If ever you’ve seen Saturn through a telescope, you’ll know why it’s often called the jewel of the Solar System. And May will be the best time this year to see this gem of a planet. Saturn is at opposition on 10th May; a time when the planet is opposite to the Sun in the sky, and perfectly placed to be admired.
The planet will reach its highest point in the sky at midnight on the 10th and will look like a bright star in the constellation of Libra. If you have a mobile device with a planetarium app showing the constellations, just point it at the night sky, line up the brightest stars, and you should be able to pinpoint Saturn. If you don’t have the technology, it’s still easy to spot: look due south around midnight, at the point in the sky where you’d expect to see the Sun at midday, Saturn will be the pale yellow dot shining bright amongst the background stars.
Saturn is a gas giant, second only to Jupiter in girth, and made of similar stuff – mainly hydrogen and helium – and is the most distant planet visible with the naked eye. It’s thought to have a small rocky core, perhaps the size of Earth. It also has a large portfolio of natural satellites, currently 62 moons and rising. Saturn’s most spectacular feature though, has to be its enigmatic rings.
Galileo Galilei was one of the first astronomers to observe the rings of Saturn in 1610, describing them as ears or handles! Galileo was using possibly the most advanced telescope of the age (designed and built by himself), but it was only capable of revealing the crudest of images, with much left to the imagination. Modern astronomical instruments, both ground-based telescopes and those in space, together with a handful of space exploration missions, have provided us with breathtaking images of Saturn, its ring plane, and its many fascinating moons. We now know that the rings, moons and moonlets are held in place by gravity and, much like our host star, Saturn sits at the heart of a complex orbital system.
The outer planets, residing beyond the asteroid belt, are all bestowed with rings; but none as beautiful (or visible) as Saturn’s fine collection. This cosmic bling, shining and glinting in the light of the Sun, is made almost entirely of ice crystals. The main rings measure about 280,000km across, and a mere 20m deep on average; so thin that they occasionally disappear from view altogether when seen edge-on from our distant viewpoint on Earth. The icy particles range in size from tiny hail stones to small icebergs. They are held in place by the strong gravity exerted by Saturn and inhabit an area known as the Roche limit: an orbital neighbourhood where Saturn’s gravity overrides the gravity of the ring structure and stops the ice forming a single satellite. The ring plane has many interesting features, from strange optical spokes which appear to rotate at different speeds to the ice particles themselves, to ripples and waves within individual rings caused by moonlets orbiting within empty gaps.
There have been many theories of how Saturn acquired its rings. The most popular idea is that an icy moon, possibly several, strayed too close to the planet and was torn apart in the Roche limit; the ice formed the ring plane, and the denser, rocky material plunged into Saturn’s core. Due to the pristine whiteness of the ring material, some planetary scientists believe this may have happened as recently as a million years ago.
Saturn has a plethora of fascinating moons; two of which have caused great excitement among astronomers in recent years. The Cassini Huygens (pronounced hoy-gens) mission, a cooperative project of NASA, the European Space Agency and the Italian Space Agency, sent a cosmic space lab to study the saturnian system in 1997, arriving in 2004. One of the mission objectives was to investigate the largest moon, Titan: the only moon in the Solar System with a substantial atmosphere. The Huygens probe separated from the Cassini orbiter and descended through the clouds of this mysterious world. What was captured by the on-board camera was mysterious indeed: mountains, valleys and plains, rivers, lakes and seas – vistas similar to those on Earth. But instead of water, the lakes and seas are filled with liquid hydrocarbons – methane and ethane – environments where life could exist, as it does in similar habitats on Earth.
Another discovery came when the orbiter made its first flyby of the icy moon Enceladus (en-sell-a-duss). It imaged huge geysers erupting from the south polar region, blasting hot water vapour into space, creating a circular veil of ice known as the E Ring. Closer flybys have revealed a dynamic region with vast fissures coated with complex organic compounds. This could be a hint that a warm, salty ocean lies beneath the icy surface, with active geology and hydrothermal vents – a habitat where life could exist, as it does on Earth.
To find out more about this planetary jewel, go to www.nasa.gov and visit the Cassini Mission web page.
© Melanie Davies 2014