Geophysics of Icy Saturnian Geophysics of Icy Saturnian SatellitesSatellites

Torrence V. JohnsonTorrence V. Johnson

Jet Propulsion Laboratory, CaltechJet Propulsion Laboratory, Caltech

Bulk Density and Composition

Pc ~ 1 – 10 MPa

Satellite Densities

Effects of Solar Abundance of O and C

Effects of amount of C in CO

Wong et al. in Oxygen in the Solar System, 2008

Satellite Densities

Effects of amount of C in solids

Mass Fraction

Water Ice

Rock + Metal

Solid Carbon

Wong et al. in Oxygen in the Solar System, 2008

Conclusions

• Collisional fractionation – i.e. removing icy layers from solar composition, differentiated planetesimals – is one way to explain the range of densities seen, analogous to production of meteorites from differentiated parent bodies.

• This probably requires very early formation of relatively large parent objects with live short lived radionuclides (e.g. 26Al)– within the first 1-2 Myr after the earliest solids (CAIs)

The “Nice” Model and LHB

• The ‘Nice’ model of outer solar system evolution explains several important aspects of the current dynamical state of the outer solar system (e.g. Morbidelli et al., 2005).

• In this context, it has been suggested that the passage of Jupiter and Saturn through their 2:1 orbital resonance can produce the 3.9 Ga Late Heavy Bombardment and that this should have been a solar-system wide event (Gomes et al. Nature 435, 466-469, 2005).

Satellite Thermal Histories and Short Lived Radioactive Isotopes• Castillo-Rogez et al. (2007) have

suggested that early heating from SLRI is required to explain the two key aspects of the thermal and dynamical evolution of Saturn’s satellite, Iapetus:– Synchronous rotation state– Retention of a large ‘fossil’ rotation bulge

Time of formation constrained by required heat for successful models

Heat required for successful models

C0(60Fe/56Fe) = 10-6

26Al only

Speculations on Chronology

• Iapetus is perhaps the most heavily cratered object in the solar system, with multiple large basins

• Our thermal models require ~ 100 My to produce a thick lithosphere capable of sustaining large basins

• The equatorial ridge may be associated with despinning 200 to 900 My after formation – some basins post-date ridge

• Can we tie Iapetus’ cratering record to the inner solar system and the Nice model for LHB?

McKeegan and Davis, Treatise on Geochemistry, Vol 1, 2006 ed

McKeegan and Davis, Treatise on Geochemistry, Vol 1, 2006 ed

Iapetus forms

Solar System Chronology(simplified, after McKeegan and Davis, 2006)

455045554560456545704575

Pb-Pb Age, Ma

Eucrites

Angrites

Phosphates

Chondrules

CAIs

Iapetus

SN shock

Solar System Chronology(simplified, after McKeegan and Davis, 2006)

4400445045004550

Pb-Pb Age, Ma

Iapetus

Earth/Moon Iapetus Retains Large Basins

Solar System Chronology(simplified, after McKeegan and Davis, 2006)

38003900400041004200430044004500

Pb-Pb Age, Ma

Iapetus

Earth/Moon

Iapetus Retains Large Basins

Iapetus DespinningLHB - Moon

Conclusions

• Early formation of the Saturn system (2.5 – 5 Myr after CAI’s) and Iapetus’ thermal history are consistent with the large basins seen on Iapetus being part of the proposed solar-system-wide LHB at 3.9 Ga.

• Other implications include possible disruption and re-accretion of Saturn’s innermost satellites during the LHB

Leading side topography from Giese et al., Icarus, 193, 2008

Crater size range required to break synchronous rotation fromChapman and McKinnon in Satellites, 1986

Conclusion:Iapetus may have despunseveral times during the course of the LHB.

Trailing side basins

200 km

Large Basins on Iapetus – LHB?