Saturn, King of the Night

Look toward the south any night around 9:30 from now to mid-June, about 1/2 way from horizon to overhead, and you will find two bright stars, one above and slightly to the left of the other. Point a pair of binoculars or a telescope at the higher star to reveal the majesty of the planet Saturn.

Of all of the dozens of objects we observe during the astronomy course I run here in Southbury, Saturn has always been the one that draws exclamations of awe from my students. Even the smallest telescope or binoculars will show the beautiful rings surrounding the planet. A larger telescope will show more ring detail, and as many as five of its moons. Every night that we observe Saturn during class we see something new, and more than once a student has been looking at Saturn when a moon has suddenly re-appeared from behind the planet or ring, or vanished into Saturn’s shadow.

Galileo was the first to point a telescope at Saturn in 1610. Because of the poor quality and small size of his telescope, Galileo described the rings as looking like ears. Later he concluded that they were two very large moons. But we'll return to Galileo's confusion shortly.

Saturn is the second largest planet in the solar system, located 750 million miles from Earth between the orbits of Jupiter and Uranus. It is about nine times the diameter of Earth, with a mass nearly 100 times that of our planet. It was the furthest planet known to ancient Man. A gas giant similar in composition to Jupiter, it lacks a solid surface (it may have a very small rocky core), and displays a continually changing structure of clouds and storms in its outer atmosphere — similar, though smaller and fainter to what is seen on Jupiter.

The planet itself however, takes a minor role in our fascination when we turn to the host of moons and rings orbiting Saturn. As of this writing, Saturn has an amazing 62 known moons, 48 of which are smaller than 30 miles in diameter, and were discovered by visiting spacecraft. The system of rings consists of a countless number of ice particles, mixed with smaller moons and separated into a dozen or so major structures, with a fine structure resembling the lines of an old LP record.

Though the rings are enormous in extent — 46,000 miles in width — they are shockingly thin, at about ½ mile in depth. Due to the tilt of Saturn relative to its orbit, the tilt of Saturn’s rings continuously changes as viewed from Earth.  Twice every 14 years, the rings appear edge-on to Earth, vanishing from view for months at a time.  The last such occurrence was in 2008 — from personal experience Saturn is much less majestic without his crown.  This was a major source of confusion to Galileo — when he went back to observe Saturn in 1612, the “ears” or large moons had mysteriously vanished, only to reappear within the next year.

The date of formation of the ring system is an active area of research. The original belief that the rings formed with the planet some 4.5 billion years ago has been challenged recently by computer simulations suggesting that the composition of fine ice particles would not be stable over billions of years.  It is possible that the rings are a more recent adornment — perhaps only 100 million years old — caused by the disintegration of a single small moon.

The largest of Saturn’s moons is Titan, which is a world in and of itself. Titan is larger than the planet Mercury, and retains a very dense and dynamic atmosphere — the only moon in our solar system to hold an atmosphere.  In January, 2005 the Huygens Probe — a lander equipped with a parachute system – entered Titan’s atmosphere and safely landed on the surface.  In its descent, it produced photos showing an amazing landscape resembling the Californian coast, showing an ocean coastline, complete with river beds running down to the sea.  The difference would be the temperature and liquid involved at Titan. At distance of nearly a billion miles from the Sun, and a temperature of -290 degrees Fahrenheit, it is the gas methane that is liquid on Titan and forms a “methane cycle” similar (but much slower and more violent) to the “water cycle” here on Earth.

Lastly, one of Saturn’s moons has taken on an additional level of interest.  Enceladus, the sixth largest moon, has recently been discovered to have a subsurface liquid water ocean. Heating caused by gravitational tidal forces of Saturn and other moons has created this ocean, and explosive eruptions of geysers through the frozen surface of the moon have created a plume of material which has been very recently sampled by the Cassini spacecraft. Organic compounds found in this plume of material suggest that the subsurface ocean on Enceladus may be the most habitable region in our solar system outside of our planet. Missions to land on Enceladus are currently in the planning stages in Europe and at NASA.