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