Titan’s internal structureDominic Fortes APEX January 22nd 2009

Titan Radio Science Gravity field measurements

MethodMeasurement 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.

Objectives1.Measure Titan’s mass (actually GM) to high precision bulk density2.Determine quadrupole coefficients, J2 and C22 moment of inertia factor3.Measure second-degree tidal Love number, k2 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 Titan’s gravity field have been completed and analysed.

GM (km3 s-2) 8978.1337 ± 0.0025 J2 32.54 ± 0.34 ×10-6

M (kg) 1.345184 ± 0.000135 ×1023 C22 9.900 ± 0.027 ×10-6

Radius (km) 2575.0 ± 0.5 10C22/J2 3.04 ± 0.03

(kg m-3)* 1880.9 ± 1.1 qr 3.9555 ± 0.0023 ×10-5

(s-1) 4.560686 ± 0.000001 ×10-6 kf 1.0011 ± 0.0028

C/MR2 0.3402 ± 0.0002

*Based on the volume of a sphere with radius 2575.0 ± 0.5 km.

Results of first four Cassini radio science flybys of Titan

T11: Feb 27th 2006

T22: Dec 28th 2006

T33: Jun 29th 2007

T45: July 31st 2008

Model gravity field to degree and order threeModel gravity field to degree and order three

Full multi-arc solution (all four flybys)Full multi-arc solution (all four flybys)

New Titan spin vector included (from Radar data)New Titan spin vector included (from Radar data)

Quadrupole field is hydrostatic use Darwin-Radau approximation to obtain C/MR2

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 Titan’s core density is due to a CI-chondrite mineralogy (serpentine + clays):

Core temperature must be lower than ~ 800 K to avoid dehydration

(2)If Titan’s 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 Titan’s coreCeres may be a good analogue for Titan’s 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 Titan’s core is much cooler than expected

Accretion too slow for short-lived isotopes (e.g., 26Al) to provide heating

A metallic core is probably ruled out

Titan’s young surface might be attributed to processes other than volcanism (Fall 2008 AGU abstract by Jeff Moore)

Secular cooling Secular cooling contractional tectonics contractional tectonics mass wasting and sediment build-up 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 Titan’s atmosphere may have come from:

•Primordial gas accreted in clathrates•Abiotic generation during hydration of silicates by CO2-bearing waters•Biotic generation by methanogens in a subsurface ocean

These can be tested (in principle) by measuring the 12C/13C isotope ratio in the atmosphere

The observed The observed 1212C/C/1313C ratio varies between C-bearing species, and with latitude.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 Titan’s stratosphere is masking the input of light methane from the interior.

Serpentinization as a source of Titan’s methane

Work to do here on Earth

Experimental studies of chondrite hydration at 1 – 5 GPa

Modelling of possible hydrothermal circulation in Titan’s 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 methane’s 12C/13C ratio

Positive confirmation of cryovolcanism

Seismometer to identify depth of major density discontinuities in Titan’s interior:

•Detect subsurface ocean, roof and floor

•Detect core-mantle boundary – CRITICAL !!

TitanBulk density 1881 kg m-3

Radius 2575 kmNo intrinsic magnetic fieldThick N2-CH4 atmosphereGeologically young surfaceEvidence of outgassing (40Ar)

GanymedeBulk density 1942 kg m-3

Radius 2631 kmIntrinsic magnetic fieldNo significant atmosphereSurface at least 3 billion years old

MoI = 0.34

MoI = 0.31

An interesting comparison – Titan versus Ganymede