Daniel Gautier

Titan’s atmosphere from ISO mid-infrared spectroscopy

Abstract We have analyzed Titan observations performed by the Infrared Space Observatory (ISO) in the range 7–30 μm. The spectra obtained by three of the instruments on board the mission (the short wavelength spectrometer, the photometer, and the camera) were combined to provide new and more precise thermal and compositional knowledge of Titan’s stratosphere. With the high spectral resolution achieved by the SWS (much higher than that of the Voyager 1 IRIS spectrometer), we were able to detect and separate the contributions of most of the atmospheric gases present on Titan and to determine disk-averaged mole fractions. We have also tested existing vertical distributions for C2H2, HCN, C2H6, and CO2 and inferred some information on the abundance of the first species as a function of altitude. From the CH3D band at 1161 cm−1 and for a CH4 mole fraction assumed to be 1.9% in Titan’s stratosphere, we have obtained the monodeuterated methane-averaged abundance and retrieved a D/H isotopic ratio of 8.7−1.9+3.2 × 10−5. We discuss the implications of this value with respect to current evolutionary scenarios for Titan. The ν5 band of HC3N at 663 cm−1 was observed for the first time in a disk-averaged spectrum. We have also obtained a first tentative detection of benzene at 674 cm−1, where the fit of the ISO/SWS spectrum at R = 1980 is significantly improved when a constant mean mole fraction of 4 × 10−10 of C6H6 is incorporated into the atmospheric model. This corresponds to a column density of ∼ 2 × 1015 molecules cm−2 above the 30-mbar level. We have also tested available vertical profiles for HC3N and C6H6 and adjusted them to fit the data. Finally, we have inferred upper limits of a few 10−10 for a number of molecules proposed as likely candidates on Titan (such as allene, acetonitrile, propionitrile, and other more complex gases).

Titan's atmosphere and hypothesized ocean: A reanalysis of the Voyager 1 radio-occultation and IRIS 7.7-μm data

Voyager1 radio-occultation refractivity profiles of Titan in the 0–200 km altitude range are reanalyzed in order to derive limiting profiles for the equatorial thermal structure, taking into account uncertainties on the mean molecular weight m. The major atmospheric constituents are assumed to be N2 and CH4 (already detected) and Ar which is a plausible additional constituent when m is greater than 28. The stratospheric abundance of CH4 is assumed to be limited by saturation at the tropopause. Spectra calculated from the infrared thermal profiles in the CH4 ν4 band at 7.7 μm are compared with the Voyager IRIS observations. This allows us to constrain both the abundances of major atmospheric components and the stratospheric temperatures. The methane mole fraction is between 0.5 and 3.4% in the stratosphere but may be as high as 21% at the surface, whereas the atmospheric Ar/N2 ratio can be anything between 0 and 0.27. The temperature is between 92.5 and 101°K at the ground level, and between 70.5 and 74.5°K at the tropopause. The maximum uncertainty (±4°K) occurs between 150 and 200 km altitude. A “nominal” temperature and composition profile of Titans atmosphere between the ground and 200 km is given with tabulated values. The implications of composition and surface temperature uncertainties on the abundances of the expected major oceanic constituents are discussed, as well as the consequences for the solubility of noncodenssable atmospheric constituents (H2 and CO) and organic compounds raining out from the atmosphere.

Titan's atmosphere from voyager infrared observations: I. The gas composition of Titan's equatorial region

The abundances of minor atmospheric constituents in Titans equatorial region have been inferred from Voyager 1 infrared spectra. A temperature profile for the stratosphere was retrieved from the best fit of the 7.7-μm methane band in which the emission originates mainly from pressure levels between 0.01 and 3 mbar (150 and 400 km). Assuming uniform vertical distributions for all the absorbers above their condensation levels, the analysis of three different selections yields stratospheric mole fractions for the following species: C2H2 (2.2+0.7−0.9 × 10−6), C2H4 (9.0+3−5 × 10−8), C2H6 (1.3+0.5−0.7 × 10−5), C3H4 (4.4+1.7−2.1 × 10−9), C3H8 (7.0+4−4 × 10−7), C4H2 (1.4+0.6−0.7 × 10−9), HCN (1.6+0.4−0.6 × 10−7), and CO2 (1.4+0.3−0.5 × 10−8). Upper limits were obtained for the abundances of C2N2 (1.5 × 10−9) and HC3N (1.5 × 10−9). An altitude-dependent profile of CO2 was tested against observations but no conclusive information as to the vertical distribution of carbon dioxide could be extracted. The formation of emission lines for all of the minor components is found to originate from pressure levels between 1 and 20 mbar (75 and 200 km). There is no evidence for longitudinal variations in the atmospheric composition. The results are finally compared to previous studies and to current photochemical models.

Titan's atmosphere from voyager infrared observations: III. Vertical distributions of hydrocarbons and nitriles near Titan's North Pole

Abstract The temperature structure and vertical distributions of minor stratospheric constituents in Titans north polar region have been inferred from the study of Voyager 1 infrared spectra. The data were recorded at grazing incidence over the limb, and cover an approximate vertical range of 100–300 km. The upper stratospheric temperature is retrieved from an analysis of the 7.7-μm methane band in which the emission originates mainly from pressure levels between 0.01 and 5 mbar (120 and 375 km). We find the thermal profile at 70°N to be significantly colder (by ∼ 20°K) than the equatorial one. Vertical distributions of the following absorbers were derived from the best fits obtained in the spectral region 200–1400 cm −1 for three different data sets (corresponding to three different altitudes above the satellites surface): C 2 H 2 , C 4 H 2 , C 2 H 6 , C 3 H 4 , HCN, HC 3 N, and C 2 N 2 . The mixing ratios of these species increase with height, suggesting their formation in the upper stratosphere and above. The abundances of C 3 H 8 and C 2 H 4 could only be obtained near the 1.5-mbar level. When compared to the abundances derived at the equator, we find the nitriles HC 3 N and C 2 N 2 and some of the hydrocarbons (C 2 H 4 , C 3 H 4 , and C 4 H 2 ) to be considerably enhanced in the north polar region. The upper limit of the CO 2 abundance obtained near the north pole indicates a depletion of this gas with respect to the equator (by a factor of at least ∼2). Current available photochemical models are insufficient to interpret these observations.

A TWO-DIMENSIONAL MODEL FOR THE PRIMORDIAL NEBULA CONSTRAINED BY D/H MEASUREMENTS IN THE SOLAR SYSTEM: IMPLICATIONS FOR THE FORMATION OF GIANT PLANETS

Using the density and temperature profiles resulting from a two-dimensional turbulent model of the solar nebula as well as an appropriate law for the time variation of the disk accretion rate, we integrate the equation of diffusion that rules the evolution of the D/H ratio in H2O and HCN throughout the nebula. By fitting D/H measured in LL3 meteorites and comets or inferred in proto-Uranian and proto-Neptunian ices, we constrain the parameters of the model, namely, the initial accretion rate (0), the initial radius of the turbulent disk RD, and the α-coefficient of turbulent viscosity, and we find 2 × 10-6 < (0) < 10-5 M☉ yr-1, 12.8 < RD < 39 AU, and 0.006 < α < 0.04. Under the assumption that cometary cores are homogeneous, the microscopic icy grains that subsequently formed cometesimals were produced in the Uranus-Neptune region and no later than 3.5 × 105 yr. The epochs of the formation of Jupiter and Saturn cannot be lower than 0.7 and 5.7 Myr, respectively, after the formation of the Sun. Uranus and Neptune were completed after the dissipation of the nebula. The enrichment in volatiles with respect to the solar abundance measured by the Galileo probe in Jupiter may result from the trapping of these gases in the form of clathrate hydrates in the feeding zone of the forming planet.

ENRICHMENTS IN VOLATILES IN JUPITER: A NEW INTERPRETATION OF THE GALILEO MEASUREMENTS

Using an evolutionary model of the solar nebula, we fit all enrichments in volatiles with respect to the solar abundance measured in Jupiter by the Galileo probe. We argue that volatiles were trapped in the form of solid clathrate hydrates in the cooling feeding zone of Jupiter while the gas mass of the nebula was continuously decreasing with time. Enrichments in Jupiter are those acquired in planetesimals at the time of the hydrodynamic collapse of the feeding zone. The O/H ratio in Jupiter is predicted to be at least 8 times solar.

Titan's atmosphere from Voyager infrared observations

Abstract We have studied the 900–1200 cm−1 range in three different selections of ∼30 Voyager 1 IRIS spectra recorded in Titans equatorial region. In particular, we have reanalyzed the 8.6-μm emission feature attributed to both C3H8 and CH3D bands. Observations were compared to synthetic spectra generated from an atmospheric model which incorporates recent results on the thermal structure and gas composition (in particular on the abundances of methane and propane) at Titans equator from A. Coustenis, B. Bezard, and D. Gautier (1989 , Icarus, 80, 54–76.). The best fit approach and the error analysis yielded a CH3D mole fraction of 1.1+0.7−0.6 × 10−5 in the stratosphere and a CH3D/ CH4 ratio of 6+5.6−2.1 × 10−4. A value of D/H was derived, 1.5+1.4−0.5 × 10−4, which is in conflict with the results of the previous analysis by S.J. Kim and J. Caldwell (1982 , Icarus 52, 473–482)—but consistent with their revised value ( T. Owen, B. L. Lutz, and C. de Bergh 1986 , Nature (London) 320, 244–246)—and in excellent agreement with the estimation deduced from observations in the 1.6-μm CH3D band by C. de Bergh, B. L. Lutz, T. Owen, and J. Chauville (1988 , Astrophys. J. 329, 951–955). The associated error bars result mainly from instrumental noise and uncertainties in the thermal profile. Deuterium appears to be clearly enriched in Titan, but present uncertainties in the D H ratio, both in the satellite and in the protosolar nebula, preclude a firm interpretation of the origin of the observed enrichment.

The helium abundance of Saturn from Voyager measurements

Voyager radio-occultation and IR spectroscopy measurements are combined to infer an He mole fraction in the upper troposphere of Uranus of 0.152 + or - 0.033; the corresponding mass fraction is Y = 0.262 + or - 0.048. This value is in agreement with recent estimates of the solar He abundance, suggesting that He differentiation has not occurred on Uranus. Comparisons with values previously obtained for Jupiter and Saturn imply that migration of He toward the core began long ago on Saturn and may also have recently begun on Jupiter. The protosolar He abundance inferred from the Uranus measurements and from recent solar evolutionary models is used along with an assumed primordial He mass fraction of 0.23-0.24 to estimate a 3-4-percent enrichment of He in the interstellar medium between the big bang and the origin of the solar system. The result is in agreement with galactic chemical evolution models which include a substantial decrease in D during the evolutionary process.

An intense stratospheric jet on Jupiter.

The Earths equatorial stratosphere shows oscillations in which the east–west winds reverse direction and the temperatures change cyclically with a period of about two years. This phenomenon, called the quasi-biennial oscillation, also affects the dynamics of the mid- and high-latitude stratosphere and weather in the lower atmosphere. Ground-based observations have suggested that similar temperature oscillations (with a 4–5-yr cycle) occur on Jupiter, but these data suffer from poor vertical resolution and Jupiters stratospheric wind velocities have not yet been determined. Here we report maps of temperatures and winds with high spatial resolution, obtained from spacecraft measurements of infrared spectra of Jupiters stratosphere. We find an intense, high-altitude equatorial jet with a speed of ∼140 m s-1, whose spatial structure resembles that of a quasi-quadrennial oscillation. Wave activity in the stratosphere also appears analogous to that occurring on Earth. A strong interaction between Jupiter and its plasma environment produces hot spots in its upper atmosphere and stratosphere near its poles, and the temperature maps define the penetration of the hot spots into the stratosphere.

An interpretation of the nitrogen deficiency in comets

Abstract We propose an interpretation of the composition of volatiles observed in comets based on their trapping in the form of clathrate hydrates in the solar nebula. The formation of clathrates is calculated from the statistical thermodynamics of Lunine and Stevenson (1985 , Astrophys. J. Suppl. 58, 493–531), and occurs in an evolutionary turbulent solar nebula described by the model of Hersant et al. (2001 , Astrophys. J. 554, 391–407). It is assumed that clathrate hydrates were incorporated into the icy grains that formed cometesimals. The strong depletion of the N2 molecule with respect to CO observed in some comets is explained by the fact that CO forms clathrate hydrates much more easily than does N2. The efficiency of this depletion, as well as the amount of trapped CO, depends upon the amount of water ice available in the region where the clathration took place. This might explain the diversity of CO abundances observed in comets. The same theory, applied to the trapping of volatiles around 5 AU, explains the enrichments in Ar, Kr, Xe, C, and N with respect to the solar abundance measured in the deep troposphere of Jupiter Gautier et al 2001a , Gautier et al 2001b .