S A T U R N : Part 5
MOONS of SATURN
Saturn has many moons, but only eight are bright enough to be seen in typical amateur telescopes. In order of distance from Saturn, these include; Mimas, Enceladus, Tethys, Dione, Rhea, the largest Titan, Hyperion and Iapetus. Together, these eight are often referred as the major Saturnian satellites. Two other are possible to see, but are much fainter. The furthest out is Phoebe, which takes one year five month to orbit once and requires a sizeable telescope to see. More more difficult is Janus. Its orbit lies close to the edge of the rings and is only visible when the rings are placed edgewise towards Earth.
The remainder of the lesser satellites are mostly small, being an assortment of rocky and icy worlds. Six of these are associated within the rings, these moons contribute much of the long-term stability of the rings by “shepherding” the broken ring structure. Most of the smaller moons are totally invisible to amateur instruments. In recent times the number of known Saturnian moons has increased to more than thirty, with many being discovered between 2000 and 2002. In September 2003 the number of moons had increased to 31, and by mid-2005, Saturn has listed 50 in all. By 2009, the number has increased to 62, being now only one satellite behind Jupiter. Of these, 52 has names, which is tabulated in Table 3 and 4 at the end of this webpage.
TABLE 1 : MAIN SATELLITES of SATURN
***************************************************** No. Name Period Distance Diameter Mag. (days) (km.) (km.) (vis.) ***************************************************** I Mimas 0.94 185 520 392 12.9 II Enceladus 1.37 238 020 500 11.7 III Tethys 1.89 294 660 1 060 10.2 IV Dione 2.74 377 400 1 120 10.4 V Rhea 4.52 527 040 1 530 9.7 VI Titan 15.95 1 221 830 5 150 8.3 VII Hyperion 21.28 1 481 100 205x130x110 14.2 VIII Iapetus 79.33 3 561 300 1 060 11.1 IX Phoebe 550.48 12 952 000 220 16.5 X Janus 0.69 151 472 110x100x80 14 XI Epimetheus 0.69 151 422 17x60x50 15 XII Helene 2.74 377 400 18x16x15 18 XIII Telesto 1.89 294 660 17x14x13 18.5 XIV Calypso 1.89 294 660 17x11x11 18.7 XV Atlas 0.6 137 670 20x10 18 XVI Prometheus 0.61 139 353 70x50x40 16 XVII Pandora 0.63 141 700 55x45x35 16 *****************************************************
DESCRIPTION of the
Fig. 5-5. MIMAS : An Almost Destroyed
Fig. 5-6. ENCELADUS
ENCELADUS lies between the orbits of Mimas and Tethys at a distance of some four Saturn radii or 238,020 km and orbits Saturn once every 1.37 days. With an average 11.8 magnitude, it is visible in apertures as small as 15cm or 20cm. It was discovered by William Herschel in 1789, being named in Greek mythology after the giant who died in battle and was buried under Mt. Edna by Athena.
This slightly brownish-yellow moon is believed to be the most reflective body in the Solar System. The moon itself is roughly spherical whose shapes is given as 56×247×245km. Enceladus show evidence of more recent geological activity including recent volcanism (as water fountains) which is likely caused by the tidal forces exerted by Saturn and nearby Dione. Some regions on Enceladus display varied surface features that are both cratered and fairly smooth. Evidence also exists that Enceladus has a tenuous atmosphere which is being being replenished by the presumed geological activity.
TETHYS is the third major satellite out from Saturn whose name derives from Greek mythology from one of the six Titanesses, and she was commonly associated with the fertile sea. In legend she born from the union of Uranus and Gaia, and was both the wife and sister of the Titan, Oceanus.
Tethys was discovered telescopically by Gian Domenico Cassini in 1684. This particular Saturnian moon bears remarkable resemblance to the next outermost satellite Dione, whose diameter is about 1,060 km and whose composition is mainly frozen water ice. The moon orbits some 294 660 km from Saturn in 1.8878 days. Tethys appears as third brightest moon of Saturn, shining at opposition at 10.2 magnitude, following Titan then Rhea.
Fig. 5-7. THREE VIEWS of TETHYS
A huge 250km sized crater named Odysseus that appears on its surface, however unlike Mimas, the impact did not shatter the moon. Opposing this crater is a huge 2,000 km long canyon named Ithaca Casma as swathing across Tethys ’ equator. In places the canyon is 100 km wide and sinks to a depth of 4 km.
Tethys has two smaller companions named Telesto (XIII) and Calypso (XIV) lying about 60° both ahead and behind, respectively. These occupy the so-called Lagrangian points, within the same orbit as Tethys itself, and this is similar to the Trojan asteroids that appear before and after Jupiter’s orbit.
DIONE in Greek mythology was born either as a daughter of Oceanus and Tethys or as one of the original Titans (as one of the six Titanesses). Her child was Aphrodite and her husband was Zeus. Dione lies within six Saturn radii of the planet or 377 400 km from Saturn itself — equivalent to the distance between the Earth and the Moon. It orbits the planet in 2.737 days — the same as the rotational period. The moon was discovered by Gian Domenico Cassini in 1684 has a diameter of 1,120km — second only to Titan. Dione at opposition is quite bright at 10.4 magnitude, following slight behind by the closer moon to Saturn, 10.2 magnitude Tethys. Sometimes amateurs can be confused in which moon they are looking, and it is wise to confirm each moon with an ephemeris of the Saturnian moons.
Dione is brownish coloured world and is likely a mixed slurry of water and ices but does have a significant rocky materials like silicates. In some ways, Dione looks likely less damaged by impact craters than nearby Rhea. The moon also has some odd features, such as being heavily cratered on the trailing side hemisphere rather than the typical leading side as seen on the other Saturnian moons.
Fig. 5-8. Two Faces of DIONE
A relatively poor first image was made by Voyager 1 on 11th November 1980. However, the improved images taken by Cassini in 2005 clearly show more intricate patterns of white material. To me it looks like some child scrawling its hand randomly across the surface, digging up the pristine ice from Dione underground.
Dione also has a companion moon, Helene, which precedes 60° preceding at the Lagrangian point. Helene was found by French astronomers P. Laques and J. Lecacheaux from ground-based telescopes in 1980, in size this “co-moon” is a small iregular 36×32×30 kilometres.
RHEA is the second largest moon of Saturn, which it shares almost equally with the next outermost satellite Iapetus. Discovered by Italian-French astronomer Giovanni Domenico Cassini in 1672, this interesting moon is some 1,530 kilometres in diameter, whose period is some 4.517 days in length. Like the Earth ’ own Moon, Rhea keeps the same face always pointing towards Saturn and orbits some 527,000 kilometres above Saturnian cloud tops.
As one of the Titans in Greek mythology is either the wife and sister of Cronus (Saturn) and the mother of Zeus (Jupiter) who is identified as a fertility goddess. In some versions of ancient Roman mythology she was known as Ops and worshipped as magna mater — the Great mother goddess. Rhea was also one of the principal gods of Crete. Her six children were the male gods; Zeus, Poseidon, Pluto, and the female goddesses of Hestia, Hera, and Demeter.
When Voyager 1 passed with 80,000km of its surface, it found to be a heavily cratered world that appears slightly bluish in colour. Cassini again observed the moon in much detail in early August 2005. Here the craters are seemingly eroded by water ice flows with the addition of some significant resurfacing in its polar regions. Evidence suggests this has been recently formed. Some regions (See Fig. 5-9.) show a swath of white icy material across the surface.
Fig. 5-9. Three Faces of RHEA
Rhea is littered with many craters on its leading face and seems to be composed of much darker material than the moon’s trailing side. Some have correctly pointed out that in some ways Rhea looks more like our own Moon or even Mercury. One thing about the craters that is different from our Moon is that the walls of the craters are not as tall and nor are the central peaks inside them. As the mean density is 1.3 g.cm-3, this indicates mainly water ice mixed with some rocky material. It is quite likely the impacts caused the watery material to flow differently than impacts into solid rock. One of the largest crater on Rhea is Izanagi, which is positioned near the polar regions. On the opposing trailing side of the moon find bright rays of white highly reflective material that looks awfully like some of the rays seen on the lunar surface. One of these looks like someone has just thrown a large snowball towards Rhea, with the resultant ‘splat ’ strewing material falling across the surface from the impact site. Yet another white material arc encircles about one-third the diameter of Rhea. On this side are fewer craters and quite different terrain.
Like Dione, Rhea is detectable through 7.5cm., though any 10.5cm. would be far more suitable.
TITAN was discovered by Christaan Hugyens in 1655 and is the largest moon in the Saturnian system. It is the second largest in the Solar System — just behind Jupiter’s Galilean satellite Ganymede, and is larger than Mercury. Visible even in 7.5cm, Titan’s true diameter is 5 150 kilometres across and is the only known satellite that has a substantial atmosphere. This was found by Gerard Kuiper in 1944 during the Second World War who later became an early expert and discoverer about the regions containing the outer planets. Before the Voyager 1 and 2 spacecraft reached Saturn, no one knew what surface of Titan was like but it was assumed that we could just see through the cloud tops — feature in the imaginative artwork of the Solar System made during the mid-20th Century. Voyager 1 passed in November 1980 within only 7,000 km of the moon but revealed no surface details. Instead found the 100 km thick orangery coloured atmosphere descending to surface pressures some 1.6 times of Earth. Particular interest for planetary scientists is this orange-coloured atmosphere, but why it has survived is not known — especially when it is generally believed that long ago all the other satellites in the Solar System had quickly lost their atmospheres. This unique atmosphere is mainly composed of nitrogen (83%), methane and argon, with traces of carbon monoxide and other hydrocarbon and nitrogen compounds. The atmosphere’s colour would be expected to be nearly transparent or pure white with this composition, but it is likely tainted with aerosol layers of organic chemical compounds.
Variations in the atmospheric colour were also found to be divided into two distinct parts. The southern hemisphere was noticeably brighter and the north hemisphere pole appeared much dark. Although we do not know the orientation of Titan’s axis regrading the ecliptic, the division is the atmosphere is thought to correspond roughly with this moon’s equator. It also seems unlikely that this division is caused by seasonal effects or by Titan’s actual surface as it is too far from the Sun. These variations may be caused by the amount of crystallised methane in the cloud layer which changes depending on the inclination of the Sun in Saturn’s orbit. This may also account for the slight observed changes in Titan’s visual magnitude observed over the last twenty years that has remained unexplained.
Some presently believe Titan atmosphere maybe very similar to the Earth’s some three-and-a-half billion years ago. Titan is considered important to study because the material is likely still in pristine state and unaffected since the solar system’s formation. Investigation may reveal much about the composition of the original solar nebula — something we still know so little about.
Lying so far away from the Sun in the cold depths of the Solar System makes the mean temperature of Titan’s cloud tops to be around -200°C (70K). Some areas of the surface may possibly be composted of a mixture of the liquids of nitrogen, ethane and methane combined with floating pieces ‘iceberg ’ sized blocks of methane and carbon dioxide (dry ice)! Evidence of some organic materials and hydrocarbons have also been found such as acetylene, ethane, ethylene and hydrogen cyanide. This makes Titan a veritable rich organic soup — having all the necessary ingredients to make the amino acids for life. Had Titan been closer to the Sun the evolution of the moon may have had a much different story.
(a.) TRUE COLOUR IMAGE of TITAN OBSERVED CASSINI
(b.) HUYGENS DESCENDS
(c.) HUYGENS LANDING PLACE on TITAN
(d.) LAKE DISCOVERED on TITAN
The lake here is probably composed of liquid hydrocarbons at about 90K. (Image July 2005) Courtesy : NASA/JPL
Cassini took this combined image (Fig.5-10b.) using the wide-angle camera and red, green and blue spectral filters, showing the true colour of active upper atmosphere of Titan. They were taken some 9,000 km above from Titan on a Cassini fly-by on 31st March 31 2005. Here methane (CH4) is being bombarded by the solar ultra-violet radiation, which in turn forms more complex organic molecules like acetylene (C2H2) and ethane (C2H6) Although the image seems Earth-like, the blue haze is caused by light scattered by ethane in the blue and ultraviolet wavelengths of light. The lower atmosphere is a thick orange-brown haze which is at a higher atmospheric pressure than Earth. Comprised of many complex organic molecules, the materials at ground level produces a haze that is only as bright as a full moon illumination seen on Earth at night. (See 5-10c.)
Surprisingly what is quite absent in the atmosphere, and likely any oceans, is water. At the these low temperatures water vapour cannot exist, yet it is likely from the known low density of Titan, that much of the inner bulk of the satellite is composed of water. The atmosphere also acts like a greenhouse, raising the mean surface temperature by twenty-odd degrees higher (94K) than what is expected this far away from the Sun. Life likely does not exist there now. Possibly several billion years from now when the Sun has swollen into a red giant, the substantial increase in temperature may make the environment of Titan more suitable for life — but this time will only be brief.
Titan is easily seen in small telescopes orbiting in the same plane orientation as the rings. Orbiting Saturn every 15d 23h, Titan appears as a rosy-yellow 8.4 magnitude “star” up to 3.5′ east or west of the planet and roughly five times distance of the rings or twenty times the diameter of the planetary disk. The Cassini spacecraft mission has investigate Titan in detail as a special dedicated target that includes the dedicated ESO’s surface and atmospheric probe, Huygens. Cassini will reach Saturn sometime in June 2004 and Huygens to descend into the atmosphere in December of the same year. Much anticipation awaited the arrival of this craft and images recieved proved this world as truly bizarre and alien.
Fig. 5-10. HYPERION
HYPERION in Greek mythology is named after one of the original Titans; being the son of Gaea and Uranus. He is often described as either a Sun-god, the God of Observations or sometimes even the Titan of Light. In the Greek mythological tale, has does not seem as important role — bring more remembered for his children and his father than himself. He later married his sister Theia also known as Athera or Euryphaessa, who became known also as the goddess of Sight. She was also able to make divinations and prophesying the future and her eyes were said to shine light like a glowing lamp. Three of their children became important Greek Pantheon of gods; being Helius (Sun), Selene (Moon) and Eos / Aurora (Dawn).
His son, Helios was given in the early Greek myths as Sun god, who is later times was instead replaced by the Sun worship of the god Apollo. Helios, other than the Sun, has not been used as a name the Solar System.
Hyperion lies some 1,481,000 million kilometres from Saturn, and some 500,000 km outside the orbit of Titan — taking just over twenty-one days for one whole orbit. According to Roger Sinnott (S&T., pg.101; October (2001)) Titan and Hyperion line up every six-four days on the eastern side of Saturn, as the synchronised orbital periods are the ratio of 3:4.
Discovered by William Bond, George Phillips Bond and William Lassell on the 06th September 1848, this oddly sized potato-shaped moon is about 400×220km. with the mean diameter of 266 km. During the distant fly-by by Voyager 2 in late August 1982 not much was seen, but was far more closely investigated by Cassini in September 2005.
Hyperion seems to be one of the largest bodies in the Solar System to be found as non-spherical. Unlike most of the larger moons that are continuously facing the same side towards Saturn, Hyperion tumbles end over end in its orbit, in what can only be described as being chaotic and unpredictable. Some planetary astronomers suspect Hyperion maybe a piece of another moon that was destroyed or fragmented by some long-ago collision. Evidence appears with the surface features like the huge 120 km. crater named Helios, after the Sun god, which is about 120 km in diameter and perhaps 10 km deep. Yet another large crater is Bahloo — the moon god, named after the ancient legend known by the indigenous Australian Aborigines. Hyperion also has a very low density of 1.1 g.cm-3, suggesting it is composed of nearly pure water ice. This seems to be contradicted by its low albedo from dark material across the surface of the moon — a problem also faced by neighbouring Iapetus. Some have speculated Hyperion might even be partly hollow world!
Furthermore, as although the moon is about the same size as the next outermost satellite Phoebe, the moon itself appears six times brighter than it. Some have even claimed that Hyperion is easier to find than the much larger Mimas, even though the latter is somewhat brighter. The likely reason is likely that Hyperion for visual observers remains well away from the glare of Saturn. A 20cm telescope is needed to see Hyperion visually.
Fig. 5-11. IAPETUS
IAPETUS, sometimes referred in older astronomical texts as Japetus, is named in Greek mythology as one of the Titans and is believed as an ancestor of the human race. He was the father Prometheus, Atlas Epimetheus, and Menoetius. Iapetus orbits Saturn in a 79.33 day period at the mean distance of 3,561,000 km. It is third largest of the Saturnian system behind Titan and Rhea.
Iapetus itself is an unusual moon because it changes by about two magnitudes throughout its orbit. It appears that the bright surface on one side, the one favouring the western elongations, have a reflectance (or albedo) of about 50%, while the darkened side is merely 2% or 3%. Jean Dominique Cassini first observed this in 1706, when he could not see the moon during the opposing eastern elongations. Like our Moon, Iapetus has been locked into synchronous orbit, but this does not align with the dark and light portions of the satellite.
We know Iapetus as a 718km sized icy world having the mean density is about the same as pure distilled water ice, so we suspect that the surface is sprayed or coated with dark surface material — possibly from a rouge comet that has hit it in the past, or even by a substantial collision with the nearby dark, 5% albedo satellite of Phoebe. Another theory is that Iapetus brought the material to the surface from past geological activity.
An interesting image was obtained by Cassini that shows both polar regions as white ice with a swath of material distributed across the equatorial and temperate regions. The image attached shows also a large dark crater which looks even larger than the crater Herschel on Mimas.
Iapetus has the mean orbital distance of some 3.5 million kilometres. It takes 2 months 2 weeks or 79.3 days for one full orbit. The orbit of Iapetus is inclined 14.7° to the ecliptic, and every 14 of 16 years, some two years before the rings appear edgewise. Iapetus can either eclipse or occultate the ring plane as it did in 1978. Observations of this phenomenon are very rare.
Fig. 5-12. PHEOBE
PHOEBE is the second last of the main Saturnian moons. In mythology, she is also known under the alternative name Artemis, the goddess of the Earth’s own Moon. Phoebe was the daughter of Uranus and Gaia, who married her brother Coeus. Phoebe had several children, and the most renown was the goddess of the stars, Asteria, and her twin brother, Apollo the sun god. It is also the feminine form of name Phoebus — largest of the two Martian satellites. With Phoebe’s high lineage in regards the heavens, it is a pity her name was not reserved for a much bigger moon!
I have never read any observational reports of amateurs finding or even seeing Phoebe; likely I suspect because of its faint 16.6v magnitude. This moon was discovered well away from Saturn by W. Pickering in 1898 using 60cm, so I do suspect the larger Dobsonians could see it with care. Phoebe’s distance from Saturn is some 12.9 million km and takes some 1 year 5 months or 550.48 days for one whole orbit. Its true size it is only 110km across and is placed in an highly eccentric retrograde orbit.
Pheobe has been approached by the Cassini spacecraft revealing an oddly round rocky body littered with variously sized craters. (See image)
Fig. 5-13. Three Cassini Images of JANUS
JANUS is the closest of the main Saturnian moons and is much more difficult to see. Janus which orbits very close to the edge of the rings and some 151 470 kilometres from Saturn, and taking just 0.69 days to circle the planet. Janus can only be viewed when the rings are placed edgewise towards Earth, and was discovered only fairly recently during 1966, displayed as a round dot silhouetted upon the rings. At maximum brightness, Janus can reach 14.0 magnitude, but whose light is easily swamped by the much brighter rings. Janus only measures about 110×100×80 kilometres in size.
Since the Voyager spacecraft visited Saturn in the 1980’s, more moons have been found by ground-based observatories. Only one satellite has broken this trend, being XVIII Pan was found by M.R. Showalter on 16th July 1990. Pan is the innermost satellite, being one of the shepherd moons within the Encke Division of the rings.
Many of the new satellites were found by ground-based observatories in 1999 and 2000 by astronomers looking for new faint planetary satellites. Often these remain as alphanumeric codes bodies and are often not named until just a few years later — mainly once they have been confirmed independently.
For Saturn, the provision code is written as ‘S/2000 ’ meaning it was discovered as a Saturnian Satellite in 2000. [Incidentally, for the other planets the first letter is J for Jupiter, U for Uranus and N for Neptune.] If two are more are discovered in the same year, then this code is followed by S1, S2, S3, S4, etc. In 2000, twelve new moons were discovered, so each moon is named S/2000 S1 through to S/2000 S12.
The exclusively agreed naming rights are given authority to the International Astronomical Union (IAU) and all names for the new confirmed moons are made at their General Assembly. Coincidentally, the last one was held between the 13th and 26th July 2003 in Sydney, Australia. On the 25th the Naming Committee endorsed the new satellites of twelve for Saturn, eleven for Jupiter and one for Uranus. For Solar System objects it was common to use either the names for the Roman and Greek gods, but they are fast running out of names under mythology. One of the final ratifications is to give a Roman Numeral for the satellite in order in preference from closest to the furthest from the planet. For the Saturnian moons it was decided to use the gods of other nationalities. The new ones in this case were taken from Nordic and Inuit (Eskimo) gods. None could ever be seen in any amateur telescopes because they are very small and very faint. Table 2 below gives the names for these newer satellites as of 2003;
************************************* Roman No. Old Name New Name ************************************* XVIII 18 S/1981 S13 Pan ************************************* XIX 19 S/2000 S1 Ymir XX 20 S/2000 S2 Paaliaq XXI 21 S/2000 S4 Tarvos XXII 22 S/2000 S6 Ijiraq XXIII 23 S/2000 S12 Suttung XXIV 24 S/2000 S5 Kiviuq XXV 25 S/2000 S9 Mundilfari XXVI 26 S/2000 S11 Albiorix XXVII 27 S/2000 S8 Skadi XXVIII 28 S/2000 S10 Erriapo XXIX 29 S/2000 S3 Siarnaq XXX 30 S/2000 S7 Thrym XXXI 31 S/2003 S1 Narvi *************************************
The last thirteen (13) discovered moons found from Earth-based observatories. These have all proved to be small dark coloured bodies with tiny diameters — perhaps somewhere between two and seven kilometres across. Twelve of these later moons were discovered by deep-imaging by David Jewitt, Jan Kieyna and Scott Sheppard. All but one of these orbit Saturn in the opposite or retrograde from the planet rotation, being highly inclined to the ecliptic and with high orbital eccentricities. Four of the fifty satellites to date have been discovered by the Cassini spacecraft, one presently provisionally named S/2005 S1, was found within the Outer A ring in the so-called Keeler Gap.
Table 3 below gives the names for the newest satellites as of 2009. This appears as listed in I.A.U. CBAT Circular No.8730. (17th July 2006) and CBAT Circular No.8826. (5th April 2007).
************************************ Roman No. Old Name New Name ************************************ XXXV 35 S/2005 S1 Daphnis XXXVI 36 S/2004 S10 Aegir XXXVII 37 S/2004 S11 Bebhionn XXXVIII 38 S/2004 S15 Bergelmir XXXIX 39 S/2004 S18 Bestla XL 40 S/2004 S9 Farbauti XLI 41 S/2004 S16 Fenrir XLII 42 S/2004 S8 Fornjot XLIII 43 S/2004 S14 Hati XLIV 44 S/2004 S19 Hyrokkin XLV 45 S/2006 S2 Kari XLVI 46 S/2006 S5 Loge XLVII 47 S/2006 S8 Skoll XLVIII 48 S/2006 S7 Surtur XLIX 49 Anthe L 50 S/2006 S6 Jarnsaxa LI 51 S/2006 S4 Greip LII 52 S/2007 S1 Tarqeq LIII 53 S/2008 S1 Aegaeon ************************************* Unnamed Provisional Moons ************************************* LIV 54 LV 55 LVI 56 LVII 57 LVIII 58 LIX 59 LX 60 LXI 61 LXII 62 *************************************
Table 4 below gives the names for the newest satellites as of 2009, but as yet these have not been named as the orbits are yet to be confirmed. These provisional moons are;
***************** No. Name ***************** 48 S/2004 S4 36 S/2004 S13 43 S/2004 S17 54 S/2004 S7 42 S/2006 S1 55 S/2006 S3 30 S/2007 S2 52 S/2007 S3 ***************** Possible New Moons ***************** ?? S/2004 S6 ?? S/2004 S3/4 *****************
Last Update : 16th August 2012
Southern Astronomical Delights © (2012)