Titans internal structure Dominic Fortes APEX January 22 nd 2009

Titan Radio Science Gravity field measurements Method Measurement of Doppler shifts in radio telemetry signal (X-band and Ka-band) along arcs ± 24 hours of closest approach during targeted (<2000 km) flybys of Titan. Provides space-craft line-of-sight acceleration. Objectives 1.Measure Titans mass (actually GM) to high precision bulk density 2.Determine quadrupole coefficients, J 2 and C 22 moment of inertia factor 3.Measure second-degree tidal Love number, k 2 presence of ocean The gravity field determination at Titan is superior to Galileo measurements in the Jovian system - Improved signal frequency stability - Lower plasma noise - Lower altitude flybys - Improved spatial coverage of gravity field Four of the five flybys dedicated to measurement of Titans gravity field have been completed and analysed.

GM (km 3 s -2 ) ± J2J ± 0.34 ×10 -6 M (kg) ± ×10 23 C ± ×10 -6 Radius (km) ± 0.510C 22 /J ± 0.03 (kg m -3 )* ± 1.1qrqr ± ×10 -5 (s -1 ) ± ×10 -6 kfkf ± C/MR ± *Based on the volume of a sphere with radius ± 0.5 km. Results of first four Cassini radio science flybys of Titan T11: Feb 27 th 2006 T22: Dec 28 th 2006 T33: Jun 29 th 2007 T45: July 31 st 2008 Model gravity field to degree and order three Full multi-arc solution (all four flybys) New Titan spin vector included (from Radar data) Quadrupole field is hydrostatic use Darwin-Radau approximation to obtain C/MR 2

Simple 2-layer differentiated model, water-ice shell over rock core + low density global ocean under 100 km thick water-ice shell ( 0 = 950 kg m -3, depth = 250 km) + low density global ocean under 100 km thick water-ice shell ( 0 = 1200 kg m -3, depth = 250 km)

MoI = 0.34 implies a core density in range 2460 – 2570 kg m -3 Simple 2-layer differentiated model, water-ice shell over rock core

3-layer models with partially differentiated rocky core

3-layer differentiated models with metallic inner core Assuming CI chondrite rock density and an ocean-free shell, MoI = 0.34 allows only a very small metallic core. Fe-FeS eutectic core < 450 km radius (0.5 wt. %) Fe core < 350 km radius (0.7 wt. %)

Consequences of a low-density core inside Titan (1) If Titans core density is due to a CI-chondrite mineralogy (serpentine + clays): Core temperature must be lower than ~ 800 K to avoid dehydration (2) If Titans core density is due to an admixture of anhydrous rock +ice: Core temperature is limited by ice pressure-melting curve; lower than ~ 500 K Ceres may be a good analogue for Titans core Previous thermal models have predicted a hot interior Core temperatures > 1300 K Partial melting of Fe-FeS and metallic core segregation Extensive surface geology driven by high heat flow

Consequences of a low-density core inside Titan (1) If Titans core is much cooler than expected Accretion too slow for short-lived isotopes (e.g., 26 Al) to provide heating A metallic core is probably ruled out Titans young surface might be attributed to processes other than volcanism (Fall 2008 AGU abstract by Jeff Moore) Secular cooling contractional tectonics mass wasting and sediment build-up (2) Alternatively, a hot early core may have become hydrated by pervasive circulation of water along micro-fractures Analogous to hydrothermal alteration of CI-chondrite parent body? ( cf. Ceres??)

The methane in Titans atmosphere may have come from: Primordial gas accreted in clathrates Abiotic generation during hydration of silicates by CO 2 -bearing waters Biotic generation by methanogens in a subsurface ocean These can be tested (in principle) by measuring the 12 C/ 13 C isotope ratio in the atmosphere The observed 12 C/ 13 C ratio varies between C-bearing species, and with latitude. The ratio in methane (= 82) is nearly solar (= 89) suggesting a primordial source. Both biotic and abiotic processes produce isotopically very light methane. In fact the two processes are not distinguishable by means of carbon isotopes. However, it is possible that loss of light methane from Titans stratosphere is masking the input of light methane from the interior. Serpentinization as a source of Titans methane

Work to do here on Earth Experimental studies of chondrite hydration at 1 – 5 GPa Modelling of possible hydrothermal circulation in Titans core Further thermal evolutionary modelling of serpentinite and rock + ice cores Objectives for the Titan Saturn-System Mission (TSSM) In-situ measurement of out-gassed methanes 12 C/ 13 C ratio Positive confirmation of cryovolcanism Seismometer to identify depth of major density discontinuities in Titans interior: Detect subsurface ocean, roof and floor Detect core-mantle boundary – CRITICAL !!

Titan Bulk density 1881 kg m -3 Radius 2575 km No intrinsic magnetic field Thick N 2 -CH 4 atmosphere Geologically young surface Evidence of outgassing ( 40 Ar) Ganymede Bulk density 1942 kg m -3 Radius 2631 km Intrinsic magnetic field No significant atmosphere Surface at least 3 billion years old MoI = 0.34 MoI = 0.31 An interesting comparison – Titan versus Ganymede