Titan's Methane Research Articles

Methane, Ammonia, and Temperature Measurements of the Jovian Planets and Titan from CCD–Spectrophotometry

Full-disk albedo spectra of the jovian planets and Titan were derived from observations at the European Southern Observatory in July 1995. The spectra extend from 300 to 1050 nm wavelength. The spectral resolution is 0.4 nm between 520 and 995 nm, and 1 nm elsewhere. The accuracy of the albedo calibration is 4%. UBV magnitudes were also determined. Raman scattering was quantified for each planet. Methane and ammonia bands are shown at 0.4 nm spectral resolution, including a new band at 930 nm wavelength which is probably due to ammonia. Maps of the variation of these absorptions across the disks of Jupiter and Saturn are displayed. Saturn's spectrum is undisturbed by light from its rings due to the edge-on geometry during the observations. The albedo of Uranus near 1 μm wavelength has dropped almost 10% between 1993 and 1995, while there has been no change in the ultraviolet. The signature of light from Titan's surface yielded a path length of 4 km-am of methane in Titan's atmosphere. The temperature dependence of the width of the 890-nm methane band was used to measure temperature variations at three altitude levels, resulting in the first temperature maps of Jupiter and Saturn based on reflected sunlight. Jupiter displays a banded temperature structure with some discrete features of a few Kelvin amplitude. Saturn's north–south temperature asymmetry has reversed since the Voyager observations.

Temperature Lapse Rate and Methane in Titan's Troposphere

We have reanalyzed the Voyager radio occultation data for Titan, examining two alternative approaches to methane condensation. In one approach, methane condensation is facilitated by the presence of nitrogen because nitrogen lowers the condensation level of a methane/nitrogen mixture. The resulting enhancement in methane condensation lowers the upper limit on surface relative humidity of methane obtained from the Voyager occultation data from 0.7 to 0.6. We conclude that in this case the surface relative humidity of methane lies between 0.08 and 0.6, with values close to 0.6 indicated. In the other approach, methane is allowed to become supersaturated and reaches 1.4 times saturation in the troposphere. In this case, surface humidities up to 100% are allowed by the Voyager occultation data, and thus the upper limit must be set by other considerations. We conclude that if supersaturation is included, then the surface relative humidity of methane can be any value greater than 0.08—unless a deep ocean is present, in which case the surface relative humidity is limited to less than 0.85. Again, values close to 0.6 are indicated. Overall, the tropospheric lapse rate on Titan appears to be determined by radiative equilibrium. The lapse rate is everywhere stable against dry convection, but is unstable to moist convection. This finding is consistent with a supersaturated atmosphere in which condensation—and hence moist convection—is inhibited.

Titan's Rotational Light-Curve

Recent observations demonstrate that near-infrared spectroscopy can probe Titan's surface through its haze. In a 5-week period during September and October 1993 we observed Titan's methane windows at 1.1, 1.3, 1.6, and 2 μm. At 1.1 and 1.3 μm observations were consistent with observations in 1992 at the same phases reported by Lemmon et al. (1993, Icarus 103, 329-332). Our new observations indicate that Titan was brighter near eastern elongation than near western elongation by 23 ± 2% at 1.6 μn and 32 ± 3% at 2 μn. With almost daily observations at 2 μn during one orbit, we observed Titan to be dark near western elongation, to brighten as it approached eastern elongation, and to darken as it returned to western elongation. We determine that the observed light-curve is due to surface albedo variations and Titan's rotational period is 15.950 ± 0.025 days. By considering the work of other observers we constrain the rotational period to be 15.949 ± 0.006 days; this constraint is consistent with synchronous rotation. Models of Titan's surface reflectivity are inconsistent with the presence of a strong 2-μm water ice absorption feature and do not require the presence of any surface absorption features; however, we cannot rule out models of the surface as dirty water ice or silicates that have been suggested by others.

The reaction H + C4H2: Absolute rate constant measurement and implication for atmospheric modeling of Titan

Diacetylene (C4H2), an intermediary product in the photodecomposition of Titan's methane, has been suggested to be the catalyst in processes converting H atoms into molecular hydrogen for subsequent escape from Titan's atmosphere. This role for C4H2 has importance concerning formation and survival of the unsaturated species in the atmosphere of Titan. The reaction kinetics of H + C4H2 are also of significance in incomplete combustion processes forming higher unsaturated hydrocarbons from acetylene and its radicals in the C2H2/O/H system. The absolute rate constant for the reaction H + C4H2 has been measured over the temperature interval 210–423 K, using the technique of flash photolysis‐resonance fluorescence. At each of the five temperatures employed in this study the results were independent of variations in [C4H2], total pressure (Argon or Nitrogen), and flash intensity (i.e., initial [H]). The rate constant results for 210 ≤ T ≤ 423 K are k = (1.39±0.25) × 10−10 exp (−1184±44/T) cm³ s−1, where the error quoted is one standard deviation. This represents the first temperature study of this reaction. The Arrhenius parameters at the high pressure limit determined here for H + C4H2 are contrasted with those for the corresponding reactions of H with C2H2 and C3H4. Implications of the kinetic results, particularly those at low temperatures, are considered for models of the atmospheric hydrocarbon chemistry of Titan. The rate of this reaction, relative to that of the analogous, but slower, reaction of H + C2H2, appears to make H + C4H2 a very feasible reaction pathway for effective conversion of H atoms to molecular hydrogen in the stratosphere of Titan.

Thermodynamics of clathrate hydrate at low and high pressures with application to the outer solar system

view Abstract Citations (281) References (75) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Thermodynamics of clathrate hydrate at low and high pressures with application to the outer solar system Lunine, J. I. ; Stevenson, D. J. Abstract The thermodynamic stability of clathrate hydrate is calculated to predict the formation conditions corresponding to a range of solar system parameters. The calculations were performed using the statistical mechanical theory developed by van der Waals and Platteeuw (1959) and existing experimental data concerning clathrate hydrate and its components. Dissociation pressures and partition functions (Langmuir constants) are predicted at low pressure for CO clathrate (hydrate) using the properties of chemicals similar to CO. It is argued that nonsolar but well constrained noble gas abundances may be measurable by the Galileo spacecraft in the Jovian atmosphere if the observed carbon enhancement is due to bombardment of the atmosphere by clathrate-bearing planetesimals sometime after planetary formation. The noble gas abundances of the Jovian satellite Titan are predicted, assuming that most of the methane in Titan is accreted as clathrate. It is suggested that under thermodynamically appropriate conditions, complete clathration of water ice could have occurred in high-pressure nebulas around giant planets, but probably not in the outer solar nebula. The stability of clathrate in other pressure ranges is also discussed. Publication: The Astrophysical Journal Supplement Series Pub Date: July 1985 DOI: 10.1086/191050 Bibcode: 1985ApJS...58..493L Keywords: Clathrates; Cosmochemistry; Natural Satellites; Planetary Atmospheres; Planetary Evolution; Thermal Stability; Abundance; Dissociation; Hydrates; Jupiter Atmosphere; Pressure Dependence; Satellite Atmospheres; Solar System; Statistical Mechanics; Titan; Astrophysics full text sources ADS |

Titan ocean: Ethane, methane, nitrogen

Detection of the atmosphere of Saturn's satellite Titan by the Voyager I spacecraft indicated an abundance of only 3 mol % methane (CH4). Recently J.I. Lunine, D.J. Stevenson, and Y.L. Yung calculated that 3 mol % methane is sufficiently low to preclude the stable coexistence of liquid methane on Titan's surface, which has a temperature of 94 K (Science, 222, 1229, 1983). Instead, Lunine et al. suggest that Titan's atmospheric methane may have broken down by a catalyzed photochemical reaction to ethane (C2H6). The resulting ocean would consist of a mixture of C2H6 and CH4 in the proportion of 3 to 1.

(Abstract) The Large Abundance of Methane in Titan's Atmosphere

(Abstract) The Large Abundance of Methane in Titan's Atmosphere

The morphology of Titan's methane bands. I - Comparison with a reflecting layer model

view Abstract Citations (23) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The morphology of Titan's methane bands. I. Comparison with a reflecting layer model. Trafton, L. Abstract Titan's spectrum at moderate resolution is compared with and contrasted to Saturn's spectrum from 5800 to 12,000 A. The filling-in of Titan's bands arises partly from a curve-of-growth effect caused by the different pressures and visible-methane abundances in the two atmospheres. Most of Titan's bands are saturated in the sense that the absorption of both their stronger and weaker lines is affected by the pressure. On the other hand, the weaker lines of Saturn's bands lie essentially on the linear part of the curve of growth. In principle, this permits us to evaluate the parameters of a band model describing the mean transmission versus wavelength for a given band by using the spectra of both planets simultaneously. We find, however, that the reflecting layer model cannot be reconciled with the observations of Titan. This is true whether or not results from an analysis of the R(5) manifold of Titan's 3v3 methane band are considered. This implies that aerosol scattering plays a significant role in Titan's band formation. The most likely candidate for such an aerosol is frozen methane particles. Subject headings: abundances, planetary - line formation - Saturn - satellites - spectra, planetary Publication: The Astrophysical Journal Pub Date: February 1975 DOI: 10.1086/153385 Bibcode: 1975ApJ...195..805T Keywords: Astronomical Models; Methane; Planetary Atmospheres; Saturn (Planet); Spectral Bands; Titan; Absorption Spectra; Aerosols; Atmospheric Models; Atmospheric Stratification; Infrared Astronomy; Light Scattering; Molecular Spectra; Optical Reflection; Spectral Line Width; Lunar and Planetary Exploration full text sources ADS |