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Saturnian Supreme

by Perry Pezzolanella, MVAS

The most beautiful sight in a telescope has to be Saturn. The golden globe surrounded by the razor sharp, icy white rings set against the blackness of space is a sight to behold. Dedicated observers scrutinize Saturn closely in order to see detail on the globe and rings. There is actually much that can be seen and the following is a guide.

The Globe. Saturn’s globe is noticeably oblate and can be easily observed when the rings are edge-on. The diameter through its poles is about 10%, or about one Earth diameter less than through the equator. This is due to its low density, less than water, and its rapid rotation of about 10 hours, 38 minutes. Saturn is a completely cloudy world with no visible solid surface and the only details visible are subtle cloud features. The most prominent features are the Equatorial Zone, Equatorial Belts, and the Polar Hoods. The nomenclature for the cloud features is similar to Jupiter, but most of the features will not all be visible at once and are more subtle, so a good telescope with steady seeing are required. The belts are regions of sinking air that warms and dries as it descends clearing the atmosphere revealing the darker depths. The darker yellow, yellow-tan clouds are composed primarily of sulfur compounds and water droplets. The zones are areas of rising air that create billowing whitish clouds consisting primarily of ammonia ice. Saturn’s rapid rotation and strong winds stretch the clouds out into long belts that wrap completely around the planet.

The Equatorial Zone is a wide, bright, whitish band that wraps around the equator and is usually partially obstructed from view by the rings, but fully visible when the rings are edge-on. The Equatorial Belts are the darkest features and usually appear tan against the golden globe. These belts are the easiest features to see with a small telescope, but usually only one belt is visible as the other is hidden by the rings. The Polar Hoods are dusky tan and quite large. They appear to grow darker towards the poles.

Larger telescopes may reveal thin extensions of the belts that finger out into the brighter zones. A festoon is like a dark bridge that connects two belts across a zone. Garlands hook out from a belt and may hook far enough to close off into a loop. A rift is a bright cloud that bridges two zones across a belt. Ovals may be visible as brighter, noncircular patches in the belts. Knots are darker thickenings within the belts while spots are rounder and more sharply defined. White spots are subtle, but once in a rare while they can be seen with a small telescope. The Great White Spot erupted in 1933 and was seen again in 1990. This was a huge, temporary feature as it rapidly elongated within the Equatorial Zone within weeks and stretched around the entire planet. The Great White Spot rivaled the polar ice caps of Mars in brightness and was most likely a huge thunderstorm that erupted and billowed high into the atmosphere. Ammonia gas from the warmer depths froze into brilliant white crystals high up.

The Rings. These are the showcase features of Saturn and the most beautiful planetary feature in the Solar System. They are large, but thin and there is plenty to see. When the rings are wide open, look for the dark Cassini Division. It will look like a thin, dark line within the rings; it is hard to believe that this gap is as wide as the Atlantic Ocean! The outer A ring is not as bright as the inner B ring and it is important to note if there are any variations in brightness within the rings. The innermost ring, the C ring, or Crepe Ring, is dark and nearly transparent, but may be glimpsed when the rings are widest open as a narrow dusky band across Saturn’s globe. There is a very thin gap known as the Enke Division located near the outer edge of the A ring and is a challenge to see even in larger telescopes. The famous spokes that the Voyager and Cassini spacecraft have seen are worth looking for as amateur equipment becomes advanced. Take notice that at certain times when the rings are sufficiently open it is possible to see the shadow of the globe on the rings and the shadow of the rings on the globe.

Due to Saturn’s axial tilt, its rings appear to disappear about every fourteen years. At these times, as in 2009, edge-on rings are interesting to observe and great care should be taken to note if the rings reveal any brighter clumps. It is also interesting to note the dark line that crosses Saturn when the rings appear gone. Try to determine if the line is smooth or clumpy, but do not confuse any of the moons for a clump. With the rings nearly edge-on, it will be easier to see the fainter inner moons. It is also the time to observe transits, eclipses, occultations, and shadow transits of the moons.

The Moons. Up to eight moons can be seen orbiting Saturn: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, and Iapetus, but Mimas and Hyperion will require a large telescope as both are very dim. The easiest moon to see is Titan as it is the brightest at about magnitude +8.3. If it can be seen transiting Saturn, it might be possible to detect an orange color which is due to its smoggy, organic atmosphere. Iapetus is the odd, two-toned world that has one side as bright as snow and the other as dark as coal. This makes it easier to see when it is on one side of Saturn than the other as its magnitude ranges from +10 to +12. Rhea is closer in than Titan, but rather easy to see at 10th magnitude. Still closer to Saturn is Dione followed by Tethys and both are a little dimmer than Rhea. Enceladus is a little closer to Saturn than Tethys, but at a dimmer magnitude of about +11 it is a challenge. It can be seen with a high-quality 4” refracting telescope. Mimas is the closest and very dim at magnitude +13 and Hyperion orbits just a little beyond Titan and is very faint at magnitude +14. Observing these two moons is regarded as a major accomplishment.

Filters are valuable when observing any planet as they can bring out faint detail that would otherwise be invisible. The best filter for Saturn is yellow as it enhances the cloud belts and polar hoods. It also increases the contrast of the rings against the globe and the limb shadow along the edge of the planet.

Saturn is at opposition this year on March 8 in Leo. The rings will be very close to edge-on all this year providing an excellent opportunity to observe many of the phenomena described above that cannot be seen otherwise for at least another 15 years when the rings will be edge-on again. No matter what year it is, Saturn is always the most beautiful of all the planets and reigns supreme when observing alone or with others.