Geophysics of Icy Saturnian Satellites Torrence V. Johnson Jet Propulsion Laboratory, Caltech

Bulk Density and Composition P c ~ 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. 26 Al)– 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, , 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 C 0 (60 Fe /56 Fe) = Al only

Evidence for Early Planet Formation and Presence of SLRI “A stellar prodigy has been spotted about 450 light-years away in a system called UX Tau A by NASA's Spitzer Space Telescope. Astronomers suspect this system's central Sun-like star, which is just one million years old, may already be surrounded by young planets. “A stellar prodigy has been spotted about 450 light-years away in a system called UX Tau A by NASA's Spitzer Space Telescope. Astronomers suspect this system's central Sun-like star, which is just one million years old, may already be surrounded by young planets. “ Spitzer Science Center release 11/28/ Million Year Old Planets?! KBO densities from 1000 to 2600 kg m -3 suggest differentiated proto-KBO’s and collisional production of ice- and rock- rich bodies. Heating from SLRI would facilitate early differentiation of small ice-rock bodies.

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

Iapetus forms

Eucrites Angrites Phosphates Chondrules CAIs

Eucrites Angrites Phosphates Chondrules CAIs Iapetus SN shock

Iapetus Earth/Moon Iapetus Retains Large Basins

Iapetus Earth/Moon Iapetus Retains Large Basins Iapetus Despinning LHB - 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 from Chapman and McKinnon in Satellites, 1986 Conclusion: Iapetus may have despun several times during the course of the LHB. Trailing side basins 200 km Large Basins on Iapetus – LHB?