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Deposition of Energy and Photochemistry for the generation of Titan's Haze

Final Report Summary - DEPTH (Deposition of Energy and Photochemistry for the generation of Titan's Haze)

Since 2004, when in-situ measurements of Titan’s atmosphere by the Cassini spacecraft and Huygens probe began, a wealth of information has been provided to the scientific community in particular about the neutral gas densities, atmospheric ions and the plasma environment surrounding Titan, characterized by the dynamical magnetic fields from Saturn and high energy plasma. Theoretical calculations that pre-dated Cassini/Huygens gave a basic understanding of Titan’s upper atmosphere that assumed solar radiation forcing to be a key energy source on Titan but also discovered that energetic plasma from Saturn’s magnetosphere might potentially constitute a major source of energy as well. The exact balance of these two sources has since remained the focus of global scientific efforts that have followed since Titan’s detailed exploration by Cassini/Huygens. Unexpected discoveries of heavy neutral molecules as well as heavy positive and negative ions have added complexity to our understanding of Titan. In order to understand the ionospheric processes leading to macromolecular ions, complex photochemical models of Titan's ionospheric chemistry have to be developed. The models require ion molecule reaction rates and product branching ratios that are determined by laboratory experiments. However, kinetic data are often outdated or even lacking, especially when it comes to negative ions.
Our project aims in the broadest sense to understand the energy balance on Titan and the processes that result from energy deposition, including chemical changes, thermal heating, dynamics and global transport of neutral and ionized gases. We use a unique combination of models as well as data from a variety of instruments on board Cassini and in the laboratory to achieve our scientific goals. Our team uniquely combines expertise in analytical chemistry, kinetics and dynamics of ion-molecule reactions, photochemical modeling, kinetic modeling as well as global fluid modeling, which are normally addressed by different scientific communities. Furthermore, our team members have direct links, formal and informal, to all the Cassini instrument teams that are relevant to the science objectives.

Since the start of the project we have successfully investigated a range of key topics outlined in the following. Key results have after 4 years of concentrated efforts now been obtained and we intend to publish all our remaining results in the coming months.
1. We used very high-resolution mass spectrometers (FT-ICR in Tucson and Orbitrap in Grenoble) equipped with several ionization sources (Electrospray Ionization and Laser Desorption) to perform a structural analysis of the soluble fraction of tholins. We focused on negatively charged ions that had not been studied before by very high resolution MS and MS/MS. Their chemical composition indicates the presence of highly unsaturated (H/C < 1) species with high nitrogen content, which is characteristically different from the previously analyzed positively charged ions that are more saturated.

2. Based on tandem MS/MS experiments on tholins (item 1) and quantum chemical calculations, we proposed characteristic structural features for selected negative ions. Several structural "families" can be derived as "adducts" on the basic non-aromatic structural unit C2N3-. This relied on a preliminary research activity consisting in studying the fragmentation induced by collision of a series of 25 standard molecules, which were chosen for their similarities with the expected tholins. This work was successful as it showed very high sensitivity to structure, high reproducibility, and the possibility to interpret unknown structures from blind tests.

3. Modification of the Orbitrap instrument in order to add a reactivity cell in the system. This modification started with an experimental study of the complete electrostatic system of the Orbitrap instrument. From this study, a proof of feasibility of reactivity scheme was established, and mechanical developments, as well as instrumentation for gas mixture production were purchased and implemented. This study demonstrated that the Atmospheric Pressure Ionization source of the instrument (APCI) was more appropriate to produce large quantities of negatively charged ions of the type XCN- (where X stands for hydrocarbons).

4. The presence of oxygen species in Titan's atmosphere could be explained by the injection of O+ cations from Enceladus. We studied the reactions of state-selected O+(4S, 2D, 2P) with methane at selected collision energies, on the CERISES setup with VUV radiation delivered by the DESIRS beamline at the French synchrotron SOLEIL. We observed a reduction of the total cross section and a complete inversion of the branching ratio between the main products (CH4+ and CH3+) with increasing electronic excitation of the O+ parent ion.

5. We studied the interaction with VUV photons of nitrogen-containing negative ions that are possible candidates for the heavy anions observed by the CAPS/ELS spectrometer. The parent anions are produced as described in item (3), mass-selected and irradiated in a linear ion trap at the DESIRS beamline (see item (4)). The objective was to measure their transformation into smaller anions through photodissociation and their destruction through photodetachment.

6. Laboratory synthesized analogs of the haze surrounding Titan, the so-called « tholins », were produced in plasmas of nitrogen/methane/carbon monoxide mixtures. 18 molecules with molecular formulae that correspond to biological amino acids and nucleotide bases were found by very high-resolution mass spectrometry and gas chromatography-mass spectrometry analyses confirmed the presence of 7 of them. These results demonstrate that prebiotic molecules can be formed by the high-energy chemistry similar to that which occurs in planetary upper atmospheres and therefore identifies a new source of prebiotic material.

7. We reported the first extended high-resolution mass spectrometry study of HCN polymers, which constitute a fraction of tholins. Thanks to an automated software made in house, an elemental composition could be assigned to hundreds of peaks. The presence of amine groups in most chains was confirmed by infrared spectroscopy.

8. Development of a coupled ion-neutral photochemical model including hydrocarbons as well as nitrogen and oxygen species. The model allowed us unraveling the formation processes of ammonia and hydrogen isocyanide, which had remained elusive so far. For the first time, radiative association reactions were considered and we showed that they can significantly impact the concentration of hydrocarbons in the upper atmosphere.

9. Investigation of the observed structure and variability of Titan’s dayside ionosphere. Using a solar radiation and electron energy deposition model developed in-house as well as the latest analysis of observed neutral gas and ionospheric densities, we significantly advanced our understanding of Titan’s dayside ionosphere. We calculated dayside ionospheric production rates and successful correlated the theoretical production rate peak altitude with the peak altitude of the electron density profiles observed by the Cassini spacecraft, the first time ever that this has been done successfully for Titan.

10. Through detailed Cassini dataset analysis and comparison with laboratory measurements we have successfully investigated the electron loss rates in Titan’s ionosphere. The increasing mixing ratio of negative ions towards lower altitudes is explained by electron attachment to neutrals. The positive ions become longer-lived as the rate coefficients for ion-anion neutralization reactions are smaller than those for ion-electron dissociative recombination reactions. These investigations have confirmed a previously proposed increase in organic ion density and complexity with lower altitudes (item 14).

11. A particular feature of the density observations on Titan is a pronounced variability in densities of gases such as N2 and CH4. Using our Titan General Circulation Model we have now successfully reproduced the range of variability and shown that this could be driven by thermospheric global wind structures, which in turn cause vertical transport of gases and hence cause the observed variability. This is the first time that such a study has been undertaken for Titan.

12. From our results under (13) we calculated the effect of atmospheric haze on the density structure above 1000 km on Titan and successfully managed to reproduce the observed N2 and CH4 density trend with our calculations, suggesting that processes between 500-800 km on Titan have a fundamental effect on densities above. This is the first time that link has been established.

13. The Cassini INMS instrument over the past 10 years has observed a decrease in thermospheric neutral densities overall. Using our General Circulation Model in conjunction with latest analyses of Cassini UVIS measurements of haze opacity on Titan, we calculated the effect of atmospheric haze on the vertical temperature structure and successfully compared this with observations.

14. Items (6) and (13) show that photochemical aerosols have important astrobiological ramifications as well as direct consequences on atmospheric properties. In order to investigate the general processes underlying their nucleation and growth from atmospheric gases, we developed a model that couples the aerosol microphysics and the photochemistry in a self-consistent manner. We showed that the formation of the aerosols is directly related to ion processes and provide a complete interpretation of observed mass spectra by the Cassini instruments from small to large masses
Over the duration of this project we have addressed a large number questions and gained a considerably improved understanding of Titan as a representative of small objects with atmospheres in the outer solar system. Such work helps us better understand Earth and its atmosphere as well, it will help us improve predictions for comparable bodies that haven’t yet been explored in detail, such as Neptune’s moon Triton and also Pluto, a body which will be visited for the first time ever by the New Horizons spacecraft in 2016. The improved understanding of atmospheres within our solar system will also help us better predict the behavior of those found on extrasolar planets, which have become the focus of considerable scientific effort over the past decade, a trend which is expected to further increase over years to come, putting us into an ideal position for further science contributions in many diverse areas, and for European leadership in some of those areas.