Cassini: An update on the small satellites


Bonnie J. Buratti Senior Research Scientist Jet Propulsion Lab/Caltech Cassini Science Team

November 18, 2009 SBAG Meeting Boulder, CO The Small Saturnian in Context

Hyperion and both had targeted encounters · Cassini Satellite Scientific goals (from AO) a) Determine the general characteristics and geological histories of the satellites. b) Define the mechanisms of crustal and surface modifications, both external and internal. c) Investigate the compositions and distributions of surface materials, particularly dark, organic rich materials and low melting point condensed volatiles. d) Constrain models of the satellites’ bulk compositions and internal structures. e) Investigate interactions with the magnetosphere and ring systems and possible gas injections into the magnetosphere. f) Investigate interrelation of rings and satellites, including embedded satellites. Summary of Primary Observations during Prime Mission (2004-2008): Inner Satellites (VIMS and ISS)

Object Phase angle (º) Longitude Distance (not comprehensive) 61-130 L, T 30,000- 254,000 28, 62-63 T 669,000; 37,000 12-88 T 10,400- 60,000 89-91 L 42,000 23 T 52,000- 468,000 53-61 L 101,000

Observations by ISS only were also obtained for , , , Polydeuces, , , , and (spectrum from latter should be possible with existing data) Polydeuces and Helene ( Lagrangians)

Tethys Lagrangians

Daphnis in Keeler Gap

32 km


(Anthe) Anthe and Methone with arcs

F-ring Shepherds

Main ring Pan (Encke’s gap) Prometheus and F-ring shepherd Epimetheus The Small Saturnian Satellites: Examples of VIMS Spectra

Conclusion; The inner small satellites are coated with main ring particles, while the “outer” ones (the Lagrangians) are determined by the E-ring (Buratti et al., 2009, in press). Solar Phase Curves

Similar to the phase curves of the main icy satellites Outer Small Irregular Satellites

“Discovery of 12 Satellites of exhibiting orbital clustering”, Gladman et al., Nature 412, 163 (2001). Outer Irregular Satellites, Cont’d.

Although observations of the outer irregular satellite wasn’t part of the original mission plan, many observations of Albiorix, Erriapo, , , Mundilfari, Paaliaq, Siarnaq, Skadi, Tarvos, and were obtained (about 350 of Ymir alone). These observations are mainly good for astrometry. A future project might be to image the starfields with a groundbased telescope and tie them into Landolts. Ymir The Extended Mission

Object Time Comments

Telesto August 27, 2009 Prometheus Dec. 27, 2009 Satellite + rings Helene March 3, 2010 1800 km; good range in phase angles; best ever! Janus April 7, 2010

Pallene Oct. 16, 2010 36,000 km The Extended, Extended Mission (The XXM or Cassini Solstice Mission (CSM))

Due to the lower amount of Year Requests funding, we will not be able to do as many observations. 2011 Helene, Palene, Janus In addition, small satellites are Priority 3 (low) in relative 2012 Methone, Telesto scientific value. However, Cassini satellites scientists 2015 Polydeuces, have requested several high Epimetheus+Atlas value observations and we mutual event, may get some. Most Prometheus observations are 10-100K 2016 Daphnis, Pandora, close approaches Epimetheus, Prometheus

2017 Pandora, Janus, Atlas, Pan In plan

Must have Summary

Best ever New VIMS Rings Spectrum Object distance, <90º phase Future best ever ? Object? Atlas 42,000 21,000 Dec 6, 2015 Y Calypso 100,000 22,500 Sept 23, 20101 Daphnis 325,000 Y Epimetheus 37,000 2600 Dec 6, 2015 Helene 38,000 1800 km March 3, 2010 Yes Janus 96,000 (30K@124º) 8700 April 2, 2017 Methone 224,000 1900 km May 20, 2012 Yes Y Pallene 73,000 36,000 km Oct. 16, 2010 Yes2 Pan 52,000 25,000 March 7, 2017 Yes? Y Pandora 52,000 Y Polydeuces 64,000 Prometheus 437,000 21,000 Dec 6, 2015 Yes Y Telesto 10,000 11,000 May 20, 2012 1Need to check we really got; 2May already have data to get spectrum Methone, discovered by Cassini, with a ring arc, will be visited in 2012 at a distance of 1900 km Conclusions

• High resolution images and spectra of the small Saturnian satellites have been obtained by the Cassini spacecraft • The inner satellites are coated with particles from the main ring of Saturn, while those associated with the main satellites are coated with the E-ring. This makes it impossible to determine the provenance of the satellites • Many images, some in color, of the outer irregular satellites are in the PDS • The extended, and extended-extended mission could yield more observations to fill in gaps in longitude and solar phase coverage, and to image the smallest satellites better Janus and Prometheus against the rings