Titan's Upper Atmosphere Research Articles

Titan's exosphere and its interaction with Saturn's magnetosphere

Titan's nitrogen-rich atmosphere is directly bombarded by energetic ions, due to its lack of a significant intrinsic magnetic field. Singly charged energetic ions from Saturn's magnetosphere undergo charge-exchange collisions with neutral atoms in Titan's upper atmosphere, or exosphere, being transformed into energetic neutral atoms (ENAs). The ion and neutral camera, one of the three sensors that comprise the magnetosphere imaging instrument (MIMI) on the Cassini/Huygens mission to Saturn and Titan, images these ENAs like photons, and measures their fluxes and energies. These remote-sensing measurements, combined with the in situ measurements performed in the upper thermosphere and in the exosphere by the ion and neutral mass spectrometer instrument, provide a powerful diagnostic of Titan's exosphere and its interaction with the Kronian magnetosphere. These observations are analysed and some of the exospheric features they reveal are modelled.

Sputtering and heating of Titan's upper atmosphere

Titan is an important endpoint for understanding atmospheric evolution. Prior to Cassini's arrival at Saturn, modelling based on Voyager data indicated that the hydrogen escape rate was large (1-3x1028amus-1), but the escape rates for carbon and nitrogen species were relatively small (5x1026amus-1) and dominated by atmospheric sputtering. Recent analysis of the structure of Titan's thermosphere and corona attained from the Ion and Neutral Mass Spectrometer and the Huygens Atmospheric Structure Instrument on Cassini have led to substantially larger estimates of the loss rate for heavy species (0.3-5x1028amus-1). At the largest rate suggested, a mass that is a significant fraction of the present atmosphere would have been lost to space in 4Gyr; hence, understanding the nature of the processes driving escape is critical. The recent estimates of neutral escape are reviewed here, with particular emphasis on plasma-induced sputtering and heating. Whereas the loss of hydrogen is clearly indicated by the altitude dependence of the H2 density, three different one-dimensional models were used to estimate the heavy-molecule loss rate using the Cassini data for atmospheric density versus altitude. The solar heating rate and the nitrogen density profile versus altitude were used in a fluid dynamic model to extract an average net upward flux below the exobase; the diffusion of methane through nitrogen was described below the exobase using a model that allowed for outward flow; and the coronal structure above the exobase was simulated by presuming that the observed atmospheric structure was due to solar- and plasma-induced hot particle production. In the latter, it was hypothesized that hot recoils from photochemistry or plasma-ion-induced heating were required. In the other two models, the upward flow extracted is driven by heat conduction from below, which is assumed to continue to act above the nominal exobase, producing a process referred to as 'slow hydrodynamic' escape. These models and the resulting loss rates are reviewed and compared. It is pointed out that preliminary estimates of the composition of the magnetospheric plasma at Titan's orbit appear to be inconsistent with the largest loss rates suggested for the heavy species, and the mean upward flow extracted in the one-dimensional models could be consistent with atmospheric loss by other mechanisms or with transport to other regions of Titan's atmosphere.

Horizontal structures and dynamics of Titan's thermosphere

The Cassini Ion Neutral Mass Spectrometer (INMS) measures densities of gases including N2 and CH4 in situ in Titan's upper atmosphere. We have used data from 13 targeted flybys of Titan (T5–T32) to construct an empirical model that describes the mean state of the thermosphere in the northern hemisphere, giving the N2 and CH4 densities between 1000 and 1600 km as a function of latitude and height. The principal features in the INMS data are well reproduced by this simple model. We find a pronounced oblateness in the thermosphere, with densities above 1100 km altitude increasing by around 70% from the northern (winter) pole to the equator, resulting in isobaric surfaces being ≃45 km higher over the equator than at the northern pole. Thermospheric temperatures derived from the densities tend to decrease with height from 149 ± 10 K to 140 ± 13 K near 1600 km. Considerable latitude differences are present in the temperatures below 1200 km. Near 1000 km altitude, temperatures reach 164 ± 6 K at 20°N and 131 ± 6 K near 80°N. Using our Thermosphere General Circulation Model with this thermal structure imposed, we derive thermospheric horizontal wind speeds reaching ∼150 m s−1, with primarily poleward flow at equatorial latitudes which, northward of around 60°N, is accompanied by a band of prograde zonal winds of up to 50 m s−1. At high latitudes, converging horizontal winds generate regions of strong subsistence. We find thermospheric dynamics to be sensitive to coupling from below. CH4 abundances are enhanced in the northern polar region, which may result from transport by thermospheric winds.

Coupled ion and neutral rotating model of Titan's upper atmosphere

A one-dimensional composition model of Titan's upper atmosphere is constructed, coupling 36 neutral species and 47 ions. Energy inputs from the Sun and from Saturn's magnetosphere and updated temperature and eddy coefficient parameters are taken into account. A rotating technique at constant latitude and varying local-time is proposed to account for the diurnal variation of solar inputs. The contributions of photodissocation, neutral chemistry, ion–neutral chemistry, and electron recombination to neutral production are presented as a function of altitude and local time. Local time-dependent mixing ratio and density profiles are presented in the context of the TA and T5 Cassini data and are compared in detail to previous models. An independent and simplified ion and neutral scheme (19-species) is also proposed for future 3D-purposes. The model results demonstrate that a complete understanding of the chemistry of Titan's upper atmosphere requires an understanding of the coupled ion and neutral chemistry. In particular, the ionospheric chemistry makes significant contributions to production rates of several important neutral species.

The source of heavy organics and aerosols in Titan's atmosphere

AbstractIon-neutral chemistry in Titan's upper atmosphere (~ 1000 km altitude) is an unexpectedly prodigious source of hydrocarbon-nitrile compounds. We report observations from the Cassini Ion Neutral Mass Spectrometer (INMS; Waite et al. 2004) and Cassini Plasma Spectrometer (CAPS; Young et al. 2004) that allow us to follow the formation of the organic material from the initial ionization and dissociation of nitrogen and methane driven by several free energy sources (extreme ultraviolet radiation and energetic ions and electrons) to the formation of negative ions with masses exceeding 10,000 amu.

Titan's hydrodynamically escaping atmosphere

The upper atmosphere of Titan is currently losing mass at a rate ∼ ( 4 – 5 ) × 10 28 amu s −1 , by hydrodynamic escape as a high density, slow outward expansion driven principally by solar UV heating by CH 4 absorption. The hydrodynamic mass loss is essentially CH 4 and H 2 escape. Their combined escape rates are restricted by power limitations from attaining their limiting rates (and limiting fluxes). Hence they must exhibit gravitational diffusive separation in the upper atmosphere with increasing mixing ratios to eventually become major constituents in the exosphere. A theoretical model with solar EUV heating by N 2 absorption balanced by HCN rotational line cooling in the upper thermosphere yields densities and temperatures consistent with the Huygens Atmospheric Science Investigation (HASI) data [Fulchignoni, M., and 42 colleagues, 2005. Nature 438, 785–791], with a peak temperature of ∼185–190 K between 3500–3550 km. This model implies hydrodynamic escape rates of ∼ 2 × 10 27 CH 4 s −1 and 5 × 10 27 H 2 s −1 , or some other combination with a higher H 2 escape flux, much closer to its limiting value, at the expense of a slightly lower CH 4 escape rate. Nonthermal escape processes are not required to account for the loss rates of CH 4 and H 2, inferred by the Cassini Ion Neutral Mass Spectrometer (INMS) measurements [Yelle, R.V., Borggren, N., de la Haye, V., Kasprzak, W.T., Niemann, H.B., Müller-Wodarg, I., Waite Jr., J.H., 2006. Icarus 182, 567–576].

Ion chemistry and N-containing molecules in Titan's upper atmosphere

High-energy photons, electrons, and ions initiate ion–neutral chemistry in Titan's upper atmosphere by ionizing the major neutral species (nitrogen and methane). The Ion and Neutral Mass Spectrometer (INMS) onboard the Cassini spacecraft performed the first composition measurements of Titan's ionosphere. INMS revealed that Titan has the most compositionally complex ionosphere in the Solar System, with roughly 50 ions at or above the detection threshold. Modeling of the ionospheric composition constrains the density of minor neutral constituents, most of which cannot be measured with any other technique. The species identified with this approach include the most complex molecules identified so far on Titan. This confirms the long-thought idea that a very rich chemistry is actually taking place in this atmosphere. However, it appears that much of the interesting chemistry occurs in the upper atmosphere rather than at lower altitudes. The species observed by INMS are probably the first intermediates in the formation of even larger molecules. As a consequence, they affect the composition of the bulk atmosphere, the composition and optical properties of the aerosols and the flux of condensable material to the surface. In this paper, we discuss the production and loss reactions for the ions and how this affects the neutral densities. We compare our results to neutral densities measured in the stratosphere by other instruments, to production yields obtained in laboratory experiments simulating Titan's chemistry and to predictions of photochemical models. We suggest neutral formation mechanisms and highlight needs for new experimental and theoretical data.

Cassini Ion and Neutral Mass Spectrometer data in Titan's upper atmosphere and exosphere: Observation of a suprathermal corona

The neutral nitrogen and methane measurements made by Ion and Neutral Mass Spectrometer during Cassini flybys TA, TB, and T5 in Titan's upper atmosphere and exosphere are presented. Large horizontal variations are observed in the total density, recorded to be twice as large during TA as during T5. Comparison between the atmospheric and exospheric data show evidence for the presence of a significant population of suprathermal molecules. Using a diffusion model to simultaneously fit the N2 and CH4 density profiles below 1500 km, the atmospheric structure parameters are determined, taking into account recent changes in the calibration parameters. The best fits are obtained for isothermal profiles with values 152.8 ± 4.6 K for TA, 149.0 ± 9.2 K for TB, and 157.4 ± 4.9 K for T5, suggesting a temperature ≃5 K warmer at night than at dusk, a trend opposite to that determined by solar‐driven models. Using standard exospheric theory and a Maxwellian exobase distribution, a temperature of 20 to 70 K higher would be necessary to fit the TA, TB, and egress‐T5 data above 1500 km. The suprathermal component of the corona was fit with various exobase energy distributions, using a method based on the Liouville theorem. This gave a density of suprathermals at the exobase of 4.4 ± 5.1 × 105 cm−3 and 1.1 ± 0.9 × 105 cm−3, and an energy deposition rate at the exobase of 1.1 ± 0.9 × 102 eV cm−3 s−1 and 3.9 ± 3.5 × 101 eV cm−3 s−1 for the hot N2 and CH4 populations, respectively. The energy deposition rate allowed us to roughly estimate escape rates for nitrogen of ≃7.7 ± 7.1 × 107 N cm−2 s−1 and for methane of ≃2.8 ± 2.1 × 107 CH4 cm−2 s−1. Interestingly, no suprathermal component was observed in the ingress‐T5 data.

Titan's corona: The contribution of exothermic chemistry

The contribution of exothermic ion and neutral chemistry to Titan's corona is studied. The production rates for fast neutrals N 2, CH 4, H, H 2, 3CH 2, CH 3, C 2H 4, C 2H 5, C 2H 6, N( 4S), NH, and HCN are determined using a coupled ion and neutral model of Titan's upper atmosphere. After production, the formation of the suprathermal particles is modeled using a two-stream simulation, as they travel simultaneously through a thermal mixture of N 2, CH 4, and H 2. The resulting suprathermal fluxes, hot density profiles, and energy distributions are compared to the N 2 and CH 4 INMS exospheric data presented in [De La Haye, V., Waite Jr., J.H., Johnson, R.E., Yelle, R.V., Cravens, T.E., Luhmann, J.G., Kasprzak, W.T., Gell, D.A., Magee, B., Leblanc, F., Michael, M., Jurac, S., Robertson, I.P., 2007. J. Geophys. Res., doi:10.1029/2006JA012222, in press], and are found insufficient for producing the suprathermal populations measured. Global losses of nitrogen atoms and carbon atoms in all forms due to exothermic chemistry are estimated to be 8.3 × 10 24 N s −1 and 7.2 × 10 24 C s −1 .

The Process of Tholin Formation in Titan's Upper Atmosphere

Titan's lower atmosphere has long been known to harbor organic aerosols (tholins) presumed to have been formed from simple molecules, such as methane and nitrogen (CH4 and N2). Up to now, it has been assumed that tholins were formed at altitudes of several hundred kilometers by processes as yet unobserved. Using measurements from a combination of mass/charge and energy/charge spectrometers on the Cassini spacecraft, we have obtained evidence for tholin formation at high altitudes (approximately 1000 kilometers) in Titan's atmosphere. The observed chemical mix strongly implies a series of chemical reactions and physical processes that lead from simple molecules (CH4 and N2) to larger, more complex molecules (80 to 350 daltons) to negatively charged massive molecules (approximately 8000 daltons), which we identify as tholins. That the process involves massive negatively charged molecules and aerosols is completely unexpected.

Role of photoionization in the formation of complex organic molecules in Titan's upper atmosphere

Recent observations by the Cassini Orbiter suggest that complex organic chemistry occurs in the upper atmosphere of Titan. To understand the role of EUV‐VUV light in the complex organic chemistry of Titan's upper atmosphere, we investigate the formation of gaseous species from a N2/CH4 gas mixture as a function of irradiation wavelengths from 50 nm to 150 nm. A N2/CH4 gas mixture (= 95/5) at 0.066 mbar in a windowless photocell is irradiated using a narrowband synchrotron radiation source. The analysis of gaseous products by quadrupole mass spectrometry reveals the formation of heavy organics up to C7 to C8 by EUV light irradiation. In particular, the efficient formation of benzene and toluene is observed at wavelengths less than 80 nm, which is well correlated to photoionization of N2. In Titan's upper atmosphere, EUV radiation may play an important role in the formation of complex organic molecules through photoionization of N2.

Electron temperature of Titan's sunlit ionosphere

Titan's upper atmosphere is ionized by solar radiation and particle bombardment from Saturn's magnetosphere. The induced ionosphere plays a key role in the coupling of Titan's atmosphere with the Kronian environment. It also provides unique signatures for identifying energy sources upon Titan's upper atmosphere. Here we focus on observations from the first, close flyby by the Cassini spacecraft and assess the ionization and electron heating sources in Titan's sunlit ionosphere. We compare CAPS electron spectra with spectra produced by an electron transport model based on the INMS neutral densities and a MHD interaction model. In addition, we compare RPWS electron temperature against the models. The important terms in the electron energy equation include loss through excitation of vibrational states of N2 and CH4, Coulomb collisions with suprathermal electrons, and thermal conduction. Our analysis highlights the important role of the magnetic field line configuration for aeronomic studies at Titan.

The Nitrogen Chemistry of Titan's Upper Atmosphere Revealed

Titan's 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 Titan's 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 Titan's 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.

A tidal explanation for the Titan haze layers

Though Titan is in synchronous rotation around Saturn, it experiences gravitational tides as a consequence of its eccentric orbit. It is proposed that the vertical transport of aerosols by these tides produces the haze layers in Titan's upper atmosphere. Analysis shows that the zonal winds in Titan's superrotating atmosphere have a profound influence on which tidal components are effective in establishing the multiple detached-haze layers. If the Huygens Doppler winds are representative of the equatorial global superrotation, then the westward propagating s = 2 mode is the responsible tidal component even though its forcing is significantly weaker than that of the s = 0 and eastward s = 2 components. The eastward s = 2 tidal mode is eliminated by critical levels while the s = 0 mode is viscously damped in the strong high altitude winds. At polar latitudes, however, the gravest s = 0 mode is the one most likely to produce layering. It is also suggested that the atmospheric gravitational tides could be responsible for decelerating the superrotating atmosphere as seen in the Huygens Doppler wind velocity profile at about 80 km altitude.

The vertical structure of Titan's upper atmosphere from Cassini Ion Neutral Mass Spectrometer measurements

Data acquired by the Ion Neutral Mass Spectrometer (INMS) on the Cassini spacecraft during its close encounter with Titan on 26 October 2004 reveal the structure of its upper atmosphere. Altitude profiles of N 2, CH 4, and H 2, inferred from INMS measurements, determine the temperature, vertical mixing rate, and escape flux from the upper atmosphere. The mean atmospheric temperature in the region sampled by the INMS is 149 ± 3 K , where the variance is a consequence of local time variations in temperature. The CH 4 mole fraction at 1174 km is 2.71 ± 0.1 % . The effects of diffusive separation are clearly seen in the data that we interpret as an eddy diffusion coefficient of 4 −3 +4 × 10 9 cm 2 s −1 , that, along with the measured CH 4 mole fraction, implies a mole fraction in the stratosphere of 2.2 ± 0.2 % . The H 2 distribution is affected primarily by upward flow and atmospheric escape. The H 2 mole fraction at 1200 km is 4 ± 1 × 10 −3 and analysis of the altitude profile indicates an upward flux of 1.2 ± 0.2 × 10 10 cm −2 s −1 , referred to the surface. If horizontal variations in temperature and H 2 density are small, this upward flux also represents the escape flux from the atmosphere. The CH 4 density exhibits significant horizontal variations that are likely an indication of dynamical processes in the upper atmosphere.

Energy deposition of pickup ions and heating of Titan's atmosphere

Abstract The deposition of energy, escape of atomic and molecular nitrogen and heating of the upper atmosphere of Titan are studied using a Direct Simulation Monte Carlo method. It is found that the globally averaged flux of deflected magnetospheric atomic nitrogen ions and molecular pickup ions deposit more energy in Titan's upper atmosphere than solar radiation. The energy deposition in this region determines the atmospheric loss and the production of the nitrogen neutral torus. The temperature structure near the exobase is also calculated. It is found that, due to the inclusion of the molecular pickup ions more energy is deposited closer to the exobase than assumed in earlier plasma ion heating calculations. Although the temperature at the exobase is only a few degrees larger than it is at depth, the density above the exobase is enhanced by the incident plasma.

Extraordinary Cluster Formation and Intramolecular Ligand−Ligand Interactions in Cyanoactylene Mediated by Mg+•: Implications for the Atmospheric Chemistry of Titan and for Circumstellar Chemistry

Experimental results are reported that track the kinetics of gas-phase reactions initiated by Mg+*, (c-C5H5)Mg+ and (c-C5H5)2Mg+* in hydrogen cyanide and cyanoacetylene. The experiments were performed with a selected-ion flow tube (SIFT) tandem mass spectrometer at a helium buffer-gas pressure of 0.35 +/- 0.01 Torr and at 294 +/- 3 K. The observed chemistries of Mg+* and (c-C5H5)Mg+ are dominated by sequential ligation, while that of (c-C5H5)2Mg+* is by ligand switching. The rate-coefficient measurements for sequential addition of cyanoacetylene to Mg+* indicate an extraordinary pattern in alternating chemical reactivity while multiple-collision induced dissociation experiments revealed an extraordinary stability for the Mg(HC3N)4+* cluster ion. Molecular orbital calculations with density functional theory (DFT) at the B3LYP level, Hartree-Fock (HF) and second-order Mphiller-Plesset (MP2) levels, all performed with a 6-31+G(d) basis set, have been used to calculate structures and energies for the observed Mg(HC3N)1-4(+)* cations. These calculations indicate that the path of formation of Mg(HC3N)4+* involves ligand-ligand interactions leading to two cyclic (HC3N)2 ligands which then interact to form 2,4,6,8-tetracyanosemibullvalene-Mg+ or 1,2,5,6-tetracyano-1,3,5,7-cyclooctatetraene-Mg+ cations. A case is made for the formation of similar complex organomagnesium ions in the upper atmosphere of Titan where subsequent electron-ion recombination may produce cyano derivatives of large unsaturated hydrocarbons. In contrast, circumstellar environments with their much higher relative content of free electrons are less likely to give rise to such chemistry.

Temperature variations in Titan's upper atmosphere: Impact on Cassini/Huygens

Abstract. Temperature variations of Titan's upper atmosphere due to the plasma interaction of the satellite with Saturn's magnetosphere and Titan's high altitude monomer haze particles can imply an offset of up to ±30K from currently estimated model profiles. We incorporated these temperature uncertainties as an offset into the recently published Vervack et al. (2004) (Icarus, Vol. 170, 91-112) engineering model and derive extreme case (i.e. minimum and maximum profiles) temperature, pressure, and density profiles. We simulated the Huygens probe hypersonic entry trajectory and obtain, as expected, deviations of the probe trajectory for the extreme atmosphere models compared to the simulation based on the nominal one. These deviations are very similar to the ones obtained with the standard Yelle et al. (1997) (ESA SP-1177) profiles. We could confirm that the difference in aerodynamic drag is of an order of magnitude that can be measured by the probe science accelerometer. They represent an important means for the reconstruction of Titan's upper atmospheric properties. Furthermore, we simulated a Cassini low Titan flyby trajectory. No major trajectory deviations were found. The atmospheric torques due to aerodynamic drag, however, are twice as high for our high temperature profile as the ones obtained with the Yelle maximum profile and more than 5 times higher than the worst case estimations from the Cassini project. We propose to use the Cassini atmospheric torque measurements during its low flybys to derive the atmospheric drag and to reconstruct Titan's upper atmosphere density, pressure, and temperature. The results could then be compared to the reconstructed profiles obtained from Huygens probe measurements. This would help to validate the probe measurements and decrease the error bars.

Ion Neutral Mass Spectrometer Results from the First Flyby of Titan

The Cassini Ion Neutral Mass Spectrometer (INMS) has obtained the first in situ composition measurements of the neutral densities of molecular nitrogen, methane, molecular hydrogen, argon, and a host of stable carbon-nitrile compounds in Titan's upper atmosphere. INMS in situ mass spectrometry has also provided evidence for atmospheric waves in the upper atmosphere and the first direct measurements of isotopes of nitrogen, carbon, and argon, which reveal interesting clues about the evolution of the atmosphere. The bulk composition and thermal structure of the moon's upper atmosphere do not appear to have changed considerably since the Voyager 1 flyby.

Energetic Neutral Atom Emissions from Titan Interaction with Saturn's Magnetosphere

The Cassini Magnetospheric Imaging Instrument (MIMI) observed the interaction of Saturn's largest moon, Titan, with Saturn's magnetosphere during two close flybys of Titan on 26 October and 13 December 2004. The MIMI Ion and Neutral Camera (INCA) continuously imaged the energetic neutral atoms (ENAs) generated by charge exchange reactions between the energetic, singly ionized trapped magnetospheric ions and the outer atmosphere, or exosphere, of Titan. The images reveal a halo of variable ENA emission about Titan's nearly collisionless outer atmosphere that fades at larger distances as the exospheric density decays exponentially. The altitude of the emissions varies, and they are not symmetrical about the moon, reflecting the complexity of the interactions between Titan's upper atmosphere and Saturn's space environment.