Aeronomy of Titan and Saturn: Recent Advances From Cassini/Huygens Observations III
Presiding: M Galand, Center for Space Physics, Boston University; I Mueller-Wodarg, Imperial College London
P34A-01 15:30h
The Vertical Structure of Titan's Upper Atmosphere
We present the analysis of data acquired by the Cassini Ion Neutral Mass Spectrometer (INMS) during the close flyby of Titan on 26 October (TA) and to be acquired during the close flyby on 15 April 2005 (T5). Closest approach to Titan for TA occurred at an altitude of 1174 km, a latitude of 38.6 degrees and a local time of 16.75 hours, i.e. quite close to the terminator. Closest approach for T5 is planned to occur at an altitude of 1025 km, a latitude of 74 degrees, and a local time of 23 hours, i.e. close to midnight. The data from TA indicate that the upper atmosphere is isothermal at a temperature of 148 K, the eddy diffusion coefficient is 5E9 cm2s-1, and the H2 escape flux is 6E9 cm-2s-1, referred to the surface. A similar analysis will be presented for T5 data.
P34A-02 15:45h
Model Calculations of Titan's Ionosphere
An ionosphere is created at Titan due to photoionization, by solar radiation, or due to Saturn's magnetospheric electrons which interact with the atmosphere. We have modeled the ionosphere of Titan and will present calculated electron and ion densities. These densities depend on both solar zenith angle and altitude. In our model we use neutral densities measured in Titan's atmosphere by the Cassini Ion and Neutral Mass Spectrometer (INMS). We also use electron temperatures measured by the Cassini RPWS (Langmuir Probe) experiment. The resulting electron densities are compared with the electron densities seen by the RPWS and with ion densities measured by the INMS.
P34A-03 16:00h
Cassini Ultraviolet Imaging Spectrograph Observations of Titan's Atmosphere
During the third Cassini encounter with Titan (TB), on December 13, 2004, the Ultraviolet Imaging Spectrograph (UVIS) conducted a series of observations of Titan's atmospheric structure and composition. Included were two stellar occultations (Shaula and Spica) and a series of imaging scans of the day and night side of the moon. Analysis of the far ultraviolet (125 - 190 nm) occultation of Shaula (Lambda Sco) has provided details of the vertical distribution of hydrocarbons in the range 450-1600 km, compared to that observed by Voyager (900-1200 km). The occultation probed the night side of Titan in the southern hemisphere. Six hydrocarbon species were identified from their absorptions in the stellar spectrum, CH4, C2H2, C2H4, C2H6, HCN, and C4H2. An iterative forward synthesis model of the Titan atmosphere was constructed including these absorbers, assuming a hemispherically symmetrical atmosphere, to deduce their distribution and an atmospheric temperature profile. We find a maximum temperature of 173 K at 450 km, a minimum of 114 K at 615 km and an isothermal region at 150 K above 1100 km. The UVIS derived temperature at the top of the atmosphere agrees within about 10% with the results of the in situ Ion Neutral Mass Spectrometer obtained during the second encounter with Titan (TA) on October 26, 2004. The transition from convective to diffusive separation occurs between 800 km and 1100 km. Additionally, we find a previously unobserved 80 km layer at 1350 km, showing abundances of CH4 and C2H4 a factor of ~5 above the ambient underlying distribution.
P34A-04 16:15h
Waves in Titan's thermosphere
Observations by Cassini's INMS instrument during the Titan A flyby on Oct 26, 2004 provided first in-situ observations of Titan's upper atmosphere between around 1075 and 1800 km altitude. From the observed densities of N2 and CH4 we derived thermospheric temperatures, which on average have an isothermal value of around 148 K. However, we also found strong perturbations around this mean with amplitudes of up to 10 K and wavelengths of between 60 and 220 km. Since the spacecraft during its flyby moved both horizontally and vertically, these perturbations can in principle be interpreted as horizontally or vertically propagating waves. We will discuss this issue and present a spectral analysis to characterize their global properties. A similar investigation will be presented for the T5 flyby in April 2005 and comparisons will be made. Finally, suggestions will be presented regarding the origin of these waves and their importance as sources of momentum and energy for the background atmosphere.
P34A-05 INVITED 16:30h
Huygens ASI Measurements At Titan: A New Insight Of Titan's Atmosphere
During the Huygens probe mission at Titan on 14th January 2005, the Huygens Atmospheric Structure Instrument (HASI) obtained measurements of atmospheric properties from up above 1400 km down to the ground, thus inferring the atmospheric structure. The atmospheric profile along the Huygens probe trajectory during entry phase have been retrieved from the accelerometers data, while below 160 km direct pressure and temperature measurements have been performed. The vertical temperature profile retrieved from HASI data is in very good agreement with the model derived from Voyager's observations, confirms the evidence for a stratopause and the inversion layers in the upper atmosphere as observed during stellar occultations and yielded new details on atmospheric structure.
P34A-06 16:45h
Confirmation of Our Predictions, Based on Laboratory and Theoretical Studies, on the Findings of the Huygens Probe on Titan
1. The very low abundances of Ar, Kr and Xe in Titan's atmosphere can be easily explained by our experimental findings. These gases are trapped in the aerosols, which are formed by UV photolysis of acetylene in their presence. When the aerosols fall down to the surface, they clean the atmosphere of these gases. A continuous supply of the radiogenic produced 40Ar from the interior can explain its small abundance in the atmosphere. 2. The originally soft and sticky photochemical aerosols, as found by us experimentally, were calculated to harden by spontaneous and radiation induces chemical cross-linking. Indeed the camera and other detectors were not covered by sticky aerosols and the intake ports were not clogged. 3. As we predicted, no lightning discharges were detected in the quiescent Titan atmosphere. Therefore, Titan's atmospheric chemistry is driven mainly by solar UV irradiation and not by electrical discharges. 4. The mixing ratios of the major gas phase species produced by UV photolysis of acetylene, as found experimentally: methylacetylene ; diacetylene ; divinyl ; and benzene were observed by the Cassini spacecraft in Titan's upper atmosphere, with an agreement within better than an order of magnitude. 5. The N:C ratio in Titan's aerosols was measured by the Huygens probe, but no results were published yet. UV photolysis of gas mixtures containing C2H2:HCN=10 yield aerosols with a ratio N:C=0.007 up to 0.01. Electrical discharges through a N2:CH4~10 gas mixtures yield a much higher N:C ratio. 6. We anticipate mountains not higher than 1900 m on Titan's surface.