Outer Moons of Saturn

The outer or irregular moons of Saturn were my primary research topic during the 2nd 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 many of the irregular moons of Saturn. 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, 25 rotation periods could be determined (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. 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 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 (long) 20.42 0.47 177.7 3.00 0.7 Thr
Narvi 631 retrogr. 14.4 15.5 10.21 19.35 0.43 145.7 2.75 0.8 Nar
Aegir 636 retrogr. 15.5 15.8 ? 20.75 0.25 166.7 3.06 82.1 Aeg
Bebhionn 637 Gallic 15.0 14.0 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 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 ? 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 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 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 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 ? 19.4 0.18 168 2.8 2175 4S17
S/2006 S 1 65048 retrogr. 15.5 16.1 ? 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.

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 6th mag being 100x darker than a 1st 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 ∼6th 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 ∼18th 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 a chapter to be published in the book Enceladus and the Icy Moons of Saturn in early 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°.

 


© Tilmann Denk (2017)