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Saturn’s Small Moons and Faint Rings : The View from Cassini

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. Outline.

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Saturn’s Small Moons and Faint Rings : The View from Cassini

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  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. Cassini-Huygens • 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

  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. Aegaeon Methone Daphnis Polydeuces Pallene Anthe

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

  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 Synchronous orbit - satellite’s orbital period equals planet’s spin period. 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. 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. This provides a mechanism by which satellite orbits may evolve into a state of resonance with another satellite.

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

  15. The Shadow of Epimetheus 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. Encke Gap 16 Jan 2009

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

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

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

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

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

  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. Enceladus Plumes of water vapour emerging from south polar region

  24. The New Moons Discovered by Cassini ISS (so far) 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 • Methone - 2004 June 1 • Pallene - 2004 June 1 • Polydeuces - 2004 October 21 • Daphnis - 2005 May 1 • Anthe - 2007 June 22 • Aegaeon - 2008 August 18

  25. Orbit Determination 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. 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 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. 3.5 km The Discovery of Polydeuces (S/2004 S 5)

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

  28. The Discovery of Anthe • 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 2007 16:07 NC completes first integration, detects resonance (ME 21 detections) • Tuesday, 26th June 2007 09:46 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)

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

  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)

  33. 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

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