Satellite Titan Research Articles

Rate coefficients for reactions of ethynyl radical (C2H) with HCN and CH3CN: Implications for the formation of complex nitriles on Titan

Rate coefficients for the reactions of C2H+HCN→products and C2H+CH3CN→products have been measured over the temperature range 262–360 K. These experiments represent an ongoing effort to accurately measure reaction rate coefficients of the ethynyl radical, C2H, relevant to planetary atmospheres such as those of Jupiter and Saturn and its satellite Titan. Laser photolysis of C2H2 is used to produce C2H, and transient infrared laser absorption is employed to measure the decay of C2H to obtain the subsequent reaction rates in a transverse flow cell. Rate constants for the reaction C2H+HCN→products are found to increase significantly with increasing temperature and are measured to be (3.9–6.2)×10−13 cm3 molecules−1 s−1 over the temperature range of 297–360 K. The rate constants for the reaction C2H+CH3CN→products are also found to increase substantially with increasing temperature and are measured to be (1.0–2.1)×10−12 cm3 molecules−1 s−1 over the temperature range of 262–360 K. For the reaction C2H+HCN→products, ab initio calculations of transition state structures are used to infer that the major products form via an addition/elimination pathway. The measured rate constants for the reaction of C2H+HCN→products are significantly smaller than values currently employed in photochemical models of Titan, which will affect the HC3N distribution.

Mass loss of N 2 molecules from Triton by magnetospheric plasma interaction

Previous investigations of sputtering of molecular nitrogen from Triton's atmosphere lead to estimates of escape rates of about 10 21 N 2 molecules s −1. Here, the erosion of Triton's nitrogen atmosphere resulting from sputtering due to different plasma populations and particles from Neptune's magnetosphere is investigated. This investigation shows that sputtering from Triton's nitrogen atmosphere could lead to N 2 escape rates during the plasma sheet crossing on the order of 5 × 10 24 s −1. This calculation shows that sputtering of Triton's nitrogen atmosphere by magnetospheric particles is an efficient nonthermal escape mechanism, similar to Saturn's large satellite Titan, and is an additional important process for the power input of the Neptune aurora. The N 2 escape rates should be in a good agreement with the measured H +/N + ion ratio in Neptune's magnetosphere. The excess energy of the sputtered particles leads primarily to escape and supply to the Neptune system rather than to ballistic orbits. Sputtering will yield, however, a small N 2 corona on Triton.

Tidal effects of disconnected hydrocarbon seas on Titan.

Thermodynamic and photochemical arguments suggest that Titan, the largest satellite of Saturn, has a deep ocean of liquid hydrocarbons. At visible wavelengths, Titan's surface is obscured by a thick stratospheric haze, but radar observations have revealed large regions of high surface reflectivity that are inconsistent with a global hydrocarbon ocean. Titan's surface has also been imaged at infrared wavelengths, and the highest-resolution data (obtained by the Hubble Space Telescope) show clear variations in surface albedo and/or topography. The natural interpretation of these observations is that Titan, like the Earth, has continents and oceans. But Titan's high orbital eccentricity poses a problem for this interpretation, as the effects of oceanic tidal friction would have circularized Titan's orbit for most configurations of oceans and continents. Here we argue that a more realistic topography, in which liquid hydrocarbons are confined to a number of disconnected seas or crater lakes, may satisfy both the dynamical and observational constraints.

Technologies new to space in Huygens Probe Mission to Titan

Summary Within severely constrained financial budget, mass and energy allocations as well as reliability needs, the Huygens Probe to Saturn's satellite Titan raised the need to design, develop and validate many new subsystems. Among them are very high-temperature-protection materials and a shell/kernel operative concept that allows to eliminate all ablated contaminants immediately after entry, space and gaseous (−200°C-convection) thermal control without heaters, hyper- and transonic parachutes, numerical transponders…. These new developments are presented in relation with the low-cost, quick-development (fixed launch window) and robustness need of the Huygens program.

Planetary exploration by robotic aerovehicles

Planetary aerobots are a new type of telerobotic science platform that can fly and navigate in a dynamic 3-dimensional atmospheric environment, thus enabling the global in situ exploration of planetary atmospheres and surfaces. Aerobots are enabled by a new concept in planetary balloon altitude control, developed at JPL, which employs reversible-fluid changes to permit repeated excursions in altitude. The essential physics and thermodynamics ofreversible-fluid altitude control have been demonstrated in a series of altitude-control experiments conducted in the Earth's atmosphere, which are described. Aerobot altitude-control technology will be important in the exploration of seven planets and satellites in our solar system. Three of these objects—Venus, Mars, and the Saturnian satellite Titan—have accessible solid surfaces and atmospheres dominated by the dense gases nitrogen or carbon dioxide. They will be explored with aerobots using helium or hydrogen as their primary means of buoyancy. The other four planets—Jupiter, Saturn, Uranus, and Neptune—have deep atmospheres that are predominantly hydrogen. It may be possible to explore these atmospheres with aerobots inflated with atmospheric gas that is then radiatively heated from the hotter gaseous depths below. To fulfill their potential, aerobots to explore the planets will need autonomous state estimators to guide their observations and provide information to the altitude-control systems. The techniques of acquiring these data remotely are outlined. Aerobots will also use on board altitude control and navigation systems to execute complex flight paths including descent to the surface and exploiting differential wind velocities to access different latitude belts. Approaches to control of these systems are examined. The application of aerobots to Venus exploration is explored in some detail: The most ambitious mission described, the Venus Flyer Robot (VFR), would have the capability to make repeated short excursions to the high-temperature surface environment of Venus to acquire data and then return to the Earth-like upper atmosphere to communicate and recool its electronic systems. Finally a Planetary Aerobot Testbed is discussed which will conduct Earth atmospheric flights to validate autonomous-state-estimator techniques and flight-path-control techniques needed for future planetary missions.

An impact penetrometer for a landing spacecraft

The design, development and calibration of an impact force transducer or penetrometer, for use on the Huygens spacecraft scheduled to land on the surface of Saturn's satellite Titan, is described, The thumb-sized transducer employs a piezoelectric sensing element and is capable of working at cryogenic temperatures. Use of the sensor on a spacecraft imposes several reliability and safety constraints, as well as the desire to minimize mass (the sensor mass is 15 g). The impact force profile, measured at 10 kHz by the sensor, allows estimation of the density and cohesion of the surface material, and its particle size distribution. Sample profiles for terrestrial materials (sand, gravel and clay) are given.

Solar-system studies in the infrared range: Recent developments and future plans

A large fraction of our knowledge about the structure and composition of planetary and cometary atmospheres has come from infrared spectroscopy. In the case of giant planets, information on the thermal structure and the abundances ratios of these objects was derived from ground-based IR spectroscopy and from the VOYAGER IRIS infrared data. Saturn's satellite Titan was mostly explored by the VOYAGER mission, and in particular the IRIS instrument. Terrestrial planets, although already investigated by in-situ means, have also recently benefited from near-infrared (NIR) imaging spectroscopy. Finally the study of comets has known a very important development over the past years with the increasing use of infrared and millimeter spectroscopy. Future progress can be foreseen, for both ground-based and space observations. Ground-based studies will benefit from the use of bidimensional near-infrared cameras and their coupling with high-resolution spectrometers. Another promising field of research should be associated with the use of millimeter interferometers, and the extension of heterodyne spectroscopy toward higher frequencies. In the case of space research, a significant improvement is expected from two Earth-orbiting missions: ISO, in the infrared range, and later FIRST, in the Submillimeter region. More in-depth studies of specific objects will be performed with the infrared instruments on specific space probes: Mars Observer and MARS-94 for Mars, GALILEO for Jupiter, CASSINI for Saturn and Titan, and later ROSETTA for a long-term cometary exploration.

The new Titan's hydrogen torus

If the solar radiation pressure effect is taken into consideration, the structure of the atomic hydrogen cloud emitted from Titan would be significantly modified. Instead of a circular torus characterized by azimuthal symmetry and with density peak near the orbit of Titan, the atomic hydrogen cloud will take on a pear-shaped structure with the long-lived H atoms distributed through the whole Saturnian system.

Titan: 1-5 μm Photometry and Spectrophotometry and a Search for Variability

We report photometric and spectroscopic observations of Titan made between 1 and 5 μm in 1990 and 1991. The 4.8-μm albedo we measured is more than a factor of three lower than those found in observations in the early 1970s. Long wavelength filter leaks could have contaminated earlier measurements of Titan's 4.8-μm albedo, although we cannot rule out a change in Titan itself. Titan's 1.25-μm albedo that we measured in 1991 is 14 ± 3% greater than that measured in 1979; the 1.65- and 2.20-μm albedos are the same. There was no evidence for variations with orbital phase at any wavelength over the eastern half of Titan's orbit in 1990 and 1991. We also made new measurements of Uranus and Neptune at 4.8 μm that agree with previous observations. We acquired low-resolution ( R = 50) spectra of Titan between 3.1 and 5.1 μm. The spectra contain evidence for CO and CH 3D absorptions. The measured band depths require a reflecting layer located in the troposphere (near 200 mbar) or below. Spectra of Callisto's, Ganymede's and Europa's leading hemispheres in the 4.5-5.1 μm spectral region are featureless and have albedos of 0.094 ± 0.004, 0.044 ± 0.006, and 0.020 ± 0.002, respectively. If Titan's atmosphere is transparent near 5.0 μm, its surface albedo there is similar to Callisto's.

The nitrogen tori of Titan and Triton

As a result of electron impact dissociation and electron dissociative recombination, nitrogen atoms could be ejected from the exosphere of Titan representing an efficient way of non-thermal atmospheric escape. In this study, the corresponding formation of a nitrogen torus in the magnetospheric system of Saturn is simulated. The number density of the nitrogen atoms in a narrow gas ring near the orbit of Titan is estimated to be on the order of 3–20 atoms cm −3 while a much more diffuse nitrogen cloud covers the whole Saturnian system. Because of the small kinetic energy of the atomic nitrogen fragments at production, only a very small density can be maintained in most parts of the magnetosphere. The ionization of the nitrogen atoms in this extended gas torus could nevertheless introduce an appreciable amount of N + ions into the Saturnian magnetosphere. A similar extended structure of an atomic nitrogen cloud is produced in the Neptunian system as a result of the nonthermal escape of nitrogen atoms from Triton. The peak density near Triton's orbit is estimated to be 10–20 atoms cm −3.

The distribution of atomic hydrogen in the magnetosphere of Saturn

Three sets of previously unpublished Voyager ultraviolet spectrometer (UVS) observations in H Ly α emission reveal a complex three‐dimensional distribution of atomic hydrogen in the Saturn system. Voyager 1 observations during the postencounter period have provided a map of the distribution looking down on the equatorial plane. The reduced data show a nonuniform distribution in local time with a preponderance of emission on the duskside. The emission extends radially inward to the top of the Saturn atmosphere with stronger signals appearing close to the planet strongly suggesting that the principal source is the sunlit Saturn atmosphere. In the subsolar sector of the magnetosphere no excess emission in the vicinity of Titan's orbit is detectable. But a peak in the emissions near 20 Saturn radii (RS) appears in the antisolar sector suggesting a significant contribution from Titan. Voyager 1 and 2 preencounter observations also show H Ly α emissions increasing monotonically toward the planet but with a distinctive dawnside excess. The preencounter and postencounter results may be reconciled if there exists a complex H gas distribution with significant radial and azimuthal variations. We conclude that the preliminary results reporting a toroidal hydrogen distribution with a cavity inside 8 RS (Broadfoot et al., 1981) are invalid because of limited spatial resolution and poor statistics. The fact that the hydrogen distribution joins continuously into the atmosphere of Saturn and shows significant azimuthal variations indicates that the dominant source inside 8 RS is in the Saturn exosphere. The local time character and magnitude of the distribution could be produced by ballistic and escaping atoms originating in the sunlit exosphere. The energies required for the source atoms can be obtained by electron excitation of H2 within a scale height of and above the exobase. The total rate of production of atomic hydrogen may be as much as 3 times larger than that inferred earlier from the observed brightness of H2 EUV radiation from the equatorial atmosphere. The deposition of heat at the top of the thermosphere by the atomic hydrogen source can account for the measured temperature. We have examined the impact of the conditions imposed by the measured content of neutrals and ions in the inner magnetosphere through model calculations. We predict in a preliminary analysis that the neutral gas in the low latitude region of the inner magnetosphere is dominated by water products. A typical calculation shows [O] ∼ 400 cm−3, [OH] ∼ 40 cm−3, [H] ∼ 130 cm−3, [O+] ∼ 30 cm−3, [H+] ∼3 cm−3, in the plasma sheet near 4.5 RS; a mix of neutrals and ions of this order is required to balance the energy budget.

Herman‐Wallis Factors in the C2H2ν5 infrared fundamental near 14 μm

The presence of acetylene has been confirmed for some time in the atmospheres of the outer planets Jupiter, Saturn, Neptune, and Saturn's satellite Titan. For these atmospheres, the determination of C2H2 abundances using its strong ν5 fundamental requires laboratory line position and intensity measurements. The 1‐meter Fourier transform spectrometer at McMath solar telescope of Kitt Peak National Observatory was used to measure C2H2 at an unapodized spectral resolution of 0.0025 cm−1. Synthetic spectra are generated by convolving a Voigt line shape with an instrument function and varying intensity parameters by means of a nonlinear least squares technique. Intensities of 37 ν5 lines spanning P18 through R20 were measured using 0.123 torr of gas in a 1‐cm cell. A Herman‐Wallis intensity correction parameter A1pr = 1.3(4) × 10−3 has been derived using a least squares linear fit.

The greenhouse and antigreenhouse effects on Titan.

There are many parallels between the atmospheric thermal structure of the Saturnian satellite Titan and the terrestrial greenhouse effect; these parallels provide a comparison for theories of the heat balance of Earth. Titan's atmosphere has a greenhouse effect caused primarily by pressure-induced opacity of N2, CH4, and H2. H2 is a key absorber because it is primarily responsible for the absorption in the wave number 400 to 600 cm-1 "window" region of Titan's infrared spectrum. The concentration of CH4, also an important absorber, is set by the saturation vapor pressure and hence is dependent on temperature. In this respect there is a similarity between the role of H2 and CH4 on Titan and that of CO2 and H2O on Earth. Titan also has an antigreenhouse effect that results from the presence of a high-altitude haze layer that is absorbing at solar wavelengths but transparent in the thermal infrared. The antigreenhouse effect on Titan reduces the surface temperature by 9 K whereas the greenhouse effect increases it by 21 K. The net effect is that the surface temperature (94 K) is 12 K warmer than the effective temperature (82 K). If the haze layer were removed, the antigreenhouse effect would be greatly reduced, the greenhouse effect would become even stronger, and the surface temperature would rise by over 20 K.

Mapping the planets

With the advent of the telescope in the early 17th century mapping the main features of the lunar surface became feasible, the degree of resolution and precision of location increasing steadily, especially with the introduction of photography. With the dawn of space age attention has been increasingly directed to planetary cartography and today Pluto and Saturn's satellite Titan are the last major bodies in the solar system to remain unmapped. This article reviews the sophisticated techniques that have made possible this remarkable achievement.

Infrared spectra of the H2–N2 and H2–CO van der Waals complexes

Spectra of the weakly bound van der Waals complexes H2–N2 and H2–CO have been studied in the mid- and far-infrared regions corresponding to H2 vibrational and pure rotational frequencies, respectively. The experiments were done using a Fourier transform infrared spectrometer to study equilibrium gas samples at low temperature (77 K) with a long absorption path. The spectra of the complexes appear as fine structure located near the peaks of the lines in the collision-induced spectrum of hydrogen. Compared to earlier studies of these species, the present results have more complete coverage of the various possible transitions, better resolution, and better signal to noise. New structure is reported which corresponds to hindered rotational transitions of the N2 component of an H2–N2 complex and is reminiscent of patterns observed for the N2–Ar and (N2)2 complexes. The relation of these results to far-infrared measurements of Saturn’s satellite Titan, made by the Voyager spacecraft, is discussed.

Study of planetary atmospheres by absorptive occultations

As a spacecraft observes the setting or rising of the Sun or a star behind the limb of a planet, the absorption spectrum of the upper atmosphere can be observed as a function of height. This powerful method can give the composition and the vertical distribution of individual gases, from which temperatures can be derived. This review describes the technique and methods of data reduction. Early experiments on the Earth's atmosphere (many from sounding rockets) are briefly summarized; many of them were compromised by the need to use instruments not designed for the purpose. Emphasis is placed on observations of the other planets, by ultraviolet spectrometers able to look at the Sun as well as stars. Mariner 10 set tight upper limits on the atmosphere of Mercury (and also found tiny quantities of H and He). The Voyager instruments defined the upper atmospheres of all four Jovian planets, as well as the satellites Titan and Triton. The potential for future measurements is at least as rich as the history, especially by use of instruments and platforms optimized for the purpose.

Titan

Saturn's satellite Titan is the second-largest in the solar system. Its dense atmosphere is mostly molecular nitrogen with an admixture of methane, a surface pressure of 1.5 bars and a surface temperature of 94K. The fundamental driving force in the long-term evolution of Titan's atmosphere is the photolysis of methane in the stratosphere to form higher hydrocarbons and aerosols. The current rate of photolysis and undersaturation of methane in the lower troposphere suggests the presence of a massive ethane-methane-nitrogen ocean. The ocean evolves to a more ethane-rich state over geologic time, driving changes in the atmospheric thermal structure. An outstanding issue concerning Titan's earliest history is the origin of atmospheric nitrogen: was it introduced into Titan as molecular nitrogen or ammonia? Measurement of the argon-to-nitrogen ratio in the present atmosphere provides a diagnostic test of these competing hypotheses. Many of the questions raised by the Voyager encounters about Titan and its atmosphere can be adequately addressed only by an entry probe, such as that planned for the Cassini mission. ∗ ∗ Contribution number 88-22, Theoretical Astrophysics Program, University of Arizona.

A generalized theory of sun-climate/weather link and climatic change

We generalize the theory of sun-climate/weather links and climatic change developed earlier by the author. On the basis of this generalized theory, we show mathematically that key climatic/weather parameters are continuously subjected to determinable amplitude modulations and other variations which may be useful in climatic prediction work. A number of new and known oscillations in climate and atmospheric behaviour in general are deduced from the theory and accounted for in terms of their causative physical processes. Finally we briefly discuss the possibility of applying the theory to the planets Mars and Venus as well as Saturn’s largest satellite Titan.

Origin and Evolution of Outer Solar System Atmospheres

The atmospheres of bodies in the outer solar system are distinct in composition from those of the inner planets and provide a complementary set of clues to the origin of the solar system. This article reviews current understanding of the origin and evolution of these atmospheres on the basis of abundances of key molecular species. The systematic enrichment of methane and deuterated species from Jupiter to Neptune is consistent with formation models in which significant infall of icy and rocky planetesimals accompanies the formation of giant planets. The atmosphere of the Saturnian satellite Titan has been strongly modified by photochemistry and interaction with the surface over 4.5 billion years; the combined knowledge of this moon's bulk density and estimates of the composition of the surface and atmosphere provide some constraints on this body's formation. Neptune's satellite Triton is a poorly known object for which it is hoped that substantial information will be gleaned from the Voyager 2 encounter in August 1989. The mean density of the Pluto-Charon system is well known and suggests an origin in the rather water-poor solar nebula. The recent occultation of a star by Pluto provides evidence that carbon monoxide, in addition to methane, may be present in its atmosphere.

Aurora on Triton?

If the planetary magnetic field of Neptune lies within the range of values that have been predicted for it (or even if the field is considerably smaller), Voyager 2 will find Triton immersed within the plasmasphere of a rotationally dominated magnetosphere. We expect the ionosphere of Triton to be highly conducting, which, coupled with Triton's 40 km/sec velocity relative to the magnetospheric plasma, leads to a limiting current that produces an induced magnetosphere, as occurs with solar‐wind flow past Venus or Mars. The induced magnetosphere, analogous to that observed near Saturn's satellite Titan, serves to deviate the flowing plasma and reduce the potential across Triton. The limiting current in this case can probably be carried by the local plasma with drift speeds below the thermal speed. For these conditions, we do not expect current‐driven instabilities to occur, and any auroral emissions would probably not be detectable against the background airglow of Triton's atmosphere. However, if Triton were to have a weak intrinsic magnetic field (for example, application of Blackett's law to Triton indicates a surface field of 400 nT, although a 20–80 nT surface field on Triton would suffice), the current would be funneled into an auroral zone. This would produce a concentration of current that may trigger specific auroral acceleration processes, such as double‐layer formation, and produce auroral optical and radio emissions detectable by Voyager instruments. Thus the presence of an aurora, particularly one that is localized, will argue for the presence of an intrinsic field on Triton.