Dominic Fortes APEX 28 th February 2006

1. Titan’s internal structure Dominic Fortes APEX 28th February 2006

2. Introduction Modelling Titan’s internal structure is critical to understanding the moon’s formation and evolution, the geology and composition of the surface, and the evolution of the atmosphere……so, pretty much everything about Titan! • What is the current model of Titan’s internal structure? • What’s wrong with those model? • What can it be replaced with? • What are the consequences of the new model? TALK OUTLINE

3. The current model Titan may have incorporated anywhere from 0 – 15 wt % ammonia during accretion. This ammonia may exist today as a deep subsurface ocean beneath a convective lid of ice ~100 km thick. Such an ocean might be the source for flows of aqueous ammonia cryomagmas at the surface. Read more in, for example, Tobie et al., (2005) Icarus 175, 496-502. Note that this MOI factor would be the lowest of any solid body in the solar system.

4. Ganesa macula, as seen in the TA SAR swath (October 26th 2004) Swath width is ~180 km at this point, and pixel resolution is ~350 m. Radar illumination (the look direction) is from the south (i.e., below)

5. What’s wrong with the old model? Kargel* recognised that differentiation leads to hydration of the ‘rock’ fraction and leaching of soluble compounds into the volatile fraction. These soluble components are mostly sulfates, and dominantly (~ 97 wt %) magnesium sulfate. The differentiated body then consists of a hydrated silicate core overlain by shells of ice and highly hydrated salts such as epsomite (MgSO4.7H2O), Fritzsche’s salt (MgSO4.12H2O), and mirabilite (Na2SO4.10H2O). What happens when there is abundant ammonia and methane? MgSO4+ 2NH3+ 2H2O ↔ (NH4)2SO4+ Mg(OH)2 *J. S. Kargel (1991) Icarus 94, 368-390: see also Engel & Lunine (1994) JGR 99(E2), 3745-3752.

6. Building the new model • The new model is constrained by the present-day radius and density of Titan. • The initial mass fraction of rock is varied (controls quantity of sulfates solvated). • The mass fraction of ammonia is varied to yield a final NH3 mass fraction = 0 after reaction with sulfate solutions. • It is assumed that all of the ammonia is consumed to form insoluble hydroxides* that precipitate out onto the core-mantle boundary. • The initial ratio of NH3 : CH4 is fixed = 1 (actually not too important). • We assume that all of the accreted methane is outgassed, yielding a massive atmosphere overlying a methane-saturated ocean (roughly 1 % of the original methane is in solution). *sodium and potassium will form soluble hydroxides – but Mg will likely dominate.

7. And here it is…. Future thermal modelling will indicate whether or not the eutectic ammonium sulfate ocean freezes out, or can persist to the present era. Such an ocean is, potentially, a much more attractive biological habitat than an aqueous ammonia ocean. We expect the crust to be riddled with intrusions of ammonium sulfate and ice entrapped as the crust formed, emplaced by intrusive magmatism, localised melt-throughs, and impact penetration. These bodies might be the source of partial melts that can be erupted at the surface.

8. Some consequences of the new model • Crustal ‘bedrock’ ought to consist of methane clathrate. • Fluids erupted at the surface should be partial melts of ice – ammonium sulfate bodies in the crust, or ammonium sulfate – rich liquids pumped up through the crust by tidal action. • Entrainment of clathrate xenoliths in brine magmas leads to vigorous explosive volcanism (see later). • The infrared reflectivity of the surface measured by Cassini VIMS (McCord et al., [2006] LPSC 37, #1398) in a number of discrete IR windows shows that, • The ‘dark’ areas (e.g., Shangri-La, Fensal, Aztlan) appear to consist of water ice contaminated with organic material. Radar imaging generally shows that these dark areas exhibit widespread aeolian features (e.g., Lorenz et al., [2006] LPSC 37, #1249). • The composition of the ‘brighter’ terrains (e.g., Xanadu, Tsehigi) is not clear (McCord et al., [2006] LPSC 37, #1398). The brightest features – such as Hotei Arcus - are a poor spectral match to water ice (Barnes et al., [2005] Science, doi:10.1126/Science.1117075).

9. The Cassini imaging science system (ISS) can see the surface in the 938 nm methane window.

11. This portion of the T8 SAR swath (Oct ’05) shows rough mountainous terrain embayed by ‘seas’ of ‘sand’ dunes.

12. The Cassini VIMS instrument is able to map the surface in a number of discrete near and mid-IR windows. Tui regio Tui regio Hotei arcus

13. The diffuse IR reflectance of ammonium sulfate is brighter than water ice in the near infrared, with broadly similar absorption bands. At longer wavelengths, ammonium sulfate is probably an order of magnitude brighter than ice. Spectra from the USGS and JPL spectral libraries. Huygens surface spectrum redrawn after Tomasko et al. (2006) Nature, doi:10.1038/nature04126. The spectrum of the surface measured by the Huygens lander is surprisingly featureless, except for an unexplained IR-blue slope (Tomasko et al., (2006). Ammonium sulfate might account for the IR-blue slope, the breadth of the 1.55 micron band, as well as the reflectivity of bright terrains around 5 microns.

14. Dielectric constant of ammonium sulfate Redrawn from Hoshino et al., (1958). Phys. Rev. 112(2), 405-412. At Titan’s surface temperature, the dielectric constant of ammonium sulfate is » 5.5. On cooling below the solidus (where e0 » 9.5), there is a paraelectric to ferroelectric phase transition, where e0 rises to » 165. For short periods, therefore, cooling lava flows containing (NH4)2SO4 would have a comparable radar reflectivity to metals. COMPARE organics: e0» 1.5 water ice: e0» 3.0 ammonia hydrates: e0» 4.5

15. Merged ISS 938 nm map plus corrected (for specular component) radar backscatter from Ta, T3, T4, and T8. Note correlation between ISS-bright areas and high radar backscatter. These terrains are also radiometrically cold.

16. Explosive cryovolcanism The solubility of methane in water or brine is very low. Partial melts, even in equilibrium with methane clathrate, will likely have very small quantities of solvated methane (<< 0.1 wt %). For a magma erupted at 270 K, bubble volume fractions of 75 % (leading to explosive disruption of the magma column) only occur for methane mass fractions in solution exceeding 0.3 wt %. However…. Incorporation of a few wt % methane clathrate xenoliths results in vigorous explosive activity* Physically reasonable abundances of xenoliths (up to 10 wt %) can produce ice-lava fountains 1000s of metres high. *for model details see Grindrod and Fortes (2006) LPSC 37, abstract 1294

17. Even if there is no methane in solution, critical xenolith abundances of 1-2 wt % produce explosive eruptions. It is plausible that explosive activity could be the norm for cryovolcanism on Titan.

18. What might we see if Titanicvolcanism were to be…. titanic? • Collapsing ash clouds could carve channels on the flanks of ice volcanoes, and produce long run-out ash flows (e.g., Ganesa Macula). • Denser ammonium sulfate (r = 1.77 g cm-3) will fall out of the eruption cloud first, perhaps yielding an IR-bright fan of material near the vent (e.g., Hotei Arcus). • Ice (particularly vesicular ice) will be transported more readily, mixing with light organic aerosols and filling low-lying basins to form the IR-dark units. • Highland regions are thus enriched in ammonium sulfate and appear brighter in the mid-IR. • Large volumes of ice-ammonium sulfate tephra could be a major component of Titan’s sedimentary reservoir – feeding the observed dune fields.

19. SUMMARY • Existing Titan models overlook interaction between the rock and volatile components during differentiation. • Including this element results in a model wherein ammonia is converted to ammonium sulfate. • Ammonium sulfate might explain the IR spectrum of Titan’s surface. • The model also predicts a methane clathrate crust, which can lead to violent explosive vulcanicity.