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Register a new userImpact of aerosols present in Titans atmosphere on the CASSINI radar experiment
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Added by
Sebastien Rodriguez
Publication date
2009
fields
Physics
and research's language is
English
Authors
S. Rodriguez
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Simulations of Titans atmospheric transmission and surface reflectivity have been developed in order to estimate how Titans atmosphere and surface properties could affect performances of the Cassini radar experiment. In this paper we present a selection of models for Titans haze, vertical rain distribution, and surface composition implemented in our simulations. We collected dielectric constant values for the Cassini radar wavelength ($sim 2.2$ cm) for materials of interest for Titan: liquid methane, liquid mixture of methane-ethane, water ice and light hydrocarbon ices. Due to the lack of permittivity values for Titans haze particles in the microwave range, we performed dielectric constant ($varepsilon_r$) measurements around 2.2 cm on tholins synthesized in laboratory. We obtained a real part of $varepsilon_r$ in the range of 2-2.5 and a loss tangent between $10^{-3}$ and $5.10^{-2}$. By combining aerosol distribution models (with hypothetical condensation at low altitudes) to surface models, we find the following results: (1) Aerosol-only atmospheres should cause no loss and are essentially transparent for Cassini radar, as expected by former analysis. (2) However, if clouds are present, some atmospheric models generate significant attenuation that can reach $-50 dB$, well below the sensitivity threshold of the receiver. In such cases, a $13.78 GHz$ radar would not be able to measure echoes coming from the surface. We thus warn about possible risks of misinterpretation if a textquotedblleft wet atmospheretextquotedblright $ $is not taken into account. (3) Rough surface scattering leads to a typical response of $sim -17 dB$. These results will have important implications on future Cassini radar data analysis.
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We have searched for the presence of simple P and S-bearing molecules in Titans atmosphere, by looking for the characteristic signatures of phosphine and hydrogen sulfide in infrared spectra obtained by Cassini CIRS. As a result we have placed the first upper limits on the stratospheric abundances, which are 1 ppb (PH3) and 330 ppb (H2S), at the 2-sigma significance level.
In this paper we select large spectral averages of data from the Cassini Composite Infrared Spectrometer (CIRS) obtained in limb-viewing mode at low latitudes (30S--30N), greatly increasing the path length and hence signal-to-noise ratio for optically thin trace species such as propane. By modeling and subtracting the emissions of other gas species, we demonstrate that at least six infrared bands of propane are detected by CIRS, including two not previously identified in Titan spectra. Using a new line list for the range 1300-1400cm -1, along with an existing GEISA list, we retrieve propane abundances from two bands at 748 and 1376 cm-1. At 748 cm-1 we retrieve 4.2 +/- 0.5 x 10(-7) (1-sigma error) at 2 mbar, in good agreement with previous studies, although lack of hotbands in the present spectral atlas remains a problem. We also determine 5.7 +/- 0.8 x 10(-7) at 2 mbar from the 1376 cm-1 band - a value that is probably affected by systematic errors including continuum gradients due to haze and also an imperfect model of the n6 band of ethane. This study clearly shows for the first time the ubiquity of propanes emission bands across the thermal infrared spectrum of Titan, and points to an urgent need for further laboratory spectroscopy work, both to provide the line positions and intensities needed to model these bands, and also to further characterize haze spectral opacity. The present lack of accurate modeling capability for propane is an impediment not only for the measurement of propane itself, but also for the search for the emissions of new molecules in many spectral regions.
Cassini-Huygens data are used to re-examine the potential sources of the D/H enhancement over solar, measured in methane, in Titans atmosphere. Assuming that the system is closed with respect to carbon, the use of constraints from the Huygens probe for the determination of the current mass of atmospheric methane and the most up-to-date determination of D/H from Cassini/CIRS infrared spectra allow us to show that photochemical enrichment of deuterium is not sufficient to be the sole mechanism yielding the measured D/H value. A possible fractionation between CH3D and CH4 during the escape process may slightly enhance the deuterium enrichment, but is not sufficient to explain the observed D/H value over the range of escape values proposed in the literature. Hence, alternative mechanisms such as a primordial deuterium enrichment must be combined with the photochemical enrichment in Titans atmosphere in order to explain its current D/H value.
The ionosphere of Titan hosts a complex ion chemistry leading to the formation of organic dust below 1200 km. Current models cannot fully explain the observed electron temperature in this dusty environment. To achieve new insight, we have re-analyzed the data taken in the ionosphere of Titan by the Cassini Langmuir probe (LP), part of the Radio and Plasma Wave Science package. A first paper (Chatain et al., 2021) introduces the new analysis method and discusses the identification of 4 electron populations produced by different ionization mechanisms. In this second paper, we present a statistical study of the whole LP dataset below 1200 km which gives clues on the origin of the 4 populations. One small population is attributed to photo- or secondary electrons emitted from the surface of the probe boom. A second population is systematically observed, at a constant density (~500 cm-3), and is attributed to background thermalized electrons from the ionization process of precipitating particles fom the surrounding magnetosphere. The two last populations increase in density with pressure, solar illumination and EUV flux. The third population is observed with varying densities at all altitudes and solar zenith angles except on the far nightside (SZA > ~140{deg}), with a maximum density of 2700 cm-3. It is therefore certainly related to the photo-ionization of the atmospheric molecules. Finally, a fourth population detected only on the dayside and below 1200 km reaching up to 2000 cm-3 could be photo- or thermo-emitted from dust grains.
Formation of hazes at microbar pressures has been explored by theoretical models of exoplanet atmospheres to explain Rayleigh scattering and/or featureless transmission spectra, however observational evidence of aerosols in the low pressure formation environments has proved elusive. Here, we show direct evidence of aerosols existing at $sim$1 microbar pressures in the atmosphere of the warm sub-Saturn WASP-69b using observations taken with Space Telescope Imaging Spectrograph (STIS) and Wide Field Camera 3 (WFC3) instruments on the Hubble Space Telescope. The transmission spectrum shows a wavelength-dependent slope induced by aerosol scattering that covers 11 scale heights of spectral modulation. Drawing on the extensive studies of haze in our Solar System, we model the transmission spectrum based on a scaled version of Jupiters haze density profile to show that WASP-69b transmission spectrum can be produced by scattering from an approximately constant density of particles extending throughout the atmospheric column from 40 millibar to microbar pressures. These results are consistent with theoretical expectations based on microphysics of the aerosol particles that have suggested haze can exist at microbar pressures in exoplanet atmospheres.
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