Origin and Early Evolution of a Partially Differentiated Titan

1. Origin and Early Evolution of a Partially Differentiated Titan • Amy C. Barr1, Robert I. Citron2, Robin M. Canup1 • 1Southwest Research Institute Boulder CO • 2University of Colorado, Boulder CO

2. Gas Giant Satellite Formation solar nebula planet formation initial state satellite systems form in disk around parent planet during late stages of giant planet growth, or just after. t=0 CAI formation (first solids) solar nebula dispersal t=? • Entire process lasts ~ 10 Myr, until dispersal of solar nebula • Disks around other stars last 1 - 10 Myr (Haisch et al., 2001) • Timetable suggests 26Al, 60Fe heat during formation • Planetocentric impactors with v < 3 km/s (melting, but no vaporization) • Callisto’s undifferentiated state implies tend > 4 Myr (Barr & Canup 2008) (Canup & Ward 2002; Canup & Ward 2006; McKinnon 2006; Barr & Canup 2008; Canup & Ward 2009)

3. By integrating to assume that all 26 Al heat is retained • Compare T(r) to melting temperature to determine whether the satellite melts during formation All impact energy deposited close to the surface Formation of Titan in a Gas-Starved Disk Titan forms from mixed ice/rock impactors ~ 50 to 100 meters After a layer accretes, its temperature increases due to radiogenic heating: Barr & Canup 2008; Barr, Citron and Canup (2010)

4. Timetable for Formation of an Unmelted Titan • A longer and later accretion is required if: • Large satellitesimals • Gas-rich environment • nebular gas capture • Gas densities lower than MMSN but we can’t rule this out • accretional degassing • impact velocities too low for ice vaporization • Satellitesimals that form Titan must themselves be undifferentiated (mixtures of ice and small rock particles) • Similar timetable as Callisto, NH3 has modest effect Barr, Citron and Canup (2010)

5. What Happens to Titan During the Late Heavy Bombardment?The Nice Model for Outer Solar System Evolution (Levison et al., 2001; Tsiganis et al., 2005; Gomes et al., 2005) Outer planets formed in compact configuration and closer to the sun Migrate outward due to interactions with massive disk of icy planetesimals At 700 Myr, Saturn & Jupiter cross 2:1 mean motion resonance and planetesimal disk destabilizes For every 1 gram that hits the Moon 35 g hits Titan at 10.5 km/s If 20 MEarth of comets remains in the disk at 700 Myr, outer planets end up on correct orbits and mass delivered to Moon 8x1021 g ~ lunar LHB

6. Impact-Induced Core Formation in Titan We use CTH to simulate hypervelocity ice-ice impacts (Turtle & Pierazzo 2001; Barr & Canup 2009) Where Psh > 1.8 GPa, melt fraction > 50%, ice/water slurry behaves like water Each impact melts a spherical region radius rmelt/rp ~ 5 (ui/15 km/s)0.6 depth of burial zmelt/rp ~ 3 (ui/15 km/s)0.5 (cf. Pierazzo et al., 1997) Rock suspended in melted region sinks, consolidates at base of melt pool Large fragments at base of the melt pool rapidly sink to core (Tonks & Melosh 1992;1993;1997) More impacts => larger rock core If >50% of rock by volume sinks to core, differentiation drives itself to completion. Total number of impacts scales with planetesimal disk mass and is constrained by the Nice model

7. Density (g/cm3) Monte Carlo model of Impact-Induced Core Formation During an Outer Solar System LHB Cross section Surface • Impacts randomly distributed on surface • rp sampled to match Jovian trojan size distribution • ui Rayleigh-distributed with <ui>=10.5 km/s • Impact-melted regions parameterized by CTH results Ice 50% Ice, 50% Rock Rock

8. Density (g/cm3) Monte Carlo model of Impact-Induced Core Formation During an Outer Solar System LHB Ice 50% Ice, 50% Rock Rock

9. Density (g/cm3) Monte Carlo model of Impact-Induced Core Formation During an Outer Solar System LHB Core size ~ 40% RT No Runaway Differentiation Moment of Inertia ~ 0.38 (no ice phase transitions or compression at depth, which will decrease C/MR2) Ice 50% Ice, 50% Rock Rock

10. Titan Formation & Early Evolution solar nebula planet formation satellite formation initial state late heavy bombardment t=4.5 Gyr Titan can form unmelted and undifferentiated in a gas-starved disk Timing and duration of an unmelted Titan consistent with timetable required to form an undifferentiated Callisto Presence of ammonia has a modest effect on time scales There is a > 99% probability that Titan remains partially differentiated after the Late Heavy Bombardment for a Nice-model LHB Titan’s moment of inertia post-LHB C/MR2 ~ 0.38 Iess et al., (2010) estimate C/MR2 ~ 0.342 Later, slow ice/rx separation moderated by convection Titan, Ganymede, Callisto accrete undifferentiated, and LHB differentiates & warms Ganymede, setting the stage for strong tidal heating & endogenic resurfacing Barr & Canup 2008; Barr & Canup 2010; Barr, Citron & Canup 2010

12. LHB also populates small body reservoirs in solar system, notably the Jovian Trojans (Morbidelli et al., 2005) Size distribution of icy objects ~ Jovian trojan asteroids <r> ~ 30 km The Nice Model for Outer Solar System Evolution Shower of comets + asteroids onto Earth’s Moon Mass delivered to moon 8x1021 g, comparable to estimates of lunar LHB Shower of icy objects onto outer planets & their satellites For every 1 gram of material hitting Moon 40 g hits Callisto at 15 km/s (Levison et al., 2001; Zahnle et al., 2003) 80 g hits Ganymede at 20 km/s

13. Ganymede & Callisto, Life Stories, Revised t=0 700 Myr Solar nebula Giant Planet Formation Satellites form in disk around Jupiter Heated from within by decay of radioisotopes Ganymede enters into orbital resonance with Io, Europa Late Heavy Bombardment Present-day interior states Primordial KB mass <40 Earth Masses Barr & Canup, submitted (2009)