So – You’re Still Interested in Astronomy?
Last time, we looked at the many moons of Jupiter. If you remember, Jupiter has 67 all together, by far the largest retinue of any planet in the Solar System.
Today, I thought we’d dip into 10 facts you might not have known about some of the moons of Saturn. Again, I say, some, because like Jupiter, Saturn’s moons are numerous and diverse ranging from tiny moonlets less than 1 kilometer across, to an enormous example, bigger than the planet Mercury. To date, we know there are at least 62 of them.
So, this time around, I thought it would be better to start with some of the lesser known moons.
- 1. Why is it so difficult to measure the precise amount of moons?
You might not realize this, but out of all 62 moons, only thirteen have diameters larger than 50 km (31 miles). Wow!
Also, don’t forget the rings. Saturn’s rings are very dense with complex orbital motions of their own. Within them are objects that range in size from the microscopic – to moonlets, hundreds of meters across. Thus a precise number of Saturnian moons cannot be given, because there is no objective boundary between the countless small anonymous objects that form Saturn’s ring system and the larger objects that have been named as moons.
Fact: So far, more than150 moonlets have been detected by the disturbance they create in the surrounding ring material. Even these are thought to be only a small sample of the total population of such objects.
- 2. Are Saturn’s moons divided into groups like Jupiter’s?
Most certainly. Although the boundaries may be somewhat vague, Saturn’s moons can be divided into ten groups according to their orbital characteristics. Trying to describe how they differ can be rather complex, so astronomers sometimes split the moons into 5 broad categories instead.
The major icy moons
Titan itself (which we will talk about next time)
The Inuit group
The Gallic group, and,
The Norse Group
Fact: The moons of the Norse group also orbit in the opposite direction to Saturn’s rotation.
- 3. Tell us more about the major icy moons?
Janus and Epimetheus are called co-orbital moons. They are of roughly equal size, with Janus being slightly larger than Epimetheus. Janus and Epimetheus have orbits with only a few kilometers difference in semi-major axis. (Close enough that they would collide if they attempted to pass each other) Instead of colliding, however, their gravitational interaction causes them to swap orbits every four years. – Cool!
Janus is extensively cratered with several craters larger than 30 km but few linear features. From its very low density and relatively high albedo, it seems likely that Janus is a very porous and icy rubble pile. The moon is also highly non-spherical.
- 4. Is there anything interesting about the Inuit Group?
Not really. The Inuit group includes five prograde outer moons that are similar enough in their distances from the planet (186–297 radii of Saturn), their orbital inclinations (45–50°) and their colors that they can be considered a group. The moons are Ijiraq, Kiviuq, Paaliaq, Siarnaq, and Tarqeq. The largest among them is Siarnaq with an estimated size of about 40 km.
- 5. So what about the Gallic Group?
The Gallic group consist of four prograde outer moons that are similar enough in their distance from the planet (207–302 radii of Saturn), their orbital inclination (35–40°) and their color that they can be considered a group. They are Albiorix, Bebhionn, Erriapus, and Tarvos. As of 2009, Tarvos is the most distant of Saturn’s moons with a prograde orbit. The largest among these moons is Albiorix with an estimated size of about 32 km
- 6. Is there anything to note about this group?
Albiorix is the largest member of the Gallic group of irregular satellites.
It was named in August 2003 for “Albiorix, “a Gallic giant who was considered to be the king of the world. The name is known from an inscription found near the French town of Sablet which identifies him with the Roman god Mars (an interpretatio romana).
Albiorix orbits Saturn at a distance of about 16 million km (9.9 Million miles) and its diameter is estimated at 32 kilometers (19.8 miles).
Given the similarity of the orbital elements and the homogeneity of the physical characteristics with other members of the Gallic group, it was suggested that these satellites could have a common origin in the break-up of a larger moon.
- 7. Finally, we come to the Norse Group
The Norse (or Phoebe) group consists of 29 retrograde outer moons. They are Aegir, Bergelmir, Bestla, Farbauti, Fenrir, Fornjot, Greip, Hati, Hyrrokkin, Jarnsaxa, Kari, Loge, Mundilfari, Narvi, Phoebe, Skathi, Skoll, Surtur, Suttungr, Thrymr, Ymir, S/2004 S 7, S/2004 S 12, S/2004 S 13, S/2004 S 17, S/2006 S 1, S/2006 S 3, S/2007 S 2, and S/2007 S 3. After Phoebe, Ymir is the largest of the known retrograde irregular moons, with an estimated diameter of only 18 km. The Norse group may itself consist of several smaller subgroups.
- 8. Why is this group also known as the Phoebe Group?
Now this is interesting….
The Norse group is also known by the most dominant moon of this group – Phoebe. She is roughly spherical and has a diameter of 213 kilometers (132 mi), which is equal to about one-sixteenth of the diameter of the Moon. Phoebe rotates on its axis every nine hours and it completes a full orbit around Saturn in about 18 months. Its surface temperature is 75 K (−198.2 °C).
Most of Saturn’s inner moons have very bright surfaces, but Phoebe’s is very dark. The Phoebean surface is extremely heavily scarred, with craters up to 80 kilometers across, one of which has walls 16 kilometers high.
Phoebe’s dark coloring initially led to scientists surmising that it was a captured asteroid, as it resembled the common class of dark carbonaceous asteroids. These are chemically very primitive and are thought to be composed of original solids that condensed out of the solar nebula with little modification since then.
However, images from Cassini indicate that Phoebe’s craters show a considerable variation in brightness, which indicate the presence of large quantities of ice below a relatively thin blanket of dark surface deposits some 300 to 500 meters (980 to 1,640 ft) thick. In addition, quantities of carbon dioxide have been detected on the surface, a finding that has never been replicated for an asteroid. It is estimated that Phoebe is about 50% rock, as opposed to the 35% or so that typifies Saturn’s inner moons. For these reasons, scientists are coming to believe that Phoebe is in fact a captured centaur, one of a number of icy planetoids from the Kuiper belt that orbit the Sun between Jupiter and Neptune. Phoebe is the first such object to be imaged as anything other than a dot.
Despite its small size, Phoebe is thought to have been a hot, spherical body early in its history, with a differentiated interior, before solidifying and being battered into its current, slightly non-equilibrium shape.
Material displaced from Phoebe’s surface by microscopic meteor impacts may be responsible for the dark surfaces of Hyperion. Debris from the biggest impacts may have been the origin of the other moons of Phoebe’s group—all of which are less than 10 km in diameter.
- 9. Anything else?
Well this little moon has a big name The Phoebe ring is one of the rings of Saturn. This ring is tilted 27 degrees from Saturn’s equatorial plane (and the other rings). It extends from at least 128 to 207 times the radius of Saturn; Phoebe orbits the planet at an average distance of 215 Saturn radii.
The ring is about 20 times as thick as the diameter of the planet. Since the ring’s particles are presumed to have originated from micrometeoroid impacts on Phoebe, they should share its retrograde orbit, which is opposite to the orbital motion of the next inner moon, Iapetus. Inwardly migrating ring material would thus strike Iapetus’s leading hemisphere, and is suspected to have triggered the processes that led to the two-tone coloration of that moon. Although very large, the ring is virtually invisible—it was discovered using NASA’s infrared Spitzer Space Telescope.
- 10. And Finally.
Two moons were claimed to be discovered by different astronomers but never seen again. Both moons were said to orbit between Titan and Hyperion.
Chiron which was supposedly sighted by Hermann Goldschmidt in 1861, but never observed by anyone else.
Themis was allegedly discovered in 1905 by astronomer William Pickering, but never seen again. Nevertheless it was included in numerous almanacs and astronomy books until the 1960s
So there we go. That’s our look at the ‘minor’ moons of Saturn.
Next time, much more fun – planet sized moons and strange mysteries to be explored.
(And we’re still only on Saturn)
See you then…