Outer Moons of Saturn (29 Aug 2018)

The outer or irregular moons of Saturn were my primary research topic during the 2^nd half of the Cassini mission at Saturn. This page gives an overview.

Die sogenannten irregulären oder äußeren Monde von Saturn waren mein primäres Forschungsthema während der zweiten Hälfte der Cassinimission am Saturn. Diese Seite gibt eine Übersicht (in englischer Sprache).

Observations of the outer or irregular moons with the Narrow-angle camera of the Cassini spacecraft give us information on basic properties of these distant, probably captured satellites of Saturn. Repeated shuttering over many hours allows the determination of a light curve and synodic rotation period. With several observations at various different geometries, it is also possible to derive the sidereal rotation period, the pole-axis orientation, and even calculate a convex-shape model of the object. With observations at various phase angles, a phase curve might be derived.

With the Cassini camera, I was observing 25 of the 38 irregular moons of Saturn known at end-of-mission. Because the moons are quite small and far away from the spacecraft, they appear only as light dots smaller than a pixel in the camera. From the data, rotation periods could be determined for all of them (see Table 1 ), they range from 5.45 hours (moon Hati) to 76.13 hours (Tarqeq). The mean of the spin rates of the Saturnian irregulars is significantly lower than of the main-belt asteroids, but similar to Hilda and Jupiter Trojan asteroids. The absence of very fast rotators among the irregulars indicates that these objects have rather low densities, possibly as low as comets. The very last Cassini observation of an irregular moon was performed on 05/06 Sep 2017 during an observation of object Thrymr. As an example for how a calibrated data set looks like, the images of our final four Thrymr observations can be found as an animated gif here (resized by a factor of 2 and cropped to 400×600 pixels).

The only outer moon of Saturn where high-resolution images could be taken is Phoebe. During Cassini’s approach to Saturn in June 2004, a targeted flyby at a minimum distance of 2071 km has been performed, for which my responsiblity was the imaging observation planning.

The table below gives an overview on basic properties of Saturn’s irregular moons. Very detailed information, especially about the individual moons, will be added to this page in the foreseeable future.

Table 1. Physical and astronomical properties of the irregular moons of Saturn

Moon	Spice ID¹	Group member²	Absolute magnitude H [mag]³	Best apparent magnitude in CSM [mag]⁴	Mean diameter [km]⁵	Rotation period [h]⁶	Semi-major axis [million km]⁷	Eccentricity [ ]⁷	Inclination [deg]⁷	Orbital period [a]⁷	On-sky uncertainty [ ” ]⁷	Moon (abbrev.)⁸
Phoebe	609	retrogr.	6.6	6.1	213.0	9.274	12.95	0.16	175.2	1.50	0.1	Pho
Ymir	619	retrogr.	12.3	12.9	19	11.9222	23.13	0.33	173.5	3.60	0.5	Ymi
Paaliaq	620	Inuit	11.7	10.4	25	18.79	15.20	0.33	46.2	1.88	0.3	Paa
Tarvos	621	Gallic	12.9	12.7	15	10.69	18.24	0.54	33.7	2.53	0.7	Tar
Ijiraq	622	Inuit	13.2	11.8	13	13.03	11.41	0.27	47.5	1.24	0.6	Iji
Suttungr	623	retrogr.	14.5	15.0	7	7.67	19.47	0.11	175.8	2.78	0.6	Sut
Kiviuq	624	Inuit	12.6	12.0	17	21.97	11.38	0.33	46.8	1.23	0.6	Kiv
Mundilfari	625	retrogr.	14.5	15.2	7	6.74	18.65	0.21	167.4	2.61	0.8	Mun
Albiorix	626	Gallic	11.1	9.5	33	13.33	16.39	0.48	34.1	2.15	0.3	Alb
Skathi	627	retrogr.	14.3	14.9	7½	11.10	15.64	0.27	152.6	1.99	0.7	Ska
Erriapus	628	Gallic	13.7	13.5	10	28.15	17.60	0.47	34.5	2.38	1.4	Err
Siarnaq	629	Inuit	10.6	10.5	42	10.1879	18.18	0.28	45.8	2.45	0.1	Sia

Moon	Spice ID¹	Group member²	Absolute magnitude H [mag]³	Best apparent magnitude in CSM [mag]⁴	Mean diameter [km]⁵	Rotation period [h]⁶	Semi-major axis [million km]⁷	Eccentricity [ ]⁷	Inclination [deg]⁷	Orbital period [a]⁷	On-sky uncertainty [ ” ]⁷	Moon (abbrev.)⁸
Thrymr	630	retrogr.	14.3	14.5	7½	38.79 (?)	20.42	0.47	177.7	3.00	0.7	Thr
Narvi	631	retrogr.	14.4	15.5	7½	10.21	19.35	0.43	145.7	2.75	0.8	Nar
Aegir	636	retrogr.	15.5	15.8	4½	?	20.75	0.25	166.7	3.06	82.1	Aeg
Bebhionn	637	Gallic	15.0	14.0	5½	16.33	17.12	0.47	35.1	2.29	1.3	Beb
Bergelmir	638	retrogr.	15.2	15.9	5	8.13	19.34	0.14	158.6	2.75	5.3	Ber
Bestla	639	retrogr.	14.6	13.3	6½	14.624	20.21	0.51	145.1	2.98	8.6	Bes
Farbauti	640	retrogr.	15.7	16.3	4	?	20.39	0.24	156.5	2.98	15.5	Far
Fenrir	641	retrogr.	15.9	17.0	3½	?	22.45	0.13	165.0	3.45	10.7	Fen
Fornjot	642	retrogr.	14.9	16.4	6	9.5 ?	25.15	0.21	170.4	4.09	5.7	For
Hati	643	retrogr.	15.3	15.1	5	5.45	19.87	0.37	165.8	2.85	4.1	Hat
Hyrrokkin	644	retrogr.	14.3	13.8	7½	12.76	18.44	0.34	151.5	2.55	0.9	Hyr
Kari	645	retrogr.	14.8	14.8	6	7.70	22.09	0.48	156.1	3.37	2.8	Kar
Loge	646	retrogr.	15.3	16.1	5	6.9 ?	23.06	0.19	167.7	3.59	14.3	Log
Skoll	647	retrogr.	15.4	15.1	4½	7.26 (?)	17.67	0.46	161.0	2.40	11.3	Sko
Surtur	648	retrogr.	15.8	15.9	4	?	22.94	0.45	169.7	3.55	8.9	Sur
Jarnsaxa	650	retrogr.	15.6	16.1	4	?	19.35	0.22	163.6	2.76	23.0	Jar
Greip	651	retrogr.	15.4	15.4	4½	12.75 (?)	18.46	0.32	174.8	2.56	9.5	Gre
Tarqeq	652	Inuit	14.8	15.0	6	76.13	17.96	0.17	46.3	2.43	0.5	Taq

Moon	Spice ID¹	Group member²	Absolute magnitude H [mag]³	Best apparent magnitude in CSM [mag]⁴	Mean diameter [km]⁵	Rotation period [h]⁶	Semi-major axis [million km]⁷	Eccentricity [ ]⁷	Inclination [deg]⁷	Orbital period [a]⁷	On-sky uncertainty [ ” ]⁷	Moon (abbrev.)⁸
S/2004 S 7	65035	retrogr.	15.2	14.6	5	?	21.0	0.53	166	3.1	2171	4S7
S/2004 S 12	65040	retrogr.	15.7	15.7	4	?	19.89	0.33	165.3	2.86	441	4S12
S/2004 S 13	65041	retrogr.	15.6	15.7	4	?	18.4	0.26	169	2.6	3762	4S13
S/2004 S 17	65045	retrogr.	16.0	14.6	3½	?	19.4	0.18	168	2.8	2175	4S17
S/2006 S 1	65048	retrogr.	15.5	16.1	4½	?	18.8	0.14	156	2.6	1078	6S1
S/2006 S 3	65050	retrogr.	15.6	16.5	4	?	22.43	0.38	158.6	3.44	44.3	6S3
S/2007 S 2	65055	retrogr.	15.3	15.9	5	?	16.7	0.18	174	2.2	3740	7S2
S/2007 S 3	65056	retrogr.	15.8	16.1	4	?	18.9	0.19	178	2.7	2838	7S3

Table notes:

¹…SPICE is a commonly-used information system of NASA’s Navigation and Ancillary Information Facility (NAIF). It assists engineers in modeling, planning, and executing planetary-exploration missions, and supports observation interpretation for scientists. Each planet and moon obtained a unique SPICE number. The SPICE IDs of Saturn’s irregular moons are defined in this tpc-kernel.

²…A coarse subdivision of the irregular moons into three groups is dynamically justified. The objects in the prograde “Gallic group” have an inclination close to 34°, in the prograde “Inuit group” close to 47°. All other irregulars orbit Saturn at retrograde paths, with inclinations ranging from 145° to almost 180°. The reason that no satellites have inclinations closer to 90° is the Kozai mechanism which links inclination and excentricity of an orbiting object.

³…The absolute magnitude is the magnitude (brightness) of an object if located 1 au away from the sun and observed at 0° phase angle (i.e., the observer virtually sits at the center of the sun in this definition). Smaller numbers indicate brighter objects. The magnitude scale is logarithmic, with an object of 6^th mag being 100x darker than a 1^st mag object. For this table, data from MPEC were used for the absolute magnitudes (as of July 2017).

⁴…Best apparent magnitude of each irregular moon as seen from Cassini during the Cassini Solstice Mission. Smaller numbers again indicate brighter objects. The human eye can spot stars down to ∼6^th magnitude. This means that all irregular satellites would be invisible for a human at the location of the Cassini spacecraft. The Cassini Narrow-angle camera (ISS-NAC, a small 7.5-inch or ∼19-cm Cassegrain reflector telescope) can observe objects down to ∼18^th magnitude. As seen from Earth, the irregular moons of Saturn have apparent magnitudes of ∼20 to ∼25 and are only accessible to large telescopes. Only Phoebe (16 mag) is somewhat brighter.

⁵…The conversions from H to size were calculated through $D=1 \text{ au}\cdot \frac{2}{\sqrt{A}}\cdot 10^{−0.2·(H−M_☉)}$; with the solar apparent V magnitude M_☉ = −26.71 ± 0.02 mag and the Astronomical Unit 1 au = 149 597 870.7 km. All objects’ albedos were guessed to be ∼0.06. Due to the uncertain input values, the irregular-moon sizes determined this way may be uncertain to ∼20%.

⁶…Highlighted fields indicate entries from our Cassini work. The periods are taken from our chapter on Saturn’s irregular moons to be published in the book Enceladus and the Icy Moons of Saturn in October 2018.

⁷…Data from Table 3 in Jacobson et al. (2012). [deg] = degrees, [a] = years, [ ” ] = arcseconds. The on-sky uncertainty is the statistical value for the deviation of the object’s location compared to the calculated position as seen from Earth for the epoch three orbital periods beyond January 2012. According to Jacobson et al. (2012), the approximately eight moons with values >10″ are in danger of getting lost, except for those we could observe with Cassini. The six satellites with values >1000″ are effectively lost.

⁸…These abbreviations are my own and not official.

Fig.: **Irregular moons in the sky of Cassini** (projected in geocentric coordinates). Orbit paths of five moons as seen from Cassini in the time frame somewhere between 2010 and 2014. For each satellite, one orbit around Saturn is shown: Ymir 3.60 years, Tarvos 2.54, Kiviuq 1.23, Ijiraq 1.24, Siarnaq 2.45. The small wiggles are caused by the movement of the Cassini spacecraft around Saturn, the ticks are separated by 1 day. Retrograde moon Ymir moves to the right, the other (prograde) objects to the left. The yellow band illustrates the location of the Milky Way. In the time frame shown, the sun and the Earth are near right ascension 30° / declination +10°.

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