Topside interactions with the Titan atmosphere - ESAC Trainee Project

Topside interactions with the Titan atmosphere - ESAC Trainee Project

Topside interactions with theTitan atmosphereAnne Wellbrock

Outline1. About me2. Introduction3. Introducing Titan and its atmosphere4. The UCL Titan thermosphere code5. The interaction with Saturn´smagnetosphere6. Summary

1. About me• Trainee at ESAC in 2005• BSc in Physics at• Currently on MSc in Astrophysics course at• From September 2007: 4-year STFCfunded PhD at(UCL MSSL Space and Climate Physicsand UCL Physics & Astronomy)Supervisors: Andrew Coates and Alan Aylward

2. Introduction• Titan: Saturn´s largest moon• I study the upper atmosphere(thermosphere and ionosphere) using aGeneral Circulation Model (GCM) of thethermosphere and data from theCassini-Huygens mission

3. Introducing Saturn´s moonTitan• 2nd largest moon in Solar System• Radius: 2575 km• Orbits Saturn at ~20 Saturn radii• Orbital & rotation period: 16 days(synchronous rotation)• Dense, extended nitrogen atmosphere with 1.6%methane + traces of other organic molecules, Arand CO 2• Surface pressure: 1.45 bar• Surface temperature: 94 KSource: NASA/ESA

Sources: NASA/ESA

Active methalogical cycle: Evidence for lakes and clouds!Sources: NASA/ESA

Titan´s temperature profile

Titan´s thermosphere• Steep vertical T gradient inbottom part• Then T becomes ~heightindependent => exospheric T~185K• Main cooling mechanism inthermosphere: IR cooling dueto pure rotational lines inhydrogen cyanide (byproductof ionospheric chemistry)• T and altitude range: 140K @600km to 190K @ 1400km

Titan´s ionosphereSignificant amount of particles becomeionised > 800km due to1. Solar UV radiation2. Energetic plasma from Saturn´smagnetosphere (mainly impact ionisationby fast electrons)

4. The UCL Titan thermosphere code- a general circulation model• Calculates the dynamics of thethermosphere• Solves momentum, energy andcontinuity equations• On a spherical grid of longitude,latitude and pressure level• Pressure levels correspond toaltitudes of 600km to 1400km

• Momentum equation:GravitytermP gradienttermCoriolis termViscosityterm• Continuity equation: Calculates vertical winds• Energy balance: Expressed as sum of internal andexternal energy sources and sinks (such asadiabatic heating and cooling, radiative cooling inrotational lines, heat conduction, …)

External energy sources• Only external energy source consideredin this model so far: Solar (E)UVradiation• Next step:Implementation of energetic plasmafrom Saturn´s magnetosphere as anadditional energy source

5. Titan´s interaction with Saturn´smagnetosphere• Saturn´s magnetospherecorotates with the planet• Saturn´s rotation period: ~10hours i.e.

6. Summary• To accurately model the dynamics of Titans´sthermosphere, it is necessary to include notonly solar radiation as an external energysource, but also the energetic plasma fromSaturn´s magnetosphere• This is difficult to implement since there is astrong spatial and time dependence• Therefore fast electron fluxes (and henceionisation rates) vary considerably with timeand location• Different special cases need to beinvestigated in more detail separately

ReferencesWellbrock, Anne, Topside Interactions with the Titan Atmosphere, 2007, MScresearch essay, University College London

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