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Saturn’s Small Moons and Faint Rings : The View
from CassiniNick Cooper
(with thanks to Carl Murray, Kevin Beurle, Mike Evans, Gareth Williams and the Cassini Imaging Team)
Astronomy UnitSchool of Physics and AstronomyQueen Mary University of London
19th July 2011
Outline
•The Cassini Mission
•Resonance and Tidal Evolution
•The Inner Satellites and the F ring
•Faint Rings and their Origins
•Orbit Determination
•Satellite Discoveries
•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 instrumentsand 2 out of 6 Huygens instruments
Cassini-Huygens
Cassini - The Journey to Saturn
Cassini - The Prime Mission (2004-2008)
Cassini Equinox Mission (2008-2010)Cassini Solstice Mission (2010-
2017)
Where is Cassini Now ?
Go to http://saturn.jpl.nasa.gov/mission/presentposition/
Orbital Elements
a semi-major axis
e eccentricity
i inclination
longitude of ascending node
longitude of pericentre
mean longitude
Murray & Dermott 1999
Methone
Anthe
Aegaeon
PalleneDaphnis
Polydeuces
The Saturn System (as of 1997)
4:3
2:1
2:1
The Geometry of Resonance2:1 resonance, stable configuration:
2:1 resonance, unstable configuration:
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
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)
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.
Pan, Atlas and Epimetheus
20 Jan 2009
F ring
• Encke Gap
Keeler Gap
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.
Daphnis making waves in the Keeler Gap
Keeler Gap
‘Slow lane’
‘Fast lane’ Arrows show direction of
motion of ring particles
relative to Daphnis
WAC NAC
Prometheus
Cassini SOI images Voyager image of Prometheus and Pandora
Channels in the F
ring
Prometheus forming channels in
the F ring
F ring on 2007 March 31 (270 degrees of longitude)
+750 km
-750 km
0
Prometheus
Janus and Epimetheus : ‘horseshoe’ motion
The relative radial widths of the two horseshoes are related by:
Epimetheus
Epimetheus
Janus
180 km
Atlas Ring
Pallene Ringlet
QuickTime™ and aCinepak decompressor
are needed to see this picture.
Janus-Epimetheus Ring
G Ring
Faint Rings
The G Ring arc contains a moonlet - Aegaeon
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?
Enceladus and the E ring
23
Enceladus
Plumes of water vapour emerging from south polar
region
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
Orbit Determination
Orbit ModelsFixed 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.
QuickTime™ and aH.264 decompressor
are needed to see this picture.
The Discovery of Polydeuces (S/2004 S 5)
3.5 km
The Orbit of Polydeuces
X (km)
Dione
Helene
Polydeuces
Y (km)
• 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)
The Discovery of Anthe
2004 Oct 6
Anthe
Predicted positions of
satellites shown in green, stars in
blue.
Anthe and its neighbours
Orbital elements of Anthe over a 10
year period
The oscillations are caused by
the 11:10 resonance with
Mimas
Anthe (2007 Oct 29 12:07:14.3)
Anthe (2007 Oct 29 12:07:14.3)
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
Solar Eclipse
Pale Blue Dot
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