Saturn (and a conjunction with Jupiter)

Young Astronomers Blog, Volume 28, Number 24.

Every so often, two (or more) planets appear close to each other in the night sky. We call this a conjunction. Late in December 2020, we were treated to a special conjunction where Jupiter and Saturn appeared closer together than they had since 1226. The two planets were visible low in the southwestern horizon near the constellation Capricornus just after sunset. If you missed it, Jupiter and Saturn repeat conjunctions every 20 years or so, although none will be quite this close.

To find out more about the seasons and the solstice, see Leap Year, the Solstice, and Seasons. For more about Jupiter, see Jupiter. Continue with the rest of this article for more about Saturn.

Ancient astronomers observed Saturn from the ground and included it with the five “wanderers” (planets) that moved against the background of stars. It took a few years after the invention of the telescope to discover the true nature of Saturn and its rings.

• Galileo Galilei (1610 to 1616) was the first to observe Saturn through a telescope. He noticed it had what looked like two moons on either side that disappeared and then reappeared a few years later.
• Christaan Huygens (1655 to 1659) was the first to recognize that Saturn had rings and discovered Saturn’s largest moon Titan.
• Giovanni Cassini (1675) identified the gap between the A and B rings now called the Cassini division and discovered the next four moons.

Saturn is the second largest planet in the solar system with a diameter slightly smaller than that of Jupiter and nine times that of the Earth. Like Jupiter, it rotates very quickly with a period of 10 ½ hours. Saturn is, on average, just under 900 million miles from the Sun, twice as far as Jupiter, and it takes 29 ½ years to complete one orbit.

Saturn does have a significant axial tilt at 26.7^o, slightly greater than both the Earth and Mars. This not only gives the planet seasons, but also changes the tilt of Saturn’s rings relative to the Earth as the planet orbits the Sun. The rings are tilted high to the north during the northern winter (the underside of the rings is visible) and low to the south during the northern summer (the upper side is visible). Twice a Saturnian year, during spring and autumn, the rings are flat and seem to disappear for a short time.

Some of you are familiar with the Great Red Spot on Jupiter. Well Saturn has strange weather patterns as well. This includes a hexagon shaped storm over Saturn’s northern pole, one with a huge vortex at its center.

Saturn has an extensive ring system, which is visible through a small telescope. The rings are composed of pieces of water ice that range in size from a grain of dust all the way to the size of a mountain. The main four rings (D, C, B, and A) are forty thousand miles across, but are as little as thirty feet thick. The rings of Saturn are “named” in the order they were discovered.

• The outer A ring and the inner B ring are the most visible and are separated by the Cassini Division.
• The fainter C ring is inside the B ring, and the very faint D ring is inside the C ring.
• The F ring is just outside the A ring.
• Farther out is the E ring, which is a wide diffuse ring believed to be formed from the geysers on Enceladus.
• The faint G ring is found just inside the E ring.

There is disagreement as to the age of the rings. They might be as young as a hundred million years or as old as the planet itself. There is also some debate over the formation of the rings. They might have formed from the collision of a comet or asteroid with a moon. Or a comet, asteroid, or moon could have ventured too close to Saturn and was torn apart by Saturn’s gravity. On the other hand, they could have formed with the planet itself a few billion years ago.

Two recent studies (2019 and 2022) imply that the rings might be very young (between 100 to 200 million years old) and possibly were formed from the breakup of a moon named Chrysalis.

As of July 2024, Saturn holds the record with 146 moons, the most for any planet in our solar system. There are seven moons large enough to be spherical and one large non-spherical moon.

• Mimas is the closest with a large impact crater named Herschel.
• Enceladus is believed to have an underground ocean of liquid water. It has four long canyons, nicknamed “tiger stripes” with icy geysers rising above the surface.
• Tethys is mostly water ice with the huge Odysseus Crater and the long Ithaca Chasma.
• Dione is composed of 2/3 ice with a rocky core.
• Rhea is the 2^nd largest moon of Saturn composed of a uniform mixture of ice and rock.
• Titan is by far the largest of Saturn’s moons.
• Hyperion is the largest of Saturn’s non-spherical moons and orbits just beyond Titan.
• Iapetus is much farther out with one bright hemisphere and one dark hemisphere that has a slight reddish tinge.

Between Rhea and Iapetus, we find Titan, probably the most interesting moon in the solar system.

It is the second largest moon in the solar system, a bit smaller than Jupiter’s Ganymede, but still larger than the planet Mercury. Titan has a thick hazy orange atmosphere made up mostly of nitrogen with a small amount of methane.

Titan’s surface is composed of water ice that is rock hard. Highlands such as Xanadu and lowlands filled with flowing dunes such as Shangri-La are found along its equator. It also has methane filled lakes and seas. Kraken Mare, Ligeia Mare, and Punga Mare are located at the north pole. In the south is Ontario Lacus, a lake about the size of one of the Great Lakes.

Five spacecraft have visited Saturn.

• Pioneer 11 became the first spacecraft to fly by Saturn in September 1979.
• The Voyager spacecraft flew by Saturn in (1) November 1980 and (2) August 1981 as part of their “grand tour” of the outer Solar System. Voyager 2 was given a gravity assist by Saturn and headed on to Uranus. Voyager 1’s trajectory was adjusted so that it performed a close fly-by of Saturn’s largest moon Titan before flying out of the plane of the solar system.
• Cassini was launched in October 1997 and, after two flybys of Venus and a pass by the Earth, arrived at Saturn in June 2004. Cassini explored Saturn and its moons until the mission ended in September 2017.
• Huygens rode along with Cassini and soft-landed on the surface of Titan in January 2005.