Saturn

INTRODUCTION

Saturn is the 6th planet from the Sun. It was named after the father of Jupiter by ancient astronomers. What distinguishes Saturn from the other planets in the solar system is its spectacular ring system, which you will soon learn more about. Saturn is considerably fainter than any of the inner planets or Jupiter, or several of the brightest stars for that matter.

BASIC FACTS

Saturn has a sidereal orbit of 29.5 Earth years. At opposition, Saturn can come to within 8 A.U. of the Earth. Its orbital semi-major axis of 9.54 A.U. (1430 million km) puts it at twice Jupiter's distance to the Sun. Saturn's orbital eccentricity of 0.06 means that its orbit around the Sun is elliptical. Saturn also has an axial tilt of 27 degrees.

Saturn has less than one-third the mass of Saturn, or 5.6 x 10^26 kg (95 Earth masses). Its equatorial radius is at 60000 km (9.4 Earth radii) while its polar radius is at 54000 km. Saturn has an average density of 700 kg/m^3 (0.7 g/cm^3). This means that if placed in water (1000 kg/m^3), Saturn would float!

Saturn rotates differentially, just as in the case of Jupiter. However, here in the case of Saturn, its lower average density results in a more pronounced bulge, evident from the wide difference of 6000 km between the equatorial and polar radius. It has a rotational period of 10 hrs 14 mins at the equator and 10 hrs 40 mins at the poles.

Saturn's spectacular ring system are the brightest when the rings are at right angles to the Sun during Saturn's summer or winter, and hardly visible during its spring and fall, when its edge is facing us (and thus unable to reflect enough sunlight to Earth)

ATMOSPHERE

Although Saturn has cloud belts that are one of the few distinctive features in Jupiter's atmosphere, they lack the colourful hue of Jupiter's equivalent. The lack of colour make distinguishing any weather phenomena very difficult.

Once again, spectroscopic observations played a major role in determining the composition of Saturn's atmosphere, although the lack of sensitive instruments delayed the detection of ammonia. Saturn's atmosphere is made up of predominantly molecular hydrogen (92.4%), with helium (7.4%) as the next most abundant gas, followed by methane (0.2%) and ammonia (0.2%).

The total thickness of Saturn's atmosphere is about 200 km, with the highest clouds formed 50 km below the top of the troposphere. The butterscotch colours of Saturn's cloud layers are due to the same basic cloud chemistry as on Jupiter, but because the clouds are thicker on Saturn (due to its lower gravity field), the more colourful levels are not seen.

Like on Jupiter, there is also a stable east-west zonal flow. It also has belts, oval storm systems and other atmospheric flow patterns akin to Jupiter's. These weather phenomena are powered by the convection currents in the planet's atmosphere as well as its rapid rotation rate.

A large white spot was detected by amateur astronomers in 1990 which lasted for only 1 to 3 months. Scientists believe that the white colouration stems from ammonia ice crystals formed when warm gas from the interior penetrate the cloud layers.

INTERIOR STRUCTURE

Like in the case of Jupiter, Saturn re-radiates 3 times more energy than it absorbs from the Sun. This indicates an internal heating source. However, the heating mechanism is far from similar to Jupiter's. In Saturn, helium is slowly depleted from the atmosphere as it is precipitated in the form of rain. As it sinks towards the core, the planet's gravitational field compresses it and heats it up. This energy is what powers Saturn's internal heat.

Saturn is alike to Jupiter in many ways. It has a metallic hydrogen layer just like Jupiter, but it is thinner. Saturn's core is also larger.

MAGNETOSPHERE

Saturn has a strong magnetic field because of the presence of an electrically conducting interior and its rapid rotation rate. However, it is only 1/20 times the strength of Jupiter's field, most probably because of its thinner metallic hydrogen layer.

Saturn's magnetosphere extends about 1 million km towards the Sun. Saturn also emits radio waves, but these were deflected by the Earth's ionosphere and failed to be detected until the Voyager spaceprobe approached it.

MOONS

Saturn is the planet in the solar system with the most number of moons. It has 18 named moons and at least 2, possibly 4 unnamed moons that the Hubble Space Telescope discovered in May 1995.

Saturn's moons can be divided into 3 groups.

The small moons that are less than 300 km across – Pan, Atlas, Prometheus, Pandora, Janus and Epimetheus – are primarily made up of ice.

Saturn has moons that are known as 'co-orbital satellites'. These satellites share their orbits in pairs, with one of the moons having a slightly smaller orbital radius than the other. Due to the differential orbital speeds, there will come a time when one moon 'laps' the other. As the 2 moons get close to one another, they switch orbits! This process repeats all over again.

Another feature of Saturn's moons is chaotic rotation. Hyperion, found between Titan and Iapetus, is the only moon to show this behaviour. Due to the gravitational effects of both Titan and Saturn, Hyperion constantly changes its rotation period and rotation axis.

The 6 medium sized moons – Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus were known long before the space age. Their sizes range from 400 km to 1500 km.

Discovered in 1655 by Christian Huygens, Titan is Saturn's only "large" moon. It has a diameter of 5150 km. It has an atmosphere that consists primarily of nitrogen (90%), with argon (at most 10%), methane (variable) and traces of hydrogen gas, ethane, propane and carbon monoxide.

RINGS

It is now known that all the Jovian planets have their own planetary ring system. Galileo first saw Saturn's in 1610, but he did not know what they were. It was only in 1659 that Dutch astronomer Christian Huygens finally realized that those "bumps" he saw on Saturn were actually its ring system.

Saturn's ring system is divided into 3 main rings, named A, B and C from outside in. The Cassini Division, a dark band about 270 km across lies between ring A and B. The Encke Division is located in the outer fringes of the A ring. Of the 3 rings, the B ring is the brightest, followed by the fainter A ring and the almost translucent C ring.

Voyager found the D and E ring on its visits to Saturn. The D ring is very faint and is located inside the inner edge of the C ring, stretching down almost to Saturn's cloud tops. It contains very few particles and cannot be seen from Earth. The very faint E ring lies well outside the main ring structure and is suspected to be formed from volcanism on the moon Enceladus.

The narrow F ring was discovered by Pioneer in 1979. It has an eccentric shape that resembles several strands braided together. Its shape is due to the gravitational effect of 2 shepherding moons, Prometheus and Pandora, that orbit on either side of it. Gravity plays a major role in maintaining the structure of Saturn's rings, or the rings of any Jovian planets for that matter. Saturn's moons prevent ring particles from diffusing away, as well as give the ring its elliptical shape.

The total mass of ring material has been estimated to be about 10^15 tonnes. Astronomers theorize that Saturn's ring material might have come from a satellite that had strayed too close to Saturn and broke up under Saturn's immense gravitational field. Another possibility is that the material are left-overs from Saturn's formation, about 4.5 billion years ago. However, a more plausible origin of the rings might be due to a collision between a comet and one of Saturn's moons.