P12A-01 INVITED
Titan's Winter Polar Vortex
Titan's atmosphere has provided an interesting study in contrasts and similarities with Earth's. While both have N2 as the dominant constituent and comparable surface pressures ~1 bar, Titan's next most abundant molecule is CH4, not O2, and the dissociative break-up of CH4 and N2 by sunlight and electron impact leads to a suite of hydrocarbons and nitriles, and ultimately the photochemical smog that enshrouds the moon. In addition, with a 15.95-day period, Titan is a slow rotator compared to Earth. While the mean zonal terrestrial winds are geostrophic, Titan's are mostly cyclostrophic, whipping around the moon in as little as 1 day. Despite the different dynamical regime, Titan's winter stratosphere exhibits several characteristics that should be familiar to terrestrial meteorologists. The cold winter pole near the 1-mbar level is circumscribed by strong winds (up to 190 m/s) that act as a barrier to mixing with airmasses at lower latitudes. There is evidence of enhancement of several organic species over the winter pole, indicating subsidence. The adiabatic heating associated with this gives rise to a warm anomaly at the 0.01-mbar level, raising the stratopause two scale heights above its location at equatorial latitudes. Condensate ices have been detected in Titan's lower stratosphere within the winter polar vortex from infrared spectra. The interesting question of whether there is important heterogeneous chemistry occurring within the polar vortex, analogous to that occurring in the terrestrial polar stratospheric clouds in the ozone holes, has not been addressed. The breakup of Titan's winter polar vortex has not yet been observed. On Earth, the polar vortex is nonlinearly disrupted by interaction with large-amplitude planetary waves. Large-scale waves have not been identified in Titan's atmosphere, so the decay of its polar vortex may be more gradual. Observations from an extended Cassini mission into late northern spring should provide critical data indicating whether the vortex goes away with a bang or just fades away.
P12A-02
The TitanWRF Model at the end of the Cassini Prime Mission.
TitanWRF is a global model of Titan's atmosphere from the surface to about 400km, adapted from the Earth- based, limited area WRF (Weather Research and Forecasting) model that is used extensively in terrestrial weather prediction. Work on TitanWRF began close to the start of the Cassini mission, and we will present the TitanWRF results most influenced by and relevant to Cassini observations. Although Cassini has monitored Titan for less than a third of a Titan year, it has greatly increased our knowledge of how its general circulation changes with time. Cassini CIRS provides observations of zonal mean temperatures and inferred zonal winds, adding significantly to the coverage provided by the pre- existing Voyager dataset. Notable features are the strong equatorial superrotation (also observed directly by the Huygens probe) and large temperature gradients in the winter stratosphere. These proved difficult for TitanWRF to fully reproduce, but we will present and discuss new results showing a far better match to observations. Tropospheric methane cycle experiments in TitanWRF were first motivated by ground- then Cassini-based cloud observations, and more recently by Cassini's detection of surface lakes. This enables us to test our predictions of where and when cloud formation and precipitation should occur, and we will present our latest results and predictions of what to expect next. Huygens measured tropospheric wind speeds and directions for just one location and time of year, but Cassini radar observations of what are assumed to be aeolian dune fields may allow us to infer more about the dominant surface wind directions for a range of locations. We will compare our predictions with the measured winds from Huygens and inferred wind directions from Cassini, and discuss the impact of tides due to Saturn's gravity in the TitanWRF model. This work is funded by NASA's Outer Planets Research Program and Planetary Atmospheres Research Program, and our simulations were performed on the CITerra cluster in the Geological and Planetary Sciences division at Caltech.
P12A-03
Discovery of lake-effect clouds on Titan
Images from instruments on Cassini as well as from telescopes on the ground reveal the presence of sporadic small-scale cloud activity in the cold late-winter north polar of Saturn's large moon Titan. These clouds lie underneath the previously discovered uniform polar cloud attributed to a quiescent ethane cloud at ~40 km and appear confined to the same latitudes as those of the largest known hydrocarbon lakes at the north pole of Titan. The physical properties of these clouds suggest that they are due to methane convection and condensation. Such convection has not been predicted for the cold winter pole, but can be caused by a process in many ways analogous to terrestrial lake-effect clouds. The lakes on Titan are a key connection between the surface and the meteorological cycle.
P12A-04
The Detached Haze Layer in Titan's Mesosphere
The Cassini observations reveal the presence of a detached haze layer in Titan's mesosphere at an altitude of 520 km, well above the stratosphere. Observations of scattered light made by the Imaging Science Subsystem (ISS) reveal a clearly defined layer encircling low and mid-latitude regions. The aerosol layer is also detected in stellar occultation measurements of UV extinction by the UltraViolet Imaging Spectrometer (UVIS). The haze is a global and permanent feature of Titan's atmosphere. Furthermore the location of the detached haze layer is coincident with and the likely cause of a local maximum in the temperature profile measured by the Huygens Atmospheric Structure Instrument (HASI). This temperature inversion is also permanent and global, having been detected in ground-based stellar occultations. The correlation between the extinction profile and the temperature maximum imply that the detached haze cannot be due to condensation, as previously suggested. Previously, Voyager high phase angle images at 500 nm revealed a detached haze layer near 350 km, more than 150 km lower than the Cassini layer. Close examination of the Voyager images suggests that the Cassini detached layer at 520 km is a separate phenomenon rather than a change in the Voyager detached layer. Analysis of the observed optical properties suggests that the average size of particles in the Cassini detached layer is < 45 nm, with an imaginary index k < 0.3 at 187.5 nm, while Non-LTE calculations of the temperature perturbation induced by the detached haze show that the average particle size must be greater than 35 nm for reproducing the heating rate implied by the HASI temperature profile. Calculation of the sedimentation velocity of the particles, coupled with the derived number density, imply a mass flux of 1.9-3.2 × 10-14 g cm-2 s-1. This is approximately equal to the mass flux required to explain the main haze layer and suggests that the stratospheric haze is formed primarily by sedimentation and coagulation of particles in the detached layer. This is further supported from the particle size range retrieved for the detached layer (35-45 nm), being approximately equal to the radii of ~50 nm for the monomers of the aggregate aerosols in the main haze layer. It follows that aerosols on Titan are formed primarily in the thermosphere, rather than the stratosphere as assumed in many pre-Cassini studies. This is consistent with the detection of negatively charged aerosols in the thermosphere by Cassini/CAPS. These conclusions are supported by microphysical aerosol models that couple the detached haze layer and the main haze layer and extend into the thermosphere. Our calculations suggest that the detached haze layer is due to the transition in the growth of particles from spherical shape to aggregates of fractal structure. The rapid increase in the size of the particles with the onset of fractal growth, in combination with the decrease of their settling velocity, casts them invisible in the transition region. This optical illusion process explains the well-defined and symmetric structure of the detached layer, something difficult to explain under a pure advection scenario. Further investigation of the processes defining the growth of the particles is required in order to understand why the transition takes place at this region and how the particles produced at higher altitudes by high energy radical and ion chemistry, are defining the vertical haze opacity in Titan's atmosphere.
P12A-05
Characterizing the Upper Atmosphere of Titan using the Titan Global Ionosphere- Thermosphere Model: Nitrogen and Methane.
Recently, a great deal of effort has been put forth to explain the Cassini Ion-Neutral Mass Spectrometer (Waite et al [2004]) in-situ measurements of Titan's upper atmosphere (e.g. Muller-Wodarg [2008], Strobel [2008], Yelle et al [2008]). Currently, the community seems to agree that large amounts of CH4 are escaping from Titan's upper atmosphere at a rate of roughly 2.0 x 1027 molecules of CH4/s (3.33 x 1028 amu/s), representing a significant mass source to the Kronian Magnetosphere. However, such large escape fluxes from Titan are currently not corroborated by measurements onboard the Cassini Spacecraft. Thus, we posit another potential scenario: Aerosol depletion of atmospheric methane. Using the three-dimensional Titan Global Ionosphere-Thermosphere Model (T-GITM) (Bell et al [2008]), we explore the possible removal mechanisms of atmospheric gaseous constituents by these aerosols. Titan simulations are directly compared against Cassini Ion-Neutral Mass Spectrometer in-situ densities of N2 and CH4. From this work, we can then compare and contrast this aerosol depletion scenario against the currently posited hydrodynamic escape scenario, illustrating the merits and shortcomings of both.
P12A-06 INVITED
Origin and Evolution of Titan's Nitrogen-Methane Atmosphere: A Cassini-Huygens Perspective
In its prime mission, the Cassini-Huygens Mission has provided unprecedented, synergistic remote sensing and in situ measurements from which a picture of the origin and evolution of Titan's atmosphere has begun to emerge. In particular, GCMS measurement of the height profile of methane from surface to the stratosphere, presence of methane-ethane lakes inferred from radar imaging and the VIMS spectra, abundance of heavy noble gases with the GCMS and INMS, determination of nitrogen isotope abundance in N2 by the GCMS and in HCN by CIRS, and the D/H ratio from methane by CIRS and from hydrogen by the GCMS, have been instrumental in constraining the models of Titan's atmospheric origin and evolution. The nitrogen on Titan appears to have formed early on by photolysis of ammonia, delivered from Saturn's subnebula during Titan's accretion. On the other hand, presently there are two leading hypotheses for the origin of methane - formation on Titan, and direct delivery to Titan - with their individual pros and cons. Methane on Titan displays characteristics similar to water on Earth, complete with a cycle of evaporation from surface reservoirs, cloud formation, followed by rain. It is also destroyed irreversibly by photochemistry, requiring replenishment from the interior from time to time. The fate of Titan's nitrogen is closely coupled to the photochemical-meteorological-hydrogeochemical cycle of methane.
P12A-07
Cassini/Huygens Investigations of Titan's Methane Cycle
In Titan's atmosphere, the second most abundant constituent, methane, exists as a gas, liquid and solid, and cycles between the atmosphere and surface. Similar to Earth's hydrological cycle, Titan sports clouds, rain, and lakes. Yet, Titan's cycle differs dramatically from its terrestrial counterpart, and reveals the workings of weather in an atmosphere that is ten times thicker than Earth's atmosphere, that is two orders of magnitude less illuminated, and that involves a different condensable. Measurements of Titan's troposphere, where the methane cycle plays out, are limited largely to spectral images of Titan's clouds, several temperature profiles by Voyager, Huygens and Cassini, recent Keck spectra of the surface methane humidity, and one vertical profile of Titan's methane abundance, measured on a summer afternoon in Titan's tropical atmosphere by the Huygens probe. The salient features of Titan's methane cycle are distinctly alien: clouds have predominated the northern and southern polar atmospheres; the one humidity profile precisely matches the profile (of cartoonish simplicity) used in pre-Cassini models, and surface features correlate with latitude. Data of Titan's troposphere are analyzed with thermodynamic and radiative transfer calculations, and synthesized with other studies of Titan's stratosphere and surface, to investigate the workings of Titan's methane cycle. At the end of Cassini's nominal mission, we find that Titan's weather, climate and surface-to-atmosphere exchange of volatiles vastly differs from the manifestation of these processes on Earth, largely as a result of different basic characteristics of these planetary bodies. The talk ends with a comparison between Titan and Earth's tropospheres, their fundamental properties, the energetics of their condensible cycles, their weather and climates. References: Griffith C.A. et al. Titan's Tropical Storms in an Evolving Atmosphere. Ap.J. In Press (2008). Griffith C.A. Storms, Polar Deposits, and the Methane Cycle in Titan's Atmosphere. Phil. Trans. Royal Society A. In Press (2008). Penteado, P.F. & C.A. Griffith Ground-based measurements of the methane distribution on Titan. In Preparation for submission to Icarus Griffith C.A. et al. Evidence for a Polar Ethane Cloud on Titan, Science, 313, 1620 (2006). Griffith C.A. et al. The Evolution of Titan's Mid-Latitude Clouds, Science, 310, 474 (2005).
P12A-08 INVITED
Titan's Plasma Interaction: New Perspectives from the Cassini Flybys
Titan is immersed in the sub-corotating plasma environment in Saturn's outer magnetosphere. As it orbits Saturn, its sunlit face changes from a location on the upstream side (at Saturn dusk) to the downstream side (at Saturn dawn) of the plasma flow, thus the plasma interaction depends on Saturn Local Time. Earlier measurements at Saturn and Titan by Voyager provided initial information suggesting a Venus-like interaction with Titan's massive ionosphere. This was not surprising considering our early conceptions. It was also expected that Titan would occasionally leave the magnetosphere and be immersed in the magnetosheath and solar wind. Measurements with the Cassini MAPS instruments together with numerical simulations have taught us much more about the nature of the interaction, the properties of the ionosphere, and a little about Titan's interior. The particles and fields investigations found a dynamic and structured plasma environment at 20 Rs dominated by a time-varying radial component of the Kronian magnetic field. The composition of the incident plasma was not that of a Titan-derived nitrogen and hydrogen pickup ion torus, but rather a highly variable magnetospheric hydrogen ion population with contributions from water-derived heavy ions, including an important energetic component. During one low altitude flyby, Titan had recently moved into Saturn's magnetosheath, carrying with it the memory of Saturn's magnetic field embedded in its ionosphere. In general, the new measurements of the plasma interaction and the ionosphere reflect the great variability of the external conditions, together with that introduced by the changing relative incident plasma flow and Sun angles. We provide an overview of some of these observational results, together with modeling efforts that are sorting out aspects of the complicated Titan-plasma interaction that Cassini has revealed.
P12A-09
Titan Plumes Revisited
At the time of the Voyager 1 close encounter with Titan, enhancements of density and sharp decreases of electron temperature were observed by the PLS instrument. A few smaller plumes were identified inside and outside Titan orbit and their displacements correlated with variations in solar wind dynamic pressure observed by Voyager PLS one and two corotation periods earlier. At the time, two conflicting interpretations of this phenomenon were proposed. In one interpretation (Eviatar et al., JGR, 87, 8091, 1982, the enhancements were regarded as plumes drawn out of the ionosphere of Titan by the corotation electric field. The secondary enhancements were taken to be old plumes that had been wrapped around Saturn, had begun to decay and to merge into the magnetosphere environment. An alternative interpretation, proposed by Goertz (GRL, 1983, 10, 455), viewed them as blobs of plasmas detached by flute or Kelvin-Helmholtz instability from the central body of Saturn plasma in the inner magnetosphere. The Voyager PLS instrument was unable to make a firm composition determination which would have resolved the question. In this study, we use Cassini/CAPS data to identify plumes and blobs of plasma and classify them by source by means of composition and temperature. We find that all three types of plasma bodies, primary plumes, secondary wrapped around plumes and sloughed off blobs exist in the Titan-dominated region of the magnetosphere.
P12A-10
3D Multi-fluid simulations of Titan crossing Saturn's magnetopause: comparison to T32 observations
During Cassini's T32 flyby, Titan was found in Saturn's magnetosheath. Though Titan was no longer inside the magnetosphere, mass loaded field lines of Kronian origin were found within Titan's ionosphere. This result was unexpected and indicates that Titan's interaction with solar wind plasma is affected in some way by remnant Kronian magnetic field lines, the existence of which are dependent on how recently Titan was inside Saturn's magnetosphere. We investigate the response of Titan's induced magnetosphere during a crossing of Saturn's magnetopause using both a 3D Multi-fluid coupled Saturn-Titan model and a local 3D Multi-fluid model of Titan. The 3D multi-fluid model allows us to simulate Saturn's entire magnetosphere as well as Titan's induced magnetosphere with ~460 km resolution. In this simulation we will initially place Titan within the magnetosphere and then increase the solar wind pressure until Saturn's magnetopause is forced past Titan, which will allow us to study how the coupling of Titan's ionosphere and Saturn's magnetosphere is affected on a large scale and over time by crossing Saturn's magnetopause. In addition, a local Titan simulation with resolution down to 25 km will be used to study the reaction of Titan's ionosphere to rapid changes in magnetic field strength and incident plasma composition and temperature. Results from the two models will be compared to each other as well as to Cassini magnetometer and CAPS data.