Icy Satellites: Geological Evolution and Surface Processes

Transcription

1 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

2 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

3 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)

4 Voyager Flugbahn Introduction - knowledge before Cassini (Science special issue 1981,1982) Icy satellites of Saturn: - small: 2 km Anthe km Janus - medium sized: 400 km Mimas km Rhea - low density: g/cm 3 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: Voyager 2:

5 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: Voyager 2:

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

7 Geological analysis is based on: Cassini s Exploration of Icy Satellites ISS ( µm; few m to 100m res.) morphology, topography (Porco et al., 2004) VIMS ( µm; 100m to km) composition (Brown et al., 2004) CIRS ( 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

8 Morphology and Topography The overall morphology of the icy satellites is characterized by impact Dione, ISS, 11/10/2005, m/pix

9 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) Dione, ISS, 24/12/2005, leading hemisphere, 904m/pix 150 km

10 Morphology and Topography 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 Tethys, ISS, 24/09/2005, 2.5 S 352 W, 190m/pix 20 km

11 Morphology and Topography 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

12 Morphology and Topography 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

13 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, Enceladus ISS, ISS, 02/08/2005, 09/03/2005, 400m/pix 90m/pix Dione, ISS, 14/12/2004, 1km/pix 150 km

14 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., km

15 Tectonics and Cryovolcanism 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 cryovolcanic (e.g. Moore and Schenk, 2007); Dione, ISS, 14/12/2004, 1km/pix 150 km

16 Tectonics and Cryovolcansim Dione: global network of nonrandom troughs and ridges indicate extension and compression - due to despinning/ orbital recession or impact? (e.g. Moore, 2004; Moore and Schenk 2007; Wagner et al., 2006); smooth, ridge bound, plains may originate from cryovolcanic processes (e.g.moore and Schenk 2007) Dione, ISS, Dione, ISS, 11/10/2005, 4.2 S 357 W 23m/pix 24/07/2006, Dione, Rhea, ISS, 2km/pix 24/07/2006, 22/07/2007, 2km/pix 7km/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 (Jones et al., 2008)

17 Tectonics and Cryovolcanism 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

18 Tectonics and Cryovolcanism 5 km Iapetus, ISS, 10/09/2007, 23m/pix

19 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

20 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) Cassini, MIMI, 21/06/04 Voyager Nov artifical view

21 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

22 Dark material: Surface Alterations 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 roughness indicate infilling of rough surface facets by µm-sized particles (Lee et al., 2008) Iapetus, ISS, 10/09/07, 55m/pix Iapetus

23 Dark material: Icy Satellites: Geological Evolution and Surface Processes Thermal processing Surface Alterations 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

24 Composition: Icy Satellites: Geological Evolution and Surface Processes 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) Surface Composition Visible characteristics: negative slope -> mainly water ice, minor darkening of materials Filacchione et al.,2008 Filacchione et al.,2008 flat spectrum -> some darkening of materials positive slope (reddening)-> considerable darkening of materials Cassini, VIMS, 09/03/ km Tethys, ISS false color, 24/09/2005, 4.2 S 357 W 213m/pix

25 Composition: Icy Satellites: Geological Evolution and Surface Processes 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., Surface Composition 2007, 2008) Dark material on Phoebe, Iapetus, Hyperion, Dione, the F-ring, and Epimetheus have the same basic spectral characteristics (Clark et al., 2007, 2008, Stephan et al., 2009) Dione VIMS 1.8,2-micron ice depth map Dione, VIMS/ISS Map

26 Composition: Surface 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 CO 2 (Clark et al., 2005, 2008) Dione, VIMS/ISS Map

27 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) Rayleigh scattering is observed in laboratory samples. Dione, VIMS/ISS Map

28 Composition: VIMS detects several compounds on Iapetus: (Clark et al., 2007, 2008) Icy Satellites: Geological Evolution and Surface Processes water ice (visibly bright). CO 2 (strongest signature of any surface in the Saturn system) and dominates the dark material. Surface Composition CH (organics) in the form of polycyclic aromatic hydrocarbons (PAH), trace amounts. Probably traces of ammonia (NH 3 ), though controversial within the team due to calibration. Iapetus, ISS, 31/12/2004

29 Composition: VIMS detects several compounds on Iapetus: (Clark et al., 2007, 2008) Icy Satellites: Geological Evolution and Surface Processes Several unknown absorptions for which we have no match to any compound in our spectral databases. Surface Composition 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

30 Iapetus, ISS, 31/12/2004 Icy Satellites: Geological Evolution and Surface Processes 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.

31 Surface ages: Geological Evolution 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/ m/pix

32 Evolution: Icy Satellites: Geological Evolution and Surface Processes Geological Evolution Satellite formation and heavy bombardement. Formation of Iapetus equatorial bulge by despinning (?), tectonic faulting (?) formation of old troughs, faults and ridges due to crustal stress induced by impact, freezing and/or despinning/orbit recession Iapetus, ISS, 31/12/2004, Enceladus Dione 1km/pix formation of younger troughs faults and ridges due to crustal stress induced by impact or cryovolcanism Dione, VIMS/ISS Map

33 Evolution: Geological 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 Carroll

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