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 wissenschaftliches Thema 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 can be derived.

I am doing observations of the irregular moons of Saturn with Cassini since 2007. 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. I am doing, so to speak, astronomy with the spacecraft. In the meantime, many rotation periods have been derived (see Table 1), and more are to come. A shape model has been calculated for Ymir so far. It reveals that Ymir is a relatively flat, triangularly-shaped body. I presented these results for the first time at the DPS-2012 conference; this blog from the planetary society describes a few results. Examples of orbits of five of these objects as seen from Cassini are shown in the figure below the table.

The only outer moon of Saturn where high-resolution images exist is Phoebe. During Saturn approach 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.

Fig. 1. Light curve and synodic rotational period of Albiorix.

 

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.)
Phoebe 609 retrogr. 6.6 6.1 213.0 9.274 12.94 0.163 175.77 1.50 0.1 Pho
Ymir 619 retrogr. 12.3 12.9 19 11.9222 23.13 0.334 173.50 3.60 0.5 Ymi
Paaliaq 620 Inuit 11.7 10.4 25 18.79 15.20 0.333 46.23 1.88 0.3 Paa
Tarvos 621 Gallic 12.9 12.7 15 10.69 18.24 0.538 33.73 2.54 0.7 Tar
Ijiraq 622 Inuit 13.2 11.8 13 13.03 11.41 0.272 47.48 1.24 0.6 Iji
Suttungr 623 retrogr. 14.5 15.0 7 7.67 19.47 0.114 175.81 2.78 0.6 Sut
Kiviuq 624 Inuit 12.6 12.0 17 21.97 11.38 0.332 46.77 1.23 0.6 Kiv
Mundilfari 625 retrogr. 14.5 15.2 7 6.74 18.65 0.210 167.45 2.61 0.8 Mun
Albiorix 626 Gallic 11.1 9.5 33 13.33 16.39 0.480 34.06 2.14 0.3 Alb
Skathi 627 retrogr. 14.3 14.9 11.10 15.64 0.272 152.63 1.99 0.7 Ska
Erriapus 628 Gallic 13.7 13.5 10 28.15 17.60 0.472 34.48 2.38 1.4 Err
Siarnaq 629 Inuit 10.6 10.5 42 10.1879 18.18 0.280 45.81 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.)
Thrymr 630 retrogr. 14.3 14.5 35.24 ? 20.42 0.466 177.66 2.99 0.7 Thr
Narvi 631 retrogr. 14.4 15.5 10.21 19.35 0.429 145.73 2.75 0.8 Nar
Aegir 636 retrogr. 15.5 15.8 ? 20.75 0.252 166.67 3.06 82.1 Aeg
Bebhionn 637 Gallic 15.0 14.0 16.33 17.12 0.468 35.10 2.29 1.3 Beb
Bergelmir 638 retrogr. 15.2 15.9 5 8.13 19.34 0.142 158.56 2.75 5.3 Ber
Bestla 639 retrogr. 14.6 13.3 14.624 20.14 0.520 145.16 2.98 8.6 Bes
Farbauti 640 retrogr. 15.7 16.3 4 ? 20.39 0.241 156.52 2.98 15.5 Far
Fenrir 641 retrogr. 15.9 17.0 ? 22.45 0.135 164.96 3.45 10.7 Fen
Fornjot 642 retrogr. 14.9 16.4 6 9.5 ? 25.15 0.208 170.37 4.09 5.7 For
Hati 643 retrogr. 15.3 15.1 5 5.45 19.87 0.371 165.81 2.85 4.1 Hat
Hyrrokkin 644 retrogr. 14.3 13.8 12.76 18.44 0.336 151.54 2.55 0.9 Hyr
Kari 645 retrogr. 14.8 14.8 6 7.70 22.09 0.476 156.07 3.37 2.8 Kar
Loge 646 retrogr. 15.3 16.1 5 6.9 ? 23.06 0.186 167.69 3.59 14.3 Log
Skoll 647 retrogr. 15.4 15.1 7.26 (?) 17.67 0.464 161.01 2.40 11.3 Sko
Surtur 648 retrogr. 15.8 15.9 4 ? 22.94 0.446 169.69 3.55 8.9 Sur
Jarnsaxa 650 retrogr. 15.6 16.1 4 ? 19.35 0.218 163.65 2.76 23.0 Jar
Greip 651 retrogr. 15.4 15.3 12.75 (?) 18.46 0.315 174.80 2.57 9.5 Gre
Tarqeq 652 Inuit 14.8 15.0 6 76.13 17.96 0.168 46.29 2.42 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.)
S/2004 S 7 65035 retrogr. 15.2 14.6 5 ? 21.00 0.529 165.69 3.12 2171 4S7
S/2004 S 12 65040 retrogr. 15.7 15.7 4 ? 19.89 0.327 165.26 2.86 441 4S12
S/2004 S 13 65041 retrogr. 15.6 15.7 4 ? 18.41 0.259 168.80 2.56 3762 4S13
S/2004 S 17 65045 retrogr. 16.0 14.6 ? 19.45 0.180 168.24 2.78 2175 4S17
S/2006 S 1 65048 retrogr. 15.5 16.1 ? 18.78 0.141 156.18 2.63 1078 6S1
S/2006 S 3 65050 retrogr. 15.6 16.5 4 ? 22.43 0.379 158.63 3.44 44.3 6S3
S/2007 S 2 65055 retrogr. 15.3 15.9 5 ? 16.72 0.179 174.06 2.21 3740 7S2
S/2007 S 3 65056 retrogr. 15.8 16.1 4 ? 18.94 0.185 177.59 2.68 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 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. Data from MPEC were used here for the absolute magnitudes (as of Nov 2013).

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 (16.4 mag) is somewhat brighter.

5…The conversion from absolute magnitude H to size was calculated with this formula. The object’s albedos were assumed to be ∼0.06.

6…Highlighted fields indicate entries from our Cassini work, the numbers are synodic periods taken from Denk and Mottola (2016).
A short description on Ymir’s rotation, axis, and shape is given in Denk und Mottola (2013a).

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, the six satellites with values >1000″ are effectively lost.

Fig. 2: Irregular moons in the sky of Cassini. 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)