1. Saturn’s Small Moons and Faint Rings : The View from Cassini Nick Cooper (with thanks to Carl Murray, Kevin Beurle, Mike Evans, Gareth Williams and the Cassini Imaging Team) Astronomy Unit School of Physics and Astronomy Queen Mary University of London 19th July 2011

2. Outline The Cassini Mission Resonance and Tidal Evolution The Inner Satellites and the F ring Faint Rings and their Origins Orbit Determination Satellite Discoveries

3. Joint NASA/ESA/ASI mission Announcement of Opportunity in October 1989 Instrument and team selection in November 1990 Launch in October 1997 Arrived at Saturn in July 2004 to begin a 4-year tour of the Saturn system UK involvement in Cassini- Huygens funded by SERC/PPARC/STFC UK involvement in 6 out of 12 Cassini instruments and 2 out of 6 Huygens instruments Cassini-Huygens

4. Cassini - The Journey to Saturn

5. Cassini - The Prime Mission (2004-2008) Cassini Equinox Mission (2008-2010) Cassini Solstice Mission (2010- 2017)

6. Where is Cassini Now ? Go to http://saturn.jpl.nasa.gov/mission/presentposition/

7. Orbital Elements a semi-major axis e eccentricity i inclination  longitude of ascending node  longitude of pericentre mean longitude Murray & Dermott 1999

8. Methone Anthe Aegaeon Pallene Daphnis Polydeuces

9. The Saturn System (as of 1997) 4:3 2:1

10. The Geometry of Resonance 2:1 resonance, stable configuration: 2:1 resonance, unstable configuration:

11. Examples of Resonance in the Saturn System Janus : Epimetheus (co-orbital) Dione : Helene : Polydeuces (co-orbital) Tethys : Telesto : Calypso (co-orbital) Mimas : Tethys (4:2) Titan : Hyperion (4:3) Enceladus : Dione (2:1) Mimas : Anthe (10:11), Mimas : Methone (14:15), Mimas : Aegaeon (7:6) Ring Features (gaps, edge waves, density waves) ‘Synchronous rotation’ (most regular satellites, except Hyperion). Hyperion

12. Other Selected Examples of Resonance in the Solar System Moon (1:1 spin/orbit ‘synchronous rotation’) Mercury (3:2 spin/orbit) Pluto : Charon (1:1:1 spin/spin/orbit or ‘double synchronous’) Three of the Galilean Satellites of Jupiter (Laplace Resonance) Neptune : Pluto (3:2) Asteroid Belt (Kirkwood Gaps)

13. Tidal Evolution of Orbits Outside the synchronous orbit (shown dashed), the satellite is orbiting slower than the planet is spinning and the tidal bulge is carried ahead of the satellite- planet line due to tidal dissipation. A net torque results, slowing the spin of the planet and expanding the orbit of the satellite, increasing its orbital period. This provides a mechanism by which satellite orbits may evolve into a state of resonance with another satellite. Ignoring other effects, the Earth’s spin would eventually slow to ~ 48 days and equal the orbital and spin periods of the Moon. Earth and Moon will then show the same face to each other (like Pluto and Charon). Solar tides complicate this picture. Synchronous orbit - satellite’s orbital period equals planet’s spin period.

14. Pan, Atlas and Epimetheus 20 Jan 2009 F ring Encke Gap Keeler Gap

15. The Shadow of Epimetheus Encke Gap 16 Jan 2009 Saturn takes ~29 years to orbit the Sun. On 11 August 2009, the Sun will cross Saturn’s ring plane, heading north, marking the start of spring in the northern hemisphere. Shadows on the ring plane signal the approach of the equinox.

16. Daphnis making waves in the Keeler Gap Keeler Gap ‘Slow lane’ ‘Fast lane’ Arrows show direction of motion of ring particles relative to Daphnis

17. WACNAC Prometheus Cassini SOI images Voyager image of Prometheus and Pandora Channels in the F ring

18. Prometheus forming channels in the F ring F ring on 2007 March 31 (270 degrees of longitude) +750 km -750 km 0 Prometheus

19. Janus and Epimetheus : ‘horseshoe’ motion The relative radial widths of the two horseshoes are related by: Epimetheus Janus 180 km

20. Atlas Ring Pallene Ringlet Janus-Epimetheus Ring G Ring Faint Rings The G Ring arc contains a moonlet - Aegaeon

21. Origin of Faint Rings Accretion process? - Satellite is accreting from ring material Collisional debris? - Ring/arc is result of meteoroid impacts - Satellite is big enough target but too small to retain ejecta - Ring/arc material is trapped in same resonance as the satellite; starts to fill the resonant lobe Pallene, Methone and Anthe are remnants of breakup of larger body?

22. Enceladus and the E ring

23. 23 Enceladus Plumes of water vapour emerging from south polar region

24. The New Moons Discovered by Cassini ISS (so far) Methone - 2004 June 1 Pallene - 2004 June 1 Polydeuces - 2004 October 21 Daphnis - 2005 May 1 Anthe - 2007 June 22 Aegaeon - 2008 August 18 Source of Methone arc Source of Pallene ring Co-orbital with Dione Inside Keeler gap in A ring Source of Anthe arc Source of G ring arc

25. Orbit Determination Orbit Models Fixed Ellipse 2-body Point-masses Orbital elements Precessing Ellipse 2-body Oblate primary Orbital elements Full Equations of Motion n-body Oblate primary Position/velocity Choose an appropriate mathematical model for the orbit. The model is defined by a set of parameters. The numerical values of the model parameters are initially unkown. Use the model to estimate the observed quantities. Solve for the model parameter values that generate a satisfactory match between the estimated and actual observations.

26. The Discovery of Polydeuces (S/2004 S 5) 3.5 km

27. The Orbit of Polydeuces X (km) Dione Helene Polydeuces Y (km)

28. Monday, 18th June 2007 KB delivers S34 design (for September obs.) Friday, 22nd June 2007 17:46 CM sends email re: spotting ‘Frank’ (Anthe) - 19:06 NC first precessing ellipse orbit Sunday, 24th June 200716:07 NC completes first integration, detects resonance (ME 21 detections) Tuesday, 26th June 200709:46 CM announces 2004 trail detection - 12:19 NC updates orbit - 18:35 KB delivers revised S34 design to CICLOPS - 23:00 JPL deadline for final S34 designs Thursday, 28th June 2007 02:42 Cassini closest approach to date to Anthe (32,208 km) The Discovery of Anthe

29. 2004 Oct 6 Anth e Predicted positions of satellites shown in green, stars in blue.

30. Anthe and its neighbours

31. Orbital elements of Anthe over a 10 year period The oscillations are caused by the 11:10 resonance with Mimas

32. Anthe (2007 Oct 29 12:07:14.3)

34. The observed shifts in the Anthe’s location relative to the arc are consistent with its librations around the stable point of the resonance. Hedman DPS 2008

35. Solar Eclipse

36. Pale Blue Dot

37. Cassini Websites JPL Cassini homepage: http://saturn.jpl.nasa.gov ISS Team homepage: http://www.ciclops.org Publicly available jpegs of all images (usually everything up to the previous day) : http://saturn.jpl.nasa.gov/photos/raw The full science-quality Cassini images (launch through September 2008) http://pds-rings.seti.org/cassini/iss