# Discovery of 128 New Saturnian Irregular Moons

Edward Ashton, Brett Gladman, Mike Alexandersen, Jean-Marc C. Petit

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Edward Ashton, Brett Gladman, Mike Alexandersen, Jean-Marc C. Petit. Discovery of 128 New Saturnian Irregular Moons. Research Notes of the AAS, 2025, 9 (3), pp.57. 10.3847/2515-5172/adbf87 . hal-05226681

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Discovery of 128 New Saturnian Irregular Moons

To cite this article: Edward Ashton *et al* 2025 *Res. Notes AAS* 9 57

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# Discovery of 128 new saturnian irregular moons

EDWARD ASHTON,<fnref n="1" /> BRETT GLADMAN,<fnref n="2" /> MIKE ALEXANDERSEN,<fnref n="3" /> AND JEAN-MARC PETIT<fnref n="4" />

## ABSTRACT

We highlight the announcement by the IAU Minor Planet Center of 128 new irregular moons of Saturn discovered in our CFHT imaging acquired in 2023. We were able to link 83 of the new moons to detections from our earlier 2019-2021 study. Of the 58 new retrograde moons with multiyear arcs, 46 are likely in the Mundilfari subgroup. This large fraction is in line with the steep size distribution of the subgroup and the idea that it was created by a recent collision (E. Ashton et al. in review). Also, by more than tripling the number of Kiviuq subgroup members, the new discoveries have confirmed that this subgroup is a very tightly grouped collisional family, although likely much older.

Keywords: irregular satellites — saturnian satellites — Saturn

## 1. OBSERVATIONS

In our 2019-2021 deep survey of the saturnian system we discovered 64 new moons that we tracked long enough for them to be officially recognised and announced by the Minor Planet Center (MPC). However, the number of new moons we confirmed was a minority of the total number of objects moving at Saturn-like rates that were detected. We could only label the remaining objects as moon candidates, as they had observational arcs too short to be confirmed. The large number of these candidates, along with the ongoing mysteries within the saturnian irregular moon population, motivated us to revisit the saturnian system in the second half of 2023.

We used the same technique of shifting and stacking 44 sequential images taken over 3 hours that was implemented in our previous survey (E. Ashton et al. 2021). The only change we made was to slightly shift the location of the fields relative to Saturn. We moved the east field 4.5' north and the west field 4.5' south to account for the shift in Saturn's orbital plane relative to Earth's equator. In addition we shifted 0.7' closer to Saturn in an attempt to increase the number of detected moons. Per field, we obtained one sequence in August, two in September, and $2(\text{west})/3(\text{east})$ in October. The Classical and Large-a Solar System (CLASSY) survey, a currently ongoing large program on CFHT that the authors are co-Is of, coincidentally acquired images in the vicinity of Saturn around the same time. The similar

observing strategy of CLASSY resulted in additional, and in a few cases critical, astrometric measurements of some of the new moons detected by our survey.

From the 2023 observations, 128 candidates were detected and then tracked for a long enough time to be confirmed and designated by the MPC as new moons of Saturn<fnref n="5" />. We were able to link 83 of the new moons to candidates that we found in the 2019-2021 data, of which 35 were further linked to scattered Subaru observations taken from 2004-2007.

## 2. ORBIT DISTRIBUTION

To accurately study the orbital distribution of an irregular moon population, time-averaged orbital elements are preferred. This is due to irregular moons oscillating over a moderate range of inclinations and eccentricities as a result of solar perturbations, while semimajor axes vary much less (D. Turrini et al. 2008; R. A. Jacobson et al. 2022). However, because of the amount of computation time needed, averaged orbital elements for the newly discovered moons are yet to be calculated. Although this clouds the knowledge of individual new moons, we can comment on general patterns of the distribution of the osculating orbits of the new moons. Because the new moons that were only observed

<footnote n="1">¹Institute of Astronomy and Astrophysics, Academia Sinica, No.1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan</footnote>

<footnote n="2">²Dept. of Physics and Astronomy, University of British Columbia, Vancouver BC, Canada</footnote>

<footnote n="3">³Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA</footnote>

<footnote n="4">⁴Université Marie et Louis Pasteur, CNRS, Institut UTINAM (UMR 6213), OSU THETA, F-25000 Besançon, France</footnote>

<footnote n="5">⁵ https://minorplanetcenter.net/mpec/K25/K25EF3.html, https://minorplanetcenter.net/mpec/K25/K25EF4.html, https://minorplanetcenter.net/mpec/K25/K25EF5.html</footnote>
<figure><img src="image_3.png" /></figure>

**Figure 1.** Rose diagram of the irregular moons of Saturn. For the 122 previously known irregular moons each circle represents the time-averaged inclination and semi-major axis of a moon, with the size of the circle indicating the relative size of the moon. Additionally, each moon has a line that goes from the time-averaged pericentre to the time-averaged apocentre. Color has been used to split six specific regions; the Gallic group (yellow), the Inuit group (green), the low-i (<151°) Norse members (cyan), Kari subgroup (blue), the Mundilfari subgroup (magenta), and the Phoebe subgroup (red). (E. Ashton et al. in review). The osculating orbits of the 83 new moons with multiyear arcs are indicated with black crosses.

over several months in 2023 have relatively uncertain orbits, here we will only examine the 83 new moons with multi-year arcs.

All new moons with direct orbits ($i < 90°$) appear to cluster around one of the three already-known subgroups: Kiviuq, Siarnaq, and Albiorix (T. Denk et al. 2018; E. Ashton et al. in review) (see Figure 1). Paaliaq, with it's moderate $a$ between the Kiviuq and Siarnaq subgroups, still has no small companions. This is certainly not a detection bias, because the Kiviuq subgroup with smaller semi-major axis has had a drastic increase in the number of members, with likely 12 of the new moons joining the 5 current members, tightly clustered around the orbit of Kiviuq. This large increase in the Kiviuq subgroup population, along with the highly elongated shape of Kiviuq (T. Denk et al. 2018), turns the subgroup from a potential candidate for a collisional family to a clearly confirmed one. The observational bias against finding low semi-major axes moons (see E. Ashton et al. (2022)) means that the Kiviuq subgroup is under-represented in the current sample of saturnian irregulars, and is likely larger than the present number. The large gap between S/2004 S24 and the rest of the Gallic group has not decreased with the new additions (S. S. Sheppard et al. 2023).

The vast majority (46/58) of the new moons in the retrograde Norse group appear to belong to the Mundilfari subgroup ($157° < i < 172.5°$). This is a higher

fraction compared to what is found with the previously discovered moons (63/100). With the new moons being, on average, smaller than the previously discovered moons, this difference is expected as the Mundilfari subgroup has a much steeper size distribution compared to the rest of the Norse group (E. Ashton et al. in review). One of the new Mundilfari subgroup members, S/2004 S56, has a significantly smaller semi-major axis compared to the rest of the subgroup. Although it has a semi-major axis similar to that of Phoebe, S/2004 S56 has an osculating inclination well below the range that Phoebe subgroup members have (R. A. Jacobson et al. 2022).

According to the osculating inclinations, there are 3 new moons with $i < 151°$ in the Norse group. The orbit of the new moon S/2020 S17 is about halfway between Skathi and Hyrrokin in semi-major axis. The other two, S/2020 S27 and S/2019 S37, have very similar orbits to the close pair of Narvi and S/2019 S11. The amplitude of inclination oscillations is larger the farther the averaged inclination is from $180°$. Thus, the averaged inclinations of the three new moons could easily be lower, close to that of the isolated Bestla ($i = 136.3°$), or higher, placing them in the Kari subgroup ($151° < i < 157°$).

In summary, the steady concentration of moons into collisional subgroups points to the idea that each of the giant planets initially captured a much smaller number
of moderately sized (tens of km) moons which have then
gone through a multi-generational collisional cascade (B.
Gladman et al. 2001; D. Jewitt & S. Sheppard 2005;
P. D. Nicholson et al. 2008; W. F. Bottke et al. 2010).
The large number of the smallest moons concentrating

into the Mundilfari subgroup supports the hypothesis
(E. Ashton et al. in review) that this is a relatively recent
(~ 100 Myr ago) catastrophic event that has not had
time to collisionally flatten back to a shallower steady-
state size distribution.

Facilities: CFHT

REFERENCES

Ashton, E., Gladman, B., Alexandersen, M., & Petit, J.-M.
in review, PSJ

Ashton, E., Gladman, B., & Beaudoin, M. 2021, PSJ, 2, 158

Ashton, E., Gladman, B., Beaudoin, M., Alexandersen, M., & Petit, J.-M. 2022, PSJ, 3, 107

Bottke, W. F., Nesvorný, D., Vokrouhlický, D., & Morbidelli, A. 2010, AJ, 139, 994

Denk, T., Mottola, S., Tosi, F., Bottke, W. F., & Hamilton, D. P. 2018, in Enceladus and the Icy Moons of Saturn, 409

Gladman, B., Kavelaars, J. J., Holman, M., et al. 2001,
Nature, 412, 163

Jacobson, R. A., Brozović, M., Mastroemos, N., Riedel, J. E., & Sheppard, S. S. 2022, AJ, 164, 240

Jewitt, D., & Sheppard, S. 2005, SSRv, 116, 441

Nicholson, P. D., Cuk, M., Sheppard, S. S., Nesvorny, D., & Johnson, T. V. 2008, in The Solar System Beyond Neptune, 411–424

Sheppard, S. S., Tholen, D. J., Alexandersen, M., & Trujillo, C. A. 2023, RNAAS, 7, 100

Turrini, D., Marzari, F., & Beust, H. 2008, MNRAS, 391, 1029