Icy Satellites: Geological Evolution and Surface Processes

Icy Satellites: Geological Evolution and Surface Processes Ralf Jaumann, Roger Clark, Francis Nimmo, Tilmann Denk, Amanda Hendrix, Bonnie Borutti, Jeff Moore, Paul Schenk, Ralf Srama

ISSI WS January 2010

Icy Satellites: Geological Evolution and Surface Processes Outline Introduction Cassini’s Exploration of Icy Satellites Morphology and Topography Tectonics and Cryovolcanism Surface Alterations Surface Composition Geological Evolution

Icy Satellites: Geological Evolution and Surface Processes

Introduction Pan Daphnis Atlas Prometheus Pandora Janus Epimetheus Mimas Methone Anthe Pallene (Enceladus) Thethys Telesto Calypso Thetys Polydeuces Helene Rhea (Titan) Hyperion Iapetus Phoebe + 37 small outer ones (total > 2km 61 Moons plus Titan)


Voyager Flugbahn

Introduction - knowledge before Cassini (Science special issue 1981,1982)

Icy satellites of Saturn: -  small: 2 km Anthe - 178 km Janus -  medium sized: 400 km Mimas - 1528 km Rhea -  low density: 1 -1.6 g/cm3 Impact craters are the dominant landforms (with large basins on Mimas and Tethys) Only little evidence for endogenic activitiy; Large tectonic structures are exposed only on Enceladus and Tethys Voyager 1: 12. 11. 1980 Voyager 2: 28. 08. 1981


Voyager Flugbahn

Introduction - knowledge before Cassini (Science special issue 1981,1982)

‘Wispy structures’ of unknown origin on Dione and Rhea Bright/dark dichotomy of Iapetus and ellipsoidal shape of the dark side Irregular shape of Hyperion - no proper rotational period E-ring correlated with Enceladus Resurfacing on Enceladus expected

Voyager 1: 12. 11. 1980 Voyager 2: 28. 08. 1981


Cassini’s Exploration of Icy Satellites Geological evaluation is based on 15 targeted and numerous distant fly-bys


Cassini’s Exploration of Icy Satellites Geological analysis is based on: ISS (0.3-1.1 µm; few m to 100m res.) morphology, topography (Porco et al., 2004) VIMS (0.4-5.1 µm; 100m to km) composition (Brown et al., 2004) CIRS (7-1000 µm; km resolution) thermal properties (Flasar et al., 2004) UVIS (56-190nm) composition (Esposito et al., 2004) magnetospheric, plasma particle size instruments (MAPS) for surface alterations Radio Science (RSS) for mass estimations Few RADAR observations


Morphology and Topography The overall morphology of the icy satellites is characterized by impact

Dione, ISS, 11/10/2005, 126-154m/pix


Morphology and Topography

Surface characteristics: • heavily cratered rugged surfaces (minor ejecta features) • some smooth plains (pancake (pedestal) ejecta on Dione) complex craters: • central peaks are up to 10 km high • floor uplift dominates over rim collapse (little evidence for terrace formation) (Schenk and Moore, 2007) 150 km

Dione, ISS, 24/12/2005, leading hemisphere, 904m/pix



Surface characteristics: • Scarps, troughs and ridges have extensions of several hundred km with heights of several km indicating crustal stress (e.g Giese et al., 2007) • ‘wispy streaks’ are tectonic features • equatorial ridge on Iapetus is up to 10 km high (e.g Giese et al., 2008)

Iapetus,ISS,10/09/2007 20 km

Tethys, ISS, 24/09/2005, 2.5°S 352°W, 190m/pix



Surface characteristics: •  spongy terrain on Hyperion •  impact in low density bodies

•  possible thermal erosion caused by warming of selectively accumulated dark material? 30 km

Hyperion, ISS, 26/09/2005, 197m/pix



Geological units: albedo and color variations indicate variations of morphological and compositional surface units

•  impact units (floor, rim, ejecta) •  tectonic units •  plain units •  resurfaced units

Rhea, ISS false color, 17/01/2007, 4km/pix

20 km

Tethys, ISS false color, 24/09/2005, 4.2°S 357°W 213m/pix


Tectonics and Cryovolcanism

Mimas: linear-arcuate sub-parallel troughs; related to Herschel or freeze expansion; no cryovolcanic evidence Enceladus: cross-cutting fractures and ridges; cryovolcanic activity; correlation of tectonic and volcanism (e.g.Porco et al., 2006; Nimmo et al., 2007, Jaumann et al., 2008, Spencer et al., 2009)

Mimas, ISS, Enceladus ISS, 02/08/2005, 09/03/2005, 400m/pix 90m/pix

150 km

Dione, ISS, 14/12/2004, 1km/pix


Tectonics and Cryovolcanism Enceladus: cross-cutting fractures and ridges; cryovolcanic activity; correlation of tectonic and volcanism (e.g.Porco et al., 2006; Nimmo et al., 2007, Jaumann et al., 2008, Spencer et al., 2009

150 km



Tethys ISS, 09/09/2006, 1km/pix

Tethys: global graben system Ithaca Chasma confined to a narrow zone of endogenic origin; freezing expansion? (e.g. Giese et al., 2007); Odysseus antipodal plain might be 150 km cryovolcanic (e.g. Moore and Schenk, 2007); Dione, ISS, 14/12/2004, 1km/pix


Tectonics and Cryovolcansim

Dione: global network of nonrandom troughs and ridges indicate extension and compression - due to despinning/ orbital recession or impact ? (e.g. Schenk 2007; Wagner et al., 2006); smooth, ridge bound, plains may originate from cryovolcanic processes (e.g.Moore and Schenk 2007)

Moore, 2004; Moore and

Dione, ISS, Rhea, ISS, Dione, ISS, 2km/pix 24/07/2006, 22/07/2007, 24/07/2006, 7km/pix 2km/pix

Rhea: young graben/extensional fault system trending N-S, possibly impact induced (Moore and Schenk 2007; Wagner et al., 2007) and old ridge system trending N-S; so far no surface evidence for cryovolcanic processes but faint dust ring 5 km Dione, ISS, 11/10/2005, 4.2°S 357°W 23m/pix

(Jones et al., 2008)



Iapetus: global equatorial ridge, abundantly cratered (e.g. Porco et al., 2005, Denk et al., 2008)

induced by despinning? upwraping by tectonic faulting (e.g. Giese et al., 2008)? ancient impact ring (e.g. Ip 2006)? No surface evidence for cryovolcanic processes so far.

Iapetus, ISS, 31/12/2004, 1km/pix

5 km

Iapetus, ISS, 10/09/2007, 23m/pix



5 km

Iapetus, ISS, 10/09/2007, 23m/pix


Surface Alterations

Chemical as well as structural changes of surface materials are due to: •  Charged particle bombardement (sputtering) and UV photolysis •  E-ring grain coating/bombardement and micrometeorit bombardement •  Thermal processing

Iapetus


Surface Alterations

Charged particle bombardement (sputtering) •  affects primarily the trailing hemispheres because Saturn’s magnetosphere rotates faster than the orbital speed of the moons •  although energy fluxes due to trapped plasma and solar UV are relatively low at Saturn, radiation effects occur •  decomposition of ice by sputtering and grain size alteration (e.g. Johnson et al., 2008, Newman et al., 2007)

Voyager Nov. 1980 artifical view

Cassini, MIMI, 21/06/04


Surface Alterations

E-ring grain coating/bombardement and micrometeorite bombardement •  tend to affect primarily the leading hemispheres which ‘scoops up’ incoming dust and exogenic material resulting in: •  gardening and impact excavation that expose fresh material and brighten the surface •  impact volatilization and subsequent escape of volatiles resulting in enrichment of opaque, dark material (Verbiscer et al., 2007, Hendrix, 2008)

E-Ring, ISS, 15/09/2006, 15° above the ring plain, 128km/pix


Surface Alterations

Dark material: •  The source of dark material is still unclear; outside the Saturnian system (Clark et al., 2005, 2007) or Titan impact induced (Owen et al., 2001) •  Dark material on Iapetus: - thin (several decimeter) (Ostro et al., 2006) - no breakup by craters -> relatively young - low degree of photometric Iapetus, ISS, roughness indicate 10/09/07, 55m/pix infilling of rough surface facets by µm-sized particles (Lee et al., 2008) Iapetus


Surface Alterations

Dark material: •  Thermal processing Runaway thermal segregation process whereby dark material becomes warm enough that volatiles are unstable and migrate to cold traps (e.g. Spencer, 2005; Hendrix and Hansen, 2008; Spencer and Denk, 2008; Hendrix et al., 2008)

Iapetus, ISS, 10/09/07,46m/pix

Iapetus, ISS, 10/09/07,36m/pix Iapetus


Surface Composition

Composition: The surfaces of the Saturnian satellites are mainly composed of water ice (McCord et al., 2006; Filaccione et al., 2007, 2008; Clark et al., 2005, 2007, 2008)

Visible characteristics: Filacchione et al.,2008 negative slope -> mainly Filacchione et al.,2008 water ice, minor darkening of materials flat spectrum -> some darkening of materials positive slope (reddening)-> considerable darkening of materials Tethys, ISS false color, 24/09/2005, 4.2°S 357°W 213m/pix

Cassini, VIMS, 09/03/2005

20 km


Surface Composition

Composition: • Dark material on Dione is a surface coating and external in origin, emplaced after the end of the heavy bombardment impacting the trailing side.(Clark et al., 2007, 2008)

• Dark material on Phoebe, Dione VIMS 1.8,2-micron Iapetus, Hyperion, Dione, ice depth map the F-ring, and Epimetheus have the same basic spectral characteristics (Clark et al., 2007, 2008, Stephan et al., 2009)

Dione, VIMS/ISS Map


Surface Composition

Composition: • Absorption at 2.42 µm is common and seems to be due to OH or H (Clark et al., 2005, 2008)

• Absorption at 4.25 µm is common and is due to CO2 (Clark et al., 2005, 2008)

Dione, VIMS/ISS Map


Surface Composition

Composition: • Trace sub-micron dark grains in ice produce Rayleigh scattering and a variable blue peak, explaining visible wavelength color differences observed in the Saturn system where all surfaces have similar dark material compositions (Clark et al., 2007, 2008)

Dione, VIMS/ISS Map

Rayleigh scattering is observed in laboratory samples.


Surface Composition

Composition: VIMS detects several compounds on Iapetus: (Clark et al., 2007, 2008)

–  water ice (visibly bright). –  CO2 (strongest signature of any surface in the Saturn system) and dominates the dark material.

–  CH (organics) in the form of polycyclic aromatic hydrocarbons (PAH), trace amounts.

–  Probably traces of ammonia (NH3), though controversial within the team due to calibration.

Iapetus, ISS, 31/12/2004


Surface Composition

Composition: VIMS detects several compounds on Iapetus: (Clark et al., 2007, 2008)

–  Several unknown absorptions for which we have no match to any compound in our spectral databases.

–  Rayleigh scattering is caused by dark particles embedded in the water ice causing a blue reflectance peak. - Particles must be less than 0.5 micron in diameter. - Particles must be less than about 2% by weight. Iapetus, ISS, 31/12/2004


Surface Composition

Dark material: –  The same absorption features are observed in dark material on Phoebe, Iapetus, Dione, and in Saturn’s rings and small satellites. –  This implies a common origin for the composition and the same material is pervasive throughout the Saturn system. –  Spatial patterns imply surface coatings. –  Composition, Rayleigh scattering, and surface coatings imply the origin is external to the moons, and may be external to the Saturn system. Iapetus, ISS, 31/12/2004


Geological Evolution

Surface ages: •  all satellites show heavily cratered surfaces dating back to the formation of the bodies.

•  fractured plains, faults and tectonized regions on Dione,Tethys, Rhea •  and Enceladus are significantly younger

•  bright rays on Rhea even younger

•  tectonized regions on Enceladus have been active over long periods

•  recent cryovolcanic activities on Enceladus and Dione (?) (Zahnle et al., 2003; Neukum et al., 2005, Clark et al., 2007; Jaumann et al., 2008) Iapetus, ISS, 31/12/2004 860m/pix



Evolution: •  Satellite formation and heavy bombardement. •  Formation of Iapetus’ equatorial ‘bulge’ by despinning (?), tectonic faulting (?) ISS, •  formation of old troughs, faults Iapetus, 31/12/2004, 1km/pix Enceladus Dione and ridges due to crustal stress induced by impact, freezing and/or despinning/orbit recession

•  formation of younger troughs faults and ridges due to crustal stress induced by impact or cryovolcanism Dione, VIMS/ISS Map



Evolution: •  surface contamination by dark materials •  surface alteration by micrometorite bombardement, charged particles, ring paricles and thermal processing •  recent cryovolcanic activities on Enceladus and Dione (?)

Dione, VIMS/ISS Map

Michael Michael Carroll Carroll
