Formation and evolution of Titan structure

Formation and evolution of Titan structure

Abstract

Titan is the biggest Saturn satellite. It was discover in 1655 by Huygens and it’s the first satellite observed around Saturn. For many years we think that Titan was the biggest satellite in the Solar system but some new observations proves that titan atmosphere is very large, so after a new computation the searcher proves that it’s the second biggest satellite after Ganymede (The biggest satellite of Jupiter). The most interesting characteristics of titan is its atmosphere, indeed it is the only one atmosphere rich in nitrogen( 95% dinitrogen, 5% methane) on the solar system Earth exclude. This atmosphere is very dense with a very low temperature (-100 to -200°C) which not prevent the complex meteorological process. This atmosphere is so dense that we can’t observe his surface with the visible light. Thanks to the Data collected by the Cassini-Huygens mission since its arrival to Saturn in 2004 provide information about its atmosphere composition, the surface morphology and the interior structure so it permit to restrict the possible scenarios of formation of Titan. In this article we will see the origin of Ttitan from the accretion to present with its internal structure, the creation of the ice shell and the water global ocean under this surface and the origin of its particular atmosphere.

Introduction

The data of Cassini-Huygens show that Titan is very different than the other satellite even if in term of mass and size it is similar of the Jupiter’s moons, it has the particularity of having his core not fully differentiated, a large atmosphere and probably a global ocean on its surface with an ice shell. In this article we will explain the formation of Titan and its internal structure, we will examine how the accretion phenomenon and the evolution of Titan leads to these particularity.

As the telluric planet the giant planet satellite was formed inside the circumplanetary disk by the agglomerate of material deposit on the surface by a meteoritic impact. Indeed Titan and the other Saturn moons formed from a mixture of ice and rock initially present in the vicinity of Saturn. We will see this phenomenon called Accretion and its application to Titan. Models of Titan’s formation and subsequent evolution are constrained by its present-day interior structure and dynamics.

Interior structure inferred from the gravity field data

The interpretation of the gravity data (thanks to the Cassini-Huygens spacecraft) indicates that the normalized moment of inertia of Titan is about 0.33 to 0.34. This value combined with the density of Titan indicates that the density of the core is low. It may be explained by incomplete separation of rock from ice or the presence of highly hydrated silicate minerals.

The most likely present-day structures is either a pure rock core, mostly anhydrous, surrounded by a mixture of ice and rock, surrounded by pure water ice (Fig. 1, top and middle), or a low density rock core mainly composed of hydrated silicate minerals surrounded by H2O (Fig. 1, bottom). In both cases, a liquid water layer may be present between a high-pressure ice layer and an ice shell.

Geophysical evidence for an internal water ocean

The measurements of low frequency waves and atmospheric conductivity by the PWA instrument revealed the existence of a Schumann-like resonance trapped within Titan’s atmospheric cavity. The observed signal may be triggered and sustained by strong electric currents induced in the ionosphere by Saturn’s magnetospheric plasma flow. The characteristics of this trapped-resonant mode imply the presence of a conductive layer at about 30-60 km below the surface, which may correspond to the ice/ocean interface provided that the ocean is sufficiently conductive, possibly doped with small amounts of ammonia and/or salts.

The observed topography may be explained by variations in ice shell thickness, resulting from inhomogeneous crystallization of an underlying ocean. If this analysis is correct, it implies that an internal ocean is still present in Titan’s interior and that it is currently slowly crystallizing. This constitutes additional indirect evidence for an internal ocean inside Titan.

Another technic to confirm the existence of a subsurface ocean would be to measure the tidal fluctuation from accurate gravity or topography measurements. A future mission with dedicated multiple flybys or even better in orbit around Titan will be required to confirm this detection and to get more precise information on the ocean and ice shell thicknesses.

Role of impactors on Titan structure

As described on Accretion Annex, icy moons probably accreted within the first million years of the Solar System history in circumplanetary disks from aggregates of ice and rock. The presence of its massive atmosphere suggests that Titan formed from different materials than did its Galilean cousins. In reality, the size and velocity distribution of the impactors have probably varied during the accretion phase. Two reservoirs of bodies may have contributed to the satellite growth: bodies in orbit around the planet formed or captured within the circumplanetary disk, and bodies in orbit around the Sun, colliding directly with the growing satellite. The contribution of each of these two reservoirs probably varied as the circumplanetary disk evolved. At the start of accretion process, centimeter-sized particles were probably predominant, while kilometer-sized and larger bodies became more and more frequent during the late-stage of accretion.

Different interior structures are possible, it depend on the accreted condition of Titan creation. In Figure 3 we can see the possible structure, if the accretion is very slow and the impactors are small along the accretion period, the surface should be able to cool down between successive impacts, and the differentiation may be avoided. In that case of cold accretion, no significant atmosphere would be generated. The creation of the atmosphere occur during the satellite evolution, for instance during the Late Heavy Bombardment. If the impactors are larger during the end of accretion, a surface water ocean is possible (Figure 3 middle). Finally when the impactors are large even at the end of accretion or if the accretion is faster, ice melting rapidly occurs during the growth and a massive ocean in equilibrium with a massive atmosphere would be generated.  

Differentiation and internal evolution

As already discussed previously, the moment of inertia inferred from Cassini gravity measurements suggest that Titan’s differentiation was not complete, so it probably does not have an iron core. The layer composed of a rock-ice mixture may still exist between a rocky core and a high-pressure ice layer. This possible uncompleted ice-rock separation has been proposed to be the result of cold accretion process. However, limited differentiation also implies that the convective heat transfer was efficient enough to remove heat from the interior. This is possible if the viscosity of the ice-rock mixtures is sufficiently low. For example, in the case of Callisto, if the average size of rocks was of the order of meters to tens of meters, the interior may have experienced a gradual, but incomplete unmixing of the two components (ice and rock here). Significant lateral heterogeneities among the ice/rock mixtures existing after accretion could also have improved the differentiation. In these conditions, convective motions associated with ice-rock unmixing processes may be efficient enough to avoid melting. Following this scenario, the interior of Titan would be relatively similar to Callisto, but with a thinner layer of ice-rock mixed materials, as suggested by its smaller MoI factor.

...