The Crown Jewel of the Solar System

By Irwin Horowitz, 4-01-08

	Caption: Saturn

In 1610, Galileo Galilei turned a small self-built telescope towards the heavens and for the first time humans observed the sky using something more powerful than their own eyes.  He made several key discoveries, including the presence of craters and mountains on our Moon, the phases of Venus, the uncounted multitude of stars comprising the Milky Way and most notably the four large moons orbiting around the planet Jupiter that are collectively known as the Galilean satellites.

Another “discovery” he made was of two large bodies encircling the planet Saturn, which he believed to be moons like those near Jupiter.  However, in 1612, both objects seemed to disappear, only to return the following year.  This totally confused him and he never truly understood what he had actually observed.

It wasn’t until 1655, 13 years after Galileo’s death, that Dutch astronomer Christiaan Huygens employed a more powerful telescope on Saturn and realized that what Galileo believed to be a pair of satellites was actually a ring encircling the planet.  In addition to identifying the ring, he is also credited with the discovery of the large moon Titan.  Two decades later, Giovanni Cassini noticed the presence of a large gap in the rings (a feature which would later bear his name).  Other gaps in the rings have led to their identification into several subdivisions using the first several letters of our alphabet.

The planet Saturn has long captured the imagination of people everywhere.  Known in ancient times, it was one of the seven “planetes,” Greek for wanderers (along with the Sun, Moon, Mercury, Venus, Mars and Jupiter).  It appears in the sky with a golden hue, and is usually amongst the brightest objects visible at night.  Only a few stars as well as the above list of planets outshine Saturn when it is at maximum brightness.

While numerous discoveries were made over the intervening three centuries, including the presence of about ten natural satellites, it wasn’t until the flybys by the robotic probes Pioneer 11 and Voyager 1 and 2 that we learned much greater details concerning the planet, its rings and satellites.  These flybys occurred over a two year stretch from September 1979 to August 1981.  They measured the magnetic field of the planet and found it to be stronger than Earth’s but weaker than Jupiter’s field.

The rings were shown to be composed of a vast number of particles ranging in size from tiny grains to a few meters in diameter (about the size of a small automobile).  Also, the rings appeared grooved, like the surface of a vinyl LP, with areas of varying particle density tightly confined by the gravitational interactions of some of the moons.  Perhaps the most unusual discovery concerning the rings was a series of radial spokes.  These features appear to be the result of an electromagnetic interaction between the tiniest ring particles and the planet’s magnetic field.

These missions also discovered dozens of previously unknown moons, many only a few kilometers in diameter.  A pair of moons were found to bracket the F ring (Prometheus and Pandora), shepherding the particles to ensure they remained in a tightly confined orbit.

All told, these missions vastly expanded our knowledge of this most heavenly body.  However, it wasn’t until the middle of 2004, when the Cassini spacecraft entered into orbit around the ringed planet, that many of the deepest mysteries of that world and its retinue of moons began to be revealed.

Extensive observations of the magnetic field by the probe have helped to refine the measurement of the rotation rate of the planet.  Saturn rotates faster than all of the other planets in our solar system.  A “day” on Saturn lasts less than 10 hours.  In fact, the planet rotates so fast that there is a noticeable bulge (or oblateness) at its equator.  Its radius is almost 6000 km greater at the equator than at the poles.

The atmosphere of the planet, consisting mostly of hydrogen and helium gases is buffeted by some of the fiercest winds in our solar system.  Speeds in excess of 1000 miles/hour are typical in the jet stream.

Scientists theorize that deep within the core is a rocky center, with temperatures approaching 12,000° C.  The source of the magnetic field is believed to be a magnetohydrodynamic effect caused by the rapid rotation in a layer above the core of liquid metallic hydrogen, a highly conductive material.

Four new moons have been discovered, bringing the total number at present for the planet up to sixty!  More than half of these are smaller than a city like Boise.  It is the largest of the moons, Titan that has received the greatest amount of attention.

Titan is unlike any other moon in our solar system.  It contains a thick atmosphere composed primarily of nitrogen (like Earth’s) but with various hydrocarbons like methane and ethane present in trace amounts.  The atmosphere is opaque at optical wavelengths, the result of a thick haze formed by these complex hydrocarbons.  In order to penetrate that haze, radar imaging is performed so we can “view” the surface of the moon.  At the surface, the atmosphere is even denser than our own atmosphere, but temperatures hover around -180° C (-292° F).

One of the most significant achievements of the Cassini probe has been the confirmation of liquid methane “lakes” on the surface of Titan.  Using synthetic aperture radar to image the surface during close flybys, these lakes appear much like bodies of liquid water on Earth, but exist under vastly different physical conditions.  Any water found on the moon’s surface is frozen harder than steel!

Prior to entering orbit of Saturn, the spacecraft released the Huygens probe, which descended down into the thick atmosphere of the moon.  It radioed back telemetry data regarding the physical and chemical conditions it found during its passage through that atmosphere and then soft landed on the surface.  With sufficient battery power for only a few hours, the probe radioed back the first images from the surface of that world.  These images show a “rock” strewn surface.  However, these “rocks” are composed of frozen water.

Some of Saturn’s other moons also show unique features.  I have long been fascinated by Mimas.  This image was taken in 1980 by the Voyager 1 probe.  It looks like a planet-destroying battle station from a famous 70s movie.  However, that movie was released three years before this image was taken!  Herschel Crater is named after the discoverer of Mimas, William Herschel.

Iapetus has an odd surface in which one side is very bright, while the other side is quite dark.  As it orbits around Saturn, the daytime side warms up and the water ice sublimates, leaving behind a dark residue.  At night, the vapor freezes back onto the surface.  However, it preferentially freezes on the brighter, colder material than on the warmer, darker material and over billions of years has resulted in this surface dichotomy.

The hemispheric separation originated as a result of dark material in space being swept up by the leading side of the moon as it orbited Saturn and accumulating over millennia.  The sublimation process then has further modified the contrasting surface albedo to the point we observe today.

Saturn has been well placed in the night sky for the past several months, and will continue to dominate in the early evenings into the summer.  It is located to the east of the bright star Regulus, in the constellation Leo the Lion.  To find it on a clear night, look up and locate the sickle shape (backwards question mark) consisting of six moderately bright stars.  At the base of this figure is Regulus and Saturn is the golden hued object a short distance to the left of the star.

Through even a modest sized telescope (or even a decent pair of binoculars), the ring is clearly visible under low magnification.  Saturn is currently approaching one of the nodes in its orbit.  It will reach this point in the late summer of 2009.  When it does so, the ring will seem to “disappear” for a few months, as we are viewing it nearly edge-on and it is far too thin to appear even in the largest of telescopes.

This “ring-plane” crossing is what Galileo observed back in 1612 when his two “moons” disappeared and caused him great confusion.  Saturn reaches an orbital node twice during its 29 year revolution about our Sun.  The last ring plane crossing occurred in 1996 and the next one will be in 2025.

Besides Saturn, Mars is still visible in the evening hours in the middle of Gemini, south of the twin bright stars Castor and Pollux.  Mars has a distinct orange hue making it readily identifiable under clear dark skies.  Jupiter is rising in the early morning and makes quite a show during the pre-dawn hours in April.  It is low in the southeast, easily outshining everything else in that part of the sky.  Mercury and Venus continue to be too close to the Sun for viewing.  Mercury returns to the evening sky next month, while Venus doesn’t return there until later this summer.

In the early evening on April 8th, the crescent Moon will pass just north of the Pleiades star cluster.  Unfortunately, it won’t get dark enough to see unless you have access to a telescope.  However, you can certainly view the Moon after it gets dark and see it in proximity to the cluster.

The Boise Astronomical Society will be holding its annual Messier Marathon this weekend, April 04-06 at Bruneau Dunes State Park.  Last year’s marathon was a terrific success, with clear skies and comfortable weather.  The observatory at the park has officially opened to the public for the 2008 season.  Every Friday and Saturday night there will be a public presentation and, weather permitting, observing using the 25” telescope.  There is a nominal fee of $3 per adult and $2 for children 6-11.  Under 6 are free.  If you want to experience some of the things I write about in my monthly columns, this is a good opportunity.

The BAS will be holding the monthly membership meeting on Friday, April 11th in Classroom #2 of the Discovery Center of Idaho.  This month, fellow BAS member (and BSU professor) Richard Beaver will regale us with tales of the Hertzsprung-Russell diagram and how professional astronomers use this tool to better understand the nature and lives of stars.