared, and radio portions of the electromagnetic spectrum. They were also equipped with instruments for studying magnetic fields and for detecting charged particles and interplanetary dust grains.

III

The Interior of Saturn

The mean density of Saturn is only one eighth that of the Earth, as the planet consists mainly of hydrogen. The enormous weight of Saturn's atmosphere causes the pressure to increase rapidly towards the planet’s interior, where the hydrogen first condenses into a liquid, and is then compressed into a metallic state. Electric currents in the metallic hydrogen are responsible for the planet's magnetic field. At the centre of Saturn, heavy elements have probably settled into a small rocky core with a temperature close to 15,000° C (27,000° F). Both Saturn and Jupiter are still settling gravitationally, following their accretion from the great cloud of interstellar gas and dust from which the entire solar system was formed about 4.6 billion years ago. This contraction generates heat, causing Saturn to radiate into space three times as much heat as it receives from the Sun.

IV

The Atmosphere of Saturn

Saturn's atmospheric constituents are, in order of decreasing mass, hydrogen (88 per cent), helium (11 per cent), and traces of methane, ammonia, ammonia crystals, and other gases such as ethane, ethene (ethylene), and phosphine. Voyager images showed whirls and eddies of clouds located deep in a haze that is much thicker than that of Jupiter because of Saturn's lower temperature, which at the cloud tops is close to -176° C (-285° F), about 27° C (49° F) lower than at a corresponding location on Jupiter.

The movements of Saturnian storm clouds show that near the equator the period of rotation of the atmosphere is about 10 hr 11 min. Radio emissions from Saturn indicate that the body of Saturn and its magnetosphere rotate with a period of 10 hr 39 min 25 sec. The approximately 28.5-min difference between these two times indicates that Saturn’s equatorial winds have a velocity close to 1,700 km/h (1,060 mph).

In 1988 scientists identified, from studies of Voyager photos, apuzzling atmospheric feature around Saturn's north pole. What may be a standing-wave pattern, repeated six times around the planet, makes cloud bands some distance from the pole appear to form a huge, permanent hexagon.