A Sulfate-rich Model of Titan’s Interior 2: Implications for Possible Explosive

Introduction: There has been a great deal of speculation regarding the possibility of cryovolcanic activity of Titan, traditionally involving the extrusion of aqueous ammonia solutions, perhaps from a subsurface ocean [e.g., 1]: Since the Cassini spacecraft arrived in orbit about Saturn, and having made ten Titan flybys to date (out of forty scheduled for the primary mission), it has become apparent that Titan has been (and may still be) cryovolcanically active. Features interpreted as cryovolcanic edifices and flows have been observed in synthetic aperture radar (SAR) data collected during the Ta flyby [2]. Current models predict gentle effusive volcanism, the principle volatile in the magma (methane) being insufficiently soluble in aqueous ammonia to cause explosive activity at Titan’s current atmospheric pressure [1]. In a related abstract [3] we present an alternative model of Titan’s internal structure and chemistry in which cryomagmatism on Titan involves the extrusion of aqueous ammonium sulfate through a methane clathrate crust. Here we use the model to reassess the role of magmatic volatiles in generating explosive cryovolcanism, and consider the consequences for the state of Titan’s surface and atmosphere. Modelling: In the structural model outlined in Ref. 3, we envisage ammonium sulfate solutions being extruded to the surface either from a subsurface ocean (at a depth of perhaps 150 km), or from melt pockets in the crust itself that were either intruded at the time of crust formation, or were emplaced by solid-state convection in the crust. Eutectic melts in the binary (NH4)2SO4 – H2O system will have a density of ~1240 kg m [4]; therefore, melt extraction through a clathrate crust (mean ρ = 1000 kg m) is only possible if there is a driving force other than buoyancy, or if the melt can fractionate to a more waterrich (and thus less dense) composition. Possible driving forces include tidal pumping [e.g., 5], or the pressure generated by the volume change upon partial melting. Explosive activity becomes possible if the magma contains a volatile species that is capable of exsolving as it rises towards the surface and decompresses. For Titan, plausible volatile species in the melt include methane, carbon monoxide and nitrogen; here we only consider the role of methane. As a matter of interest, we also note that explosive activity may occur even in the absence of dissolved volatiles, where a magmatic dike intrudes into volatile-laden sediments (such as those observed at the Huygens landing site [6]), or liquid methane seeps down into fractured rocks (ices), coming into contact with magmatic intrusions. There are two principle methods of introducing methane into the rising cryomagma: i) in solution; and ii) in the form of methane clathrate xenoliths. These form the basis for a range of model scenarios. For liquid sourced in an underground ocean, the methane in solution is that remaining after the extraction of methane to form the crust, and is probably of the order of 0.5 wt % (scenario 1). Melts generated in the crust might in fact contain no dissolved methane at all (scenario 2) unless they are able to equilibrate with methane clathrate country rocks (scenario 3); in this instance the quantity of dissolved methane will be very small (order 0.1 wt % [7]). It is also possible that intracrust partial melts are able to become saturated with methane (~0.5 wt % at 300 bar 270 K, for example [7]); this is scenario 4. In all instances, it is likely that rising magma will incorporate methane hydrate wall rock as xenoliths. These xenoliths play little role (other than providing a minute amount of extra buoyancy) until the clathrate decomposition depth is reached. At an assumed magma temperature of 270 K, this occurs at a pressure of 26 bars [8], or ~2000 metres depth. The decomposition of entrained xenoliths immediately liberates methane gas into the melt over and above that already exsolved (or not) from solution. We consider melts with no dissolved methane and variable xenolith abundances (scenario 5) and methane saturated melts with variable xenolith abundances (scenario 6). As the magma rises towards the surface, the confining pressure decreases, thus reducing the solubility of methane in the liquid, resulting in exsolution and the nucleation and growth of gas bubbles. If the total gas bubble volume fraction becomes large enough (~60-85%) then the magma is said to fragment, powering a Hawaiian-style explosive eruption [9-11]. If the magma viscosity is higher, bubbles may coalesce, driving strombolian-type activity. In extreme cases, choking of the vent can result in build up of gas-laden foam, or the prevention of gas exsolution. In the first case, sufficient pressure may accumulate to explosively destroy the vent blockage, and in the second case, sudden pressure release may result in violent degassing of the magma, as occurred at Mount St. Helens in 1980, for example. Experimentally determined values of the solubility of methane gas in water [7,12] are used to determine the mass fraction of methane. Using the basic model described in refs 9 and 10, the exsolved methane mass fraction (nm) provides the bulk magma density (β), and the bubble volume fraction (Vb);