V. Vuitton

The Nitrogen Chemistry of Titan’s Upper Atmosphere Revealed

Titans atmosphere is unique because dissociation of N2 and CH4, the primary atmospheric constituents, provides the H, C, and N atoms necessary for the synthesis of complex organic molecules. The first steps in the synthesis of organic molecules occur in the upper atmosphere where energetic photons and electrons dissociate N2 and CH4. We determine the abundance of a suite of nitrogen-bearing molecules in Titans upper atmosphere through analysis of measurements of the ionospheric composition made by the Ion Neutral Mass Spectrometer (INMS) on the Cassini spacecraft. We show that the density of ions in Titans upper atmosphere depends closely on the composition of the neutral atmosphere and that, for many species, measurement of associated ions coupled with simple chemical models provides the most sensitive determination of their abundance. With this technique we determine the densities of C2H4 , C4H2, HCN, HC3N, CH 3CN, NH3, C2H3CN, C2H 5CN, and CH2NH. The latter four species have not previously been detected in the gas phase on Titan, and none of these species have been accurately measured in the upper atmosphere. The presence of these species implies that nitrogen chemistry on Titan is more extensive than previously realized.

Formation and distribution of benzene on Titan

[1] We present a study of the formation and distribution of benzene (C6H6) on Titan. Analysis of the Cassini Mass Spectrometer (INMS) measurements of benzene densities on 12 Titan passes shows that the benzene signal exhibits an unusual time dependence, peaking � 20 s after closest approach, rather than at closest approach. We show that this behavior can be explained by recombination of phenyl radicals (C6H5) with H atoms on the walls of the instrument and that the measured signal is a combination of (1) C6H6 from the atmosphere and (2) C6H6 formed within the instrument. In parallel, we investigate Titan benzene chemistry with a set of photochemical models. A model for the ionosphere predicts that the globally averaged production rate of benzene by ion-molecule reactions is � 10 7 cm � 2 s �1 , of the same order of magnitude as the production rate by neutral reactions of � 4 � 10 6 cm �2 s �1 . We show that benzene is quickly photolyzed in the thermosphere, and that C6H5 radicals, the main photodissociation products, are � 3 times as abundant as benzene. This result is consistent with the phenyl/benzene ratio required to match the INMS observations. Loss of benzene occurs primarily through reaction of phenyl with other radicals, leading to the formation of complex aromatic species. These species, along with benzene, diffuse downward, eventually condensing near the tropopause. We find a total production rate of solid aromatics of � 10 � 15 gc m � 2 s �1 , corresponding to an accumulated surface layer of � 3m .

AN ESTIMATE OF THE CHEMICAL COMPOSITION OF TITAN's LAKES

Hundreds of radar-dark patches interpreted as lakes have been discovered in the north and south polar regions of Titan. We have estimated the composition of these lakes by using the direct abundance measurements from the Gas Chromatograph Mass Spectrometer aboard the Huygens probe and recent photochemical models based on the vertical temperature profile derived by the Huygens Atmospheric Structure Instrument. Thermodynamic equilibrium is assumed between the atmosphere and the lakes, which are also considered nonideal solutions. We find that the main constituents of the lakes are ethane (C2H6) (~76%-79%), propane (C3H8) (~7%-8%), methane (CH4) (~5%-10%), hydrogen cyanide (HCN) (~2%-3%), butene (C4H8) (~1%), butane (C4H10) (~1%), and acetylene (C2H2) (~1%). The calculated composition of lakes is then substantially different from what has been expected from models elaborated prior to the exploration of Titan by the Cassini-Huygens spacecraft.

Aerosol growth in Titan’s ionosphere

Photochemically produced aerosols are common among the atmospheres of our solar system and beyond. Observations and models have shown that photochemical aerosols have direct consequences on atmospheric properties as well as important astrobiological ramifications, but the mechanisms involved in their formation remain unclear. Here we show that the formation of aerosols in Titan’s upper atmosphere is directly related to ion processes, and we provide a complete interpretation of observed mass spectra by the Cassini instruments from small to large masses. Because all planetary atmospheres possess ionospheres, we anticipate that the mechanisms identified here will be efficient in other environments as well, modulated by the chemical complexity of each atmosphere.

Diurnal variations of Titan's ionosphere

of � 700 cm �3 below � 1300 km. Such a plateau is a combined result of significant depletion of light ions and modest depletion of heavy ones on Titan’s nightside. We propose that the distinctions between the diurnal variations of light and heavy ions are associated with their different chemical loss pathways, with the former primarily through ‘‘fast’’ ion-neutral chemistry and the latter through ‘‘slow’’ electron dissociative recombination. The strong correlation between the observed night-to-day ion density ratios and the associated ion lifetimes suggests a scenario in which the ions created on Titan’s dayside may survive well to the nightside. The observed asymmetry between the dawn and dusk ion density profiles also supports such an interpretation. We construct a time-dependent ion chemistry model to investigate the effect of ion survival associated with solid body rotation alone as well as superrotating horizontal winds. For long-lived ions, the predicted diurnal variations have similar general characteristics to those observed. However, for short-lived ions, the model densities on the nightside are significantly lower than the observed values. This implies that electron precipitation from Saturn’s magnetosphere may be an additional and important contributor to the densities of the short-lived ions observed on Titan’s nightside.

Formation of amino acids and nucleotide bases in a Titan atmosphere simulation experiment.

The discovery of large (>100 u) molecules in Titans upper atmosphere has heightened astrobiological interest in this unique satellite. In particular, complex organic aerosols produced in atmospheres containing C, N, O, and H, like that of Titan, could be a source of prebiotic molecules. In this work, aerosols produced in a Titan atmosphere simulation experiment with enhanced CO (N(2)/CH(4)/CO gas mixtures of 96.2%/2.0%/1.8% and 93.2%/5.0%/1.8%) were found to contain 18 molecules with molecular formulae that correspond to biological amino acids and nucleotide bases. Very high-resolution mass spectrometry of isotopically labeled samples confirmed that C(4)H(5)N(3)O, C(4)H(4)N(2)O(2), C(5)H(6)N(2)O(2), C(5)H(5)N(5), and C(6)H(9)N(3)O(2) are produced by chemistry in the simulation chamber. Gas chromatography-mass spectrometry (GC-MS) analyses of the non-isotopic samples confirmed the presence of cytosine (C(4)H(5)N(3)O), uracil (C(5)H(4)N(2)O(2)), thymine (C(5)H(6)N(2)O(2)), guanine (C(5)H(5)N(5)O), glycine (C(2)H(5)NO(2)), and alanine (C(3)H(7)NO(2)). Adenine (C(5)H(5)N(5)) was detected by GC-MS in isotopically labeled samples. The remaining prebiotic molecules were detected in unlabeled samples only and may have been affected by contamination in the chamber. 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, potentially increasing the range of planets where life could begin.

CRITICAL REVIEW OF N, N+, N-2(+), N++, And N-2(++) MAIN PRODUCTION PROCESSES AND REACTIONS OF RELEVANCE TO TITAN'S ATMOSPHERE

This paper is a detailed critical review of the production processes and reactions of N, N+, N+ 2, N++, and N++ 2 of relevance to Titans atmosphere. The review includes neutral, ion-molecule, and recombination reactions. The review covers all possible active nitrogen species under Titans atmospheric conditions, specifically N2 (A3Σ+ u), N (4 S), N (2 D), N (2 P), N+ 2, N+ (3 P), N+ (1 D), N++ 2, and N++ species, and includes a critical survey of the reactions of N, N+, N+ 2, N++, and N++ 2 with N2, H2, D2, CH4, C2H2, C2H4, C2H6, C3H8 and the deuterated hydrocarbon analogs, as well as the recombination reactions of N+ 2, N+, N++ 2, and N++. Production processes, lifetimes, and quenching by collisions with N2 of all reactant species are reviewed. The N (4 S) state is reactive with radicals and its reactions with CH2, CH3, C2H3, and C2H5 are reviewed. Metastable states N2(A3Σ+u), N (2 D), and N (2 P) are either reactive or quenched by collisions with the target molecules reviewed. The reactions of N+ (1 D) have similar rate constants as N+ (3 P), but the product branching ratios differ significantly. Temperature effects and the role of the kinetic energy content of reactants are investigated. In all cases, experimental uncertainties of laboratory data are reported or estimated. Recommended values with uncertainties, or estimated values when no data are available, are given for rate constants and product branching ratios at 300 K and at the atmospheric temperature range of Titan (150-200 K for neutral reactions and 150 K for ion reactions).

Ionization sources in Titan's deep ionosphere

We analyze a multi-instrumental data set from four Titan encounters by the Cassini spacecraft to investigate in detail the formation of the ionosphere. The data set includes observations of thermospheric and ionospheric species and suprathermal electrons. A model describing the solar and electron energy deposition is used as an organizing element of the Cassini data set. We first compare the calculated secondary electron production rates with the rates inferred from suprathermal electron intensity measurements. We then calculate an effective electron dissociative recombination coefficient, applying three different approaches to the Cassini data set. Our findings are threefold: (1) The effective recombination coefficient derived under sunlit conditions in the deep ionosphere (< 1200 km) is found to be independent of solar zenith angle and flyby. Its value ranges from 6.9 x 10(-7) cm(3) s(-1) at 1200 km to 5.9 x 10(-6) cm(3) s(-1) at 970 km at 500 K. (2) The presence of an additional, minor source of ionization is revealed when the solar contribution is weak enough. The contribution by this non-solar source-energetic electrons most probably of magnetospheric origin-becomes apparent for secondary electron production rates, due to solar illumination alone, close to or smaller than about 3 x 10(-1) cm(-3) s(-1). Such a threshold is reached near the solar terminator below the main solar-driven electron production peak (< 1050 km). (3) Our ability to model the electron density in the deep ionosphere is very limited. Our findings highlight the need for more laboratory measurements of electron dissociative recombination coefficients for heavy ion species at high electron temperatures (especially near 500 K).

RAPID ASSOCIATION REACTIONS AT LOW PRESSURE: IMPACT ON THE FORMATION OF HYDROCARBONS ON TITAN

Photochemical models of Titans atmosphere predict that three-body association reactions are the main production route for several major hydrocarbons. The kinetic rate constants of these reactions strongly depend on density and are therefore only important in Titans lower atmosphere. However, radiative association reactions do not depend on pressure. The possible existence of large rates at low density suggests that association reactions could significantly affect the chemistry of Titans upper atmosphere and better constraints for them are required. The kinetic parameters of these reactions are extremely difficult to constrain by experimental measurements as the low pressure of Titans upper atmosphere cannot be reproduced in the laboratory. However, in the recent years, theoretical calculations of kinetics parameters have become more and more reliable. We therefore calculated several radical-radical and radical-molecule association reaction rates using transition state theory. The calculations indicate that association reactions are fast even at low pressure for adducts having as few as four C atoms. These drastic changes have however only moderate consequences for Titans composition. Locally, mole fractions can vary by as much as one order of magnitude but the column-integrated production and condensation rates of hydrocarbons change only by a factor of a few. We discuss the impact of these results for the organic chemistry. It would be very interesting to check the impact of these new rate constants on other environments, such as giant and extrasolar planets as well as the interstellar medium.

Composition and chemistry of Titan's thermosphere and ionosphere.

Titan has long been known to harbour the richest atmospheric chemistry in the Solar System. Until recently, it had been believed that complex hydrocarbons and nitriles were produced through neutral chemistry that would eventually lead to the formation of micrometre sized organic aerosols. However, recent measurements by the Cassini spacecraft are drastically changing our understanding of Titans chemistry. The Ion and Neutral Mass Spectrometer (INMS) and the Cassini Plasma Spectrometer (CAPS) revealed an extraordinary complex ionospheric composition. INMS detected roughly 50 positive ions with m/z<100 and a density higher than 0.1 cm−3. CAPS provided evidence for heavy (up to 350 amu) positively and negatively charged (up to 4000 amu) ions. These observations all indicate that Titans ionospheric chemistry is incredibly complex and that molecular growth starts in the upper atmosphere rather than at lower altitude. Here, we review the recent progress made on ionospheric chemistry. The presence of heavy neutrals in the upper atmosphere has been inferred as a direct consequence of the presence of complex positive ions. Benzene (C6H6) is created by ion chemistry at high altitudes and its main photolysis product, the phenyl radical (C6H5), is at the origin of the formation of aromatic species at lower altitude.