Evidence of electrical activity on Titan drawn from the Schumann resonances sent by Huygens probe

A procedure is shown for extracting weak resonances from the responses of electromagnetic systems excited by electric discharges. The procedure, based on analysis of the late-time system response, is first checked using an analytical function and later with the data for the electric field generated by the computational simulation of Titan's atmosphere using the Transmission Line Matrix (TLM) method. Finally, the low frequency spectrum of the natural electric field in Titan's atmosphere sent by the mutual impedance sensor (MIP) included in the Huygens probe is analyzed employing this technique. The MIP sensor was initially designed to measure the horizontal component of the electric field during a quiet descent. Fortunately, the swinging that occurred during the descent allowed the MIP to measure the radial component of the electric field mixed with the horizontal one. Application of the late-time analysis technique shown in this paper confirms the signature of lightning reported by preliminary data observations, bringing out the expected eigenfrequencies of the Titan-ionosphere electromagnetic cavity, known as Schumann resonances. These resonances are the resonant frequencies of the lower TM^r (transverse magnetic to r) modes, which are quasi-transverse electromagnetic modes because they present vertical or radial components of the electric field two orders of magnitude higher than the associated horizontal, azimuthal and zenithal, components. The sequence of Schumann resonances is unique for each celestial body with an ionosphere, since these resonances are fully determined by the dimensions of the planet or satellite and the corresponding atmospheric conductivity profile. Detecting these frequencies in an atmosphere is clear proof of electrical activity, since it implies the existence of an electromagnetic-energy source, which is essential to create and maintain them.     

Thermal interactions of the Huygens probe with the Titan environment: Constraint on near-surface wind

The Huygens probe lost heat to its cold environment during its descent through Titan's atmosphere and after landing. Here I report measurements of the probe's thermal behavior and comparison with ground tests (1) to provide a context for other scientific investigations, such as the release of volatiles from the landing site, and (2) to place constraints on Titan environmental parameters directly, such as the thermal conductivity of the surface material and the strength of winds at the surface. Near-surface winds are constrained to be less than 0.2 m s⁻¹, and probably much less.     

Near-surface winds at the Huygens site on Titan: Interpretation by means of a general circulation model

This study aims at interpreting the zonal and meridional wind in Titan's troposphere measured by the Huygens probe by means of a general circulation model. The numerical simulation elucidates the relative importance of the seasonal variation in the Hadley circulation and Saturn's gravitational tide in affecting the actual wind profile. The observed reversal of the zonal wind at two altitudes in the lower troposphere can be reproduced with this model only if the near-surface temperature profile is asymmetric about the equator and substantial seasonal redistribution of angular momentum by the variable Hadley circulation takes place. The meridional wind near the surface is mainly caused by the meridional pressure gradient and is thus a manifestation of the Hadley circulation. Southward meridional wind in the PBL (planetary boundary layer) is consistent with the near-surface temperature at the equator being lower than at mid southern latitudes. Even small changes in the radiative heating profile in the troposphere can substantially affect the mean zonal and meridional wind including their direction. Saturn's gravitational tide is rather weak at the Huygens site due to the proximity to the equator, and does not clearly manifest itself in the instantaneous vertical profile of wind. Nevertheless, the simulated descent trajectory is more consistent with the observation if the tide is present. Because of a different force balance in Titan's atmosphere from terrestrial conditions, PBL-specific wind systems like on Earth are unlikely to exist on Titan.     

Sensitivity of a Titan ionospheric model to the ion-molecule reaction parameters

Various aspects of ion-molecule reactions for Titan ionospheric chemistry modeling are reviewed in this work: temperature/collision energy effects on reaction rates and, more importantly, on products distributions; differential reactivity of isomers of ions; reactivity of excited states of ions; pathways to the building of complex ions. We evaluate here the extent to which these points affect model predictions. We find that the present limiting factors to model predictivity are the model incompleteness for heavy ion production pathways; the differential reactivity of isomers; and, to a lesser degree, the temperature effects on the branching ratios of ion-molecule reactions. Extensive experimental studies are required to fill these gaps in ion-molecule reactivity knowledge.