Saturn – the ringed wonder

Saturn is the second largest planet in our Solar System. It has one striking feature that marks it out as the most beautiful object in the sky – a dazzling system of rings encircling it.

A beautiful photo of Saturn and its magnificent rings, taken with the Hubble Space Telescope on 6 June 2018, when Saturn was approximately 1.4 billion kilometres from Earth. A storm can be seen in the northern cloud tops. Image credit: NASA, ESA, and STScI

Saturn is a gas giant like its inner neighbour Jupiter. It takes 29.46 years to make one orbit of the Sun.

Saturn lacks the contrasting belts and bands of Jupiter. Instead its disk has a more uniform straw-coloured hue. The planet and its many moons – 62 at last count – were closely studied by an orbiting space probe, Cassini, from 2004 to 2017.

Around three-quarters of Saturn’s atmosphere is made up of hydrogen and a quarter is helium with traces of water, methane and ammonia. Like Jupiter, Saturn is believed to have a small rocky core, and its interior is so hot that it radiates more heat into space than it receives from the Sun.

Saturn measures 120,540 km (74,900 miles) in diameter at the equator, but 11,810 km (7,338 miles) less from pole to pole. This is due to its rapid rotation, with a day on Saturn lasting just 10 hours 40 minutes.

Saturn is 95 times more massive than Earth, but the only planet that is less dense than water. If you could place it in a large enough ocean, it would float.

Saturn’s cloud bands are less colourful than Jupiter’s and the features more subtle but they include oval storms that occasionally break out. A giant storm erupts at roughly 30-year intervals, and was first noticed in 1876. The Cassini probe saw such a storm blow up in late 2010 and encircle the whole planet within months.

The path Saturn follows from late 2019 to mid-2020. Chart by Skymania using Cartes du Ciel

Saturn’s magnificent rings

Even a small telescope will show the astonishing system of rings that encircle Saturn. They measure 282,000 km (175,225 miles) from one edge to the other, but are typically only 10km (6 miles) thick.

The famous Italian observer Galileo was first to notice them, in 1610, though he did not realise they were rings. He thought, at first, that he was seeing three separate objects, before deciding that the rings were a pair of handles.

A Dutch astronomer, Christiaan Huygens, appreciated their ring-like nature in 1655 when he observed with a more powerful telescope, but he thought they were a solid disk.

A close-up of Saturn’s B-ring from the Cassini spacecraft in 2009, showing how it is made up of countless narrower rings. A tiny moonlet is also seen in the picture. Image credit: NASA/JPL/Space Science Institute

The widest gap in the rings is known as the Cassini Division, named after the Italian astronomer Giovanni Cassini who first recorded it. It is about 4,800 km (3,000 miles) wide. A smaller break is called the Encke Gap, but close-up images show that there are numerous other divisions. The main structures are known as the A, B and C rings.

The view from Earth of the ring system varies as Saturn orbits the Sun. The planet’s axis is tilted by 27° and this makes them appear to open and close over time. Every 14 and a half years or so, the rings appear edge-on, and they briefly disappear from sight. Over the following 7.3 years, the rings open wide again.

Even a small telescope will show these incredible rings. Larger instruments will show the larger gaps within them on nights with steady atmospheric conditions.

An interesting phenomenon occurs every year, around opposition, when the rings appear brighter than usual. This is called the Seeliger effect, and is due to sunlight being reflected directly back at us. It is a similar to the way the Full Moon appears noticeably brighter than even large gibbous phases.

Saturn’s brighter moons were captured in this view taken by the Hubble Space Telescope in 2018. Image credit: NASA, ESA, and STScI

Saturn’s many moons

As well as its rings of countless particles, Saturn has many moons orbiting it. The number had reached 62 including discoveries by the Cassini space probe. The largest moons can be seen with amateur telescopes.

Saturn’s largest natural satellite is Titan. It is the second largest moon in the Solar System, after Jupiter’s Ganymede, and with a diameter of 5,150 km (3,200 miels) it is larger too than planet Mercury. Titan is also notable for being the only moon in the Solar System with a dense atmosphere.

Cassini’s companion probe Huygens, built by the European Space Agency (ESA), made a soft landing on Titan in January 2005. The Cassini/Huygens mission found that beneath Titan’s smog-like atmosphere, it is remarkably Earthlike, having liquid-carved features including seas, lakes, rivers and deltas. But these were all produced by liquid methane rather than water.

Saturn has six other sizeable moons, Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus.

Saturn’s seven largest moons, in order from the planet are (from top left) Mimas, Enceladus, Tethys, Dione, Rhea, Titan and Iapetus. Image credits: NASA/JPL-Caltech/Space Science Institute/ESA

Mimas is a moon only 418 km (26 miles) wide, that has been heavily impact-scarred. One giant crater, named Herschel, makes it look like the Death Star from Star Wars.

Enceladus is 512 km (318 miles) in diameter and covered in ice. Casisni observed jets of water spurting from cracks dubbed “tiger stripes” around its south pole. An underground ocean lies beneath the entire crust, and scientists speculate that it could be home to alien life.

Tethys is 1,066 km (662 miles) across. It has less cratering than Mimas, but is home to a 2,000 km (1,240 mile) long canyon called Ithaca Chasma.

Dione is 1,123 km (698 miles) wide. As well as being cratered, its surface has wispy markings which the Cassini probe found to be cliffs of ice.

Rhea is Saturn’s second largest moon, with a diameter of 1,528 km (949 miles). Its heavily cratered surface includes two large impact basins.

Iapetus is 1,450 km (900 miles) wide and an oddity because it has one side much darker than the other. There is also a peculiar mountain ridge running right round its equator. The Cassini mission found that the dark side is due to material deposited by one of Saturn’s minor moons, Phoebe.