At an average distance of 1.4 billion kilometers from the sun is the second largest planet in our solar system. It takes 29.5 years to complete an orbit. This gas giant is the sixth planet from the sun and the second Jovian planet.
Known since ancient times, this is among the most beautiful objects in our solar system. Its magnificent rings and largest moon, Titan can be clearly seen in a 4-inch or 100 mm telescope, with a low magnification.
In 1610 Galileo Galilei viewed the planet through his telescope and noted a peculiar shape, which confused him. As the Earth orbits Saturn it passes through the plane of Saturn’s rings, which make them seem to disappear at some time and then be viewed more clearly from a higher angle. It was first in 1656 this periodic change in the view of Saturn’s rings was understood, but it was not explained until 1659, in Systema Saturnium, by Christiaan Huygens.
Saturn has a distinct oblate shape, due to its low density and rapid rotation, along with composition. The equatorial diameter is about 120 000 km, while the polar radius is 108 000 km. This oblateness has consequences on Saturn’s rotation: The equatorial area rotates faster than on the poles. This feature is to be expected from celestial bodies that are not solid in their composition.
Saturn’s Atmosphere And Interior
As the only planet in the solar system, Saturn’s density is less than that of water: 0.7 g/cm^3. Like Jupiter, it is mainly composed of hydrogen (93%) and about 5% helium, while the rest are gases like water, ammonia, and methane. The interior of Saturn is probably very similar to Jupiter’s containing a rocky core at the center, which is enveloped by a layer of liquid, metallic hydrogen. Being located about 1.4 billion kilometers from the sun, Saturn receives very little light. Despite this, it radiates more heat into space than it received. This is explained by the hot core of Saturn which is as hot as 12 000K. The core receives this heat from the contraction of the planet.
Like Jupiter, Saturn also displays some features in its atmosphere. Though they are not as colorful or as active as Jupiter. The atmosphere displays a banded pattern, which mostly has lowly saturated colors.
It has been discovered by the spacecraft Cassini that there is molecular oxygen (O2) in both Saturn’s atmosphere and its rings. It was thought that only life could produce oxygen from other processes than fusion in stellar cores. The discovery has taught astronomers that oxygen only isn’t what they will have to look for when searching for life on other planets.
The hottest area on Saturn is not on the equator, but rather surprisingly, it is located on the south pole. This area harbors a powerful vortex, which contrary to other planets that also have them (Earth, Jupiter, Mars, and Venus) is the hottest spot on the planet. Saturn’s pole is currently facing the sun during its orbit and had it been a seasonal effect they would have seen an increment or decrement in the temperature of this area, but they didn’t. Astronomers theorize that gases that produce a greenhouse effect have been concentrated in this area by winds.
Sometimes one can see aurorae (northern/southern light) on Hubble Space Telescope images of Saturn’s poles. These aurorae perplex astronomers, because unlike on Earth, these can brighten and last for days. Sometimes they stand still while the planet rotates underneath them. Astronomers suspect that Saturn’s magnetic field plays a role in the behavior of the aurorae, which is different than on Earth and Jupiter. When Saturn’s auroras become brighter and thus more powerful, the aurora ring of energy encircling the pole shrinks in diameter.
Saturn’s Ring System
Saturn is probably best known for its planetary rings. It was thought to be the only planet in the solar system which had planetary rings, but Voyager 2 discovered rings for the other Jovian planets too, though they are nowhere in comparable size with Saturn. They were originally discovered by Galileo Galilei in 1610, but he was confused by them. Through his small telescope, they appeared as “ears”. In 1655 Christiaan Huygens determined through a more powerful telescope that they were actually ringing.
Saturn’s rings extend from a height of 6 630 km to 120 700 km above the surface (the equator). The rings consist of mainly watery ice particles, ranging in size from boulders to dust motes, but have been polluted by dust and gas from meteors and comets. But, how did they form?
There are a few theories of their origin, one stating that they are material from the original solar nebula, which never became a moon, but this theory has a flaw since the rings are unstable and of recent origin (maybe only a few hundred million years old). Another theory says that a moon came too close to the Roche limit (within this sphere of a planet an orbiting object will be torn apart by tidal forces). A third states that a moon present in orbit was smashed by a comet.
During the visits of the Voyager spacecraft, many discoveries were made. One, in particular, is that apart from the three prominent rings (A and B) and one faint ring (C) that can be seen from the Earth there are four other “rings”. The rings are named in the order they were discovered and not in the order they appear from the innermost to the outmost ring. The gap between rings A and B is called the Cassini division and the much fainter gap in the outer part of the A ring is known as the Encke Division (300 km wide), though Encke might actually never have seen this gap. The two Voyager spacecraft discovered that all these large “rings” are actually made of thousands of smaller rings (maybe as many as hundreds of thousands of rings). The albedo of the rings (reflectivity in percent) is around 0.2-0.6, which is much higher than that of the other Jovian planets. The rings that are opaque are cooler (110 Kelvin) than the rings that are more transparent (70 Kelvin).
Though they might look impressive in size (which they in a sense are!), the ring system is surprisingly thin, less than one kilometer! By using the spacecraft Cassini and a method called “stellar occultation” (an object passes in front of a star), astronomers have discovered that the edges of the rings are sharp.
In between the rings, some moons have been discovered to orbit. These moons are called Shepherd moons. Their presence in the ring system alters the shape of the rings. Moons like Pan, Atlas, Pandora, and Prometheus play a key role in the shaping. The tiny moon Pan (20 kilometers across) orbits within Encke’s gap (which is 300 km wide) and clears and maintains it. It produces knots on the rings visible in the picture to the right. Rings that are closer to Saturn than the moon orbit with higher velocities, and when they pass Pan, they receive a kick, which gives them a speed boost. Mimas may be responsible for the gap in the Cassini division. Outside the orbit of the F-ring Pandora is having a rippling effect on the ring. Though it is small in size, 84 km across, it clearly has a powerful effect on the rings.
Saturn’s Moons
On March 25th Christiaan Huygens discovered the first and largest moon of Saturn: Titan. It is visible through a medium-sized amateur telescope. When the Voyager and Pioneer spacecraft visited the Saturnian system, they sent back pictures showing an orange Titan with an impenetrable (in the visible range of light) and thick atmosphere. It took several centuries before astronomers could take a look at the surface of this mysterious world (more on this below).
Today we know of at least 34 moons that orbit Saturn. The number has steadily grown since the visit of each spacecraft.
The moons Atlas, Pan, Pandora, and Prometheus are known as the shepherd moons. They orbit within or close to the ring system and they play a key role in shaping them. More on these above.
Enceladus is a cold moon orbiting 238 020 km from Saturn and is 500 km in diameter. It is one of the most reflective objects in the solar system and reflects almost 100 percent of the amount of light falling on it. This may be due to ice volcanoes or geysers that erupt material that falls back down on the surface, or even gases escaping from the interior of the moon. It is even possible that ice volcanoes on Enceladus have erupted material enough far out to create Saturn’s E-ring. Saturn’s rings, which are close to the orbit of Enceladus consist almost entirely of pure water ice, but some of the rings have been colored by meteoric debris for millions of years. Enceladus has eluded this “miscoloring”, which suggests that something is renewing its appearance of it (maybe the volcanoes or geysers are behind this?).
Recent flybys of Enceladus by Cassini have shown that it has a small atmosphere made up of water ice. However, Enceladus’s gravity is too weak to hold on to it for a longer period of time. Astronomers are getting more and more interested in Enceladus and they will adjust some passages of the moon so they can get closer views. The next flyby will be on July 14th, 2005, and on Mars 12th, 2008.
Iapetus shows a great contrast in the brightness of its surface. Half of its surface (the polar areas) is 10 times brighter than the other, which gives the moon a suiting name (Iapetus means two-faced). One possible reason for the dark side of Iapetus may be that the material from volcanic eruptions made it dark. Another possibility is that the moon passed through a cloud of organic matter, which colored it. Many impact craters can be seen on the bright sides.
As if this dramatic feature isn’t enough, Iapetus has a seem, which runs halfway through the equator. Some astronomers believe that this seems may be of volcanic origin. Others believe that Iapetus passed through a ring and captured some of the particles as its orbit changed. Evidence points to the fact that something changed Iapetus’ orbit before colliding with ring material.
Mimas is among the innermost moons of Saturn, orbiting at a distance of approximately 185 000 km. It is 392 km in diameter. It was discovered in 1789, by William Herschel. Mimas largest crater is named after him: Herschel’s crater. The crater is about 130 km wide and 10 km deep. The impact which created it must have nearly destroyed the moon. This feature gives Mimas a strong resemblance to the Death Star from the famous “Star Wars”. Herschel crater has a central mountain, which is a relic from the impact. The height is comparable to Mount Everest on Earth. The moon in generally heavily cratered.
Phoebe orbits Saturn at a distance of 12 952 000 kilometers, and a diameter of 220 km. Phoebe has an elliptical retrograde orbit which is inclined 30 degrees to the Saturnian plane. Its relative proportions of rock and ice is quite different from the other Saturnian moon. This suggests that Phoebe is likely a captured moon from the outer solar system (from the Kuiper belt).
Titan was, as mentioned earlier, discovered by Christiaan Huygens in 1655. It is the second largest moon (Jupiter’s Ganymede is the largest) in the solar system and has a radius of 2 575 km. Its orbit takes it as far as 1.2 million km away from Saturn.
Titan is surrounded by a thick orange atmosphere containing many organic materials. The atmosphere consists of 98% nitrogen and the rest mainly methane. Because the gravity is so low on Titan, the atmosphere stretches to a height of about 600km. When these molecules rise to the upper atmosphere, they are broken apart by sunlight and the fragments form heavier organic molecules like propane, ethane, acetylene, hydrogen cyanide, and even more complex molecules.
Titan is very interesting to astronomers because it may contain the same organic substances that were present on the primordial earth – substances that gave rise to life! The temperature is around – 180 degrees Celsius on Titan, so conditions are harsh for life to exist there now, but when the sun swells and enters the red giant phase, temperatures may rise higher. But even then, the time window for life to be created will be small, since the sun will be unstable and blow off its outer layers, decreasing the temperature in the region again.
A cloud vortex has been found on Titan’s north pole, similar to the one on Saturn, but contrary to Saturn’s this one is cold, like the others found in the solar system. This vortex could allow complex chemical reactions to occur.
After a 7-year voyage (which started on the 15th of October, 1997) through the solar system, the spacecraft Cassini-Huygens entered an orbit around Saturn in the middle of 2004. The spacecraft consisted of two parts: the orbiter Cassini and the lander Huygens, which was specifically designed to enter the atmosphere of Titan and make a landing. This landing occurred on January 14th, 2005. Huygens was designed to be able to handle both a liquid and solid environment. However, it landed on solid ground and kept sending back data to Cassini for about two hours (data transmission started when it was in the atmosphere, and more than 1100 pictures were planned to be sent) before Cassini disappeared behind the horizon.
When the Huygens lander had ended its’ mission Cassini continued its four-year-long duty of studying the Saturnian system. It has for instance found out that Titan has a huge amount of ammonia and water, which could contribute to resurfacing Titan. Cassini has also discovered that there are methane volcanoes on Titan.