Extreme Ultraviolet Observations from Voyager 1 Encounter with Saturn

The Composition and Chemistry of Titan's Atmosphere

In this review I summarize the current state of knowledge about the composition of Titan's atmosphere and our current understanding of the suggested chemistry that leads to that observed composition. I begin with our present knowledge of the atmospheric composition, garnered from a variety of measurements including Cassini-Huygens, the Atacama Large Millimeter/submillimeter Array, and other ground- and space-based telescopes. This review focuses on the typical vertical profiles of gases at low latitudes rather than global and temporal variations. The main body of the review presents a chemical description of how complex molecules are believed to arise from simpler species, considering all known "stable" molecules-those that have been uniquely identified in the neutral atmosphere. The last section of the review is devoted to the gaps in our present knowledge of Titan's chemical composition and how further work may fill those gaps.

The N(2D) + CH2CHCN (Vinyl Cyanide) Reaction: A Combined Crossed Molecular Beam and Theoretical Study and Implications for the Atmosphere of Titan

The reaction of electronically excited nitrogen atoms, N(²D), with vinyl cyanide, CH₂CHCN, has been investigated under single-collision conditions by the crossed molecular beam (CMB) scattering method with mass spectrometric detection and time-of-flight (TOF) analysis at the collision energy, E_c, of 31.4 kJ/mol. Synergistic electronic structure calculations of the doublet potential energy surface (PES) have been performed to assist in the interpretation of the experimental results and characterize the overall reaction micromechanism. Statistical (Rice–Ramsperger–Kassel–Marcus, RRKM) calculations of product branching fractions (BFs) on the theoretical PES have been carried out at different values of temperature, including the one corresponding to the temperature (175 K) of Titan’s stratosphere and at a total energy corresponding to the E_c of the CMB experiment. According to our theoretical calculations, the reaction is found to proceed via barrierless addition of N(²D) to the carbon–carbon double bond of CH₂=CH–CN, followed by the formation of cyclic and linear intermediates that can undergo H, CN, and HCN elimination. In competition, the N(²D) addition to the CN group is also possible via a submerged barrier, leading ultimately to N₂ + C₃H₃ formation, the most exothermic of all possible channels. Product angular and TOF distributions have been recorded for the H-displacement channels leading to the formation of a variety of possible C₃H₂N₂ isomeric products. Experimentally, no evidence of CN, HCN, and N₂ forming channels was observed. These findings were corroborated by the theory, which predicts a variety of competing product channels, following N(²D) addition to the double bond, with the main ones, at E_c = 31.4 kJ/mol, being six isomeric H forming channels: c-CH(N)CHCN + H (BF = 35.0%), c-CHNCHCN + H (BF = 28.1%), CH₂NCCN + H (BF = 26.3%), c-CH₂(N)CCN(cyano-azirine) + H (BF = 7.4%), trans-HNCCHCN + H (BF = 1.6%), and cis-HNCCHCN + H (BF = 1.3%), while C–C bond breaking channels leading to c-CH₂(N)CH(2H-azirine) + CN and c-CH₂(N)C + HCN are predicted to be negligible (0.02% and 0.2%, respectively). The highly exothermic N₂ + CH₂CCH (propargyl) channel is also predicted to be negligible because of the very high isomerization barrier from the initial addition intermediate to the precursor intermediate able to lead to products. The predicted product BFs are found to have, in general, a very weak energy dependence. The above cyclic and linear products containing an additional C–N bond could be potential precursors of more complex, N-rich organic molecules that contribute to the formation of the aerosols on Titan’s upper atmosphere. Overall, the results are expected to have a significant impact on the gas-phase chemistry of Titan’s atmosphere and should be properly included in the photochemical models.

The Reaction N(2D) + CH3CCH (Methylacetylene): A Combined Crossed Molecular Beams and Theoretical Investigation and Implications for the Atmosphere of Titan

The reaction of excited nitrogen atoms N(²D) with CH₃CCH (methylacetylene) was investigated under single-collision conditions by the crossed molecular beams (CMB) scattering method with mass spectrometric detection and time-of-flight analysis at the collision energy (E_c) of 31.0 kJ/mol. Synergistic electronic structure calculations of the doublet potential energy surface (PES) were performed to assist the interpretation of the experimental results and characterize the overall reaction micromechanism. Theoretically, the reaction is found to proceed via a barrierless addition of N(²D) to the carbon–carbon triple bond of CH₃CCH and an insertion of N(²D) into the CH bond of the methyl group, followed by the formation of cyclic and linear intermediates that can undergo H, CH₃, and C₂H elimination or isomerize to other intermediates before unimolecularly decaying to a variety of products. Kinetic calculations for addition and insertion mechanisms and statistical (Rice-Ramsperger-Kassel-Marcus) computations of product branching fractions (BFs) on the theoretical PES were performed at different values of total energy, including the one corresponding to the temperature (175 K) of Titan’s stratosphere and that of the CMB experiment. Up to 14 competing product channels were statistically predicted, with the main ones, at E_c = 31.0 kJ/mol, being the formation of CH₂NH (methanimine) + C₂H (ethylidyne) (BF = 0.41), c-C(N)CH + CH₃ (BF = 0.32), CH₂CHCN (acrylonitrile) + H (BF = 0.12), and c-CH₂C(N)CH + H (BF = 0.04). Of the 14 possible channels, seven correspond to H displacement channels of different exothermicity, for a total H channel BF of ∼0.25 at E_c = 31.0 kJ/mol. Experimentally, dynamical information could only be obtained about the overall H channels. In particular, the experiment corroborates the formation of acrylonitrile + H, which is the most exothermic of all 14 reaction channels and is theoretically calculated to be the dominant H-forming channel (BF = 0.12). The products containing a novel C–N bond could be potential precursors to form other nitriles (C₂N₂, C₃N) or more complex organic species containing N atoms in planetary atmospheres, such as those of Titan and Pluto. Overall, the results are expected to have a potentially significant impact on the understanding of the gas-phase chemistry of Titan’s atmosphere and the modeling of that atmosphere.

The upper atmospheres of Uranus and Neptune

We review the current understanding of the upper atmospheres of Uranus and Neptune, and explore the upcoming opportunities available to study these exciting planets. The ice giants are the least understood planets in the solar system, having been only visited by a single spacecraft, in 1986 and 1989, respectively. The upper atmosphere plays a critical role in connecting the atmosphere to the forces and processes contained within the magnetic field. For example, auroral current systems can drive charged particles into the atmosphere, heating it by way of Joule heating. Ground-based observations of H3+ provides a powerful remote diagnostic of the physical properties and processes that occur within the upper atmosphere, and a rich dataset exists for Uranus. These observations span almost three decades and have revealed that the upper atmosphere has continuously cooled between 1992 and 2018 at about 8 K/year, from approximately 750 K to approximately 500 K. The reason for this trend remain unclear, but could be related to seasonally driven changes in the Joule heating rates due to the tilted and offset magnetic field, or could be related to changing vertical distributions of hydrocarbons. H3+ has not yet been detected at Neptune, but this discovery provides low-hanging fruit for upcoming facilities such as the James Webb Space Telescope and the next generation of 30 m telescopes. Detecting H3+ at Neptune would enable the characterization of its upper atmosphere for the first time since 1989. To fully understand the ice giants, we need dedicated orbital missions, in the same way the Cassini spacecraft explored Saturn. Only by combining in situ observations of the magnetic field with in-orbit remote sensing can we get the complete picture of how energy moves between the atmosphere and the magnetic field. This article is part of a discussion meeting issue 'Future exploration of ice giant systems'.

On an EUV Atmospheric Simulation Chamber to Study the Photochemical Processes of Titan's Atmosphere

The in situ exploration of Titan’s atmosphere requires the development of laboratory experiments to understand the molecular growth pathways initiated by photochemistry in the upper layers of the atmosphere. Key species and dominant reaction pathways are used to feed chemical network models that reproduce the chemical and physical processes of this complex environment. Energetic UV photons initiate highly efficient chemistry by forming reactive species in the ionospheres of the satellite. We present here a laboratory experiment based on a new closed and removable photoreactor coupled here to an Extreme Ultraviolet (EUV) irradiation beam produced by the high-order harmonic generation of a femtosecond laser. This type of EUV stable source allow long-term irradiation experiments in which a plethora of individual reactions can take place. In order to demonstrate the validity of our approach, we irradiated for 7 hours at 89.2 nm, a gas mixture based on N₂/CH₄ (5%). Using only one wavelength, products of the reaction reveal an efficient photochemistry with the formation of large hydrocarbons but especially organic compounds rich in nitrogen similar to Titan. Among these nitrogen compounds, new species had never before been identified in the mass spectra obtained in situ in Titan’s atmosphere. Their production in this experiment, on the opposite, corroborates previous experimental measurements in the literature on the chemical composition of aerosol analogues produced in the laboratory. Diazo-compounds such as dimethyldiazene (C₂H₆N₂), have been observed and are consistent with the large nitrogen incorporation observed by the aerosols collector pyrolysis instrument of the Huygens probe. This work represents an important step forward in the use of a closed cell chamber irradiated by the innovative EUV source for the generation of photochemical analogues of Titan aerosols. This approach allows to better constrain and understand the growth pathways of nitrogen incorporation into organic aerosols in Titan’s atmosphere.

The H3+ ionosphere of Uranus: decades-long cooling and local-time morphology

The upper atmosphere of Uranus has been observed to be slowly cooling between 1993 and 2011. New analysis of near-infrared observations of emission from H3+ obtained between 2012 and 2018 reveals that this cooling trend has continued, showing that the upper atmosphere has cooled for 27 years, longer than the length of a nominal season of 21 years. The new observations have offered greater spatial resolution and higher sensitivity than previous ones, enabling the characterization of the H3+ intensity as a function of local time. These profiles peak between 13 and 15 h local time, later than models suggest. The NASA Infrared Telescope Facility iSHELL instrument also provides the detection of a bright H3+ signal on 16 October 2016, rotating into view from the dawn sector. This feature is consistent with an auroral signal, but is the only of its kind present in this comprehensive dataset. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H3+, H5+ and beyond'.

Nucleic acid bases in Titan tholins and possible genetic systems in the Titan liquidosphere

Titan is the largest moon of Saturn and possesses a dense atmosphere composed of nitrogen and methane. Various types of organic compounds (hydrocarbons, nitriles, etc.) have been found on Titan, which were generated by reactions taking place in its atmosphere. These reactions are considered to provide crucial evidence for chemical reactions which may have occurred in the atmosphere of primitive Earth. Cassini discovered several lakes of liquid methane and ethane on Titan's surface; in addition, the presence of ammonia water in its sub-surface was implied. In order to simulate the chemical reactions in Titan's atmosphere, gas mixtures of nitrogen and methane have been exposed to plasma discharges to synthesize complex organic matters. In this study, we focused on the formation of nucleic acid bases and related compounds recovered from synthesized Titan tholins. The five nucleic acid bases that terrestrial life uses (adenine, cytosine, thymine, guanine, and uracil) have already been reported to be present in synthesized Titan tholins. Purines and pyrimidines, including the five aforementioned nucleic acid bases, were extracted from synthesized Titan tholins and analyzed by HPLC and LC/MS. As a result, the pyrimidine bases of isocytosine and 2, 4-diaminopyrimidine were detected together with the terrestrial nucleic acid bases of adenine, uracil, and cytosine. The results obtained in conjunction with those from previous studies show that some nucleic acid bases and related pyrimidine bases are found in synthesized Titan tholins, suggesting that chemical evolutions toward xenogenetic systems could occur in Titan's environment.

Infrared study of matrix-isolated ethyl cyanide: simulation of the photochemistry in the atmosphere of Titan

Low-temperature Ar matrix isolation has been carried out to investigate the infrared spectrum of ethyl cyanide (CH₃CH₂CN), a molecule present in the atmosphere of Titan.

Planetary surface photometry and imaging: progress and perspectives

Spacecraft have visited and returned many thousands of images and spectra of all of the planets, many of their moons, several asteroids, and a few comet nuclei during the golden age of planetary exploration. The signal in each pixel of each image or spectral channel is a measurement of the radiance of scattered sunlight into a specific direction. The information on the structure and composition of the surface that is contained in variation of the radiance with scattering geometry and wavelength, including polarization state, has only just begun to be exploited and is the topic of this review. The uppermost surfaces of these bodies are mainly composed of particles that are continuously generated by impacts of micrometeoroids and larger impactors. Models of light scattering by distributions of sizes and irregular shapes of particles and by closely packed particles within a surface are challenging. These are active topics of research where considerable progress has recently been made. We focus on the surfaces of bodies lacking atmospheres.These surfaces are diverse and their morphologies give evidence of their evolution by impacts and resurfacing by a variety of processes including down slope movement and electrostatic transport of particles, gravitational accumulation of debris, volatile outgassing and migration, and magnetospheric interactions. Sampling of scattering geometries and spatial resolution is constrained by spacecraft trajectories. However, the large number of archived images and spectra demand more quantitative interpretation. The scattering geometry dependence of the radiance is underutilized and promises constraints on the compositions and structure of the surface for materials that lack diagnostic wavelength dependence. The general problem is considered in terms of the lunar regolith for which samples have been returned to Earth.

Is titan wet or dry?

Titan's dense and cold nitrogen atmosphere contains a small amount of methane under conditions at least approaching those at which one or both constituents would condense. The possibility of methane and nitrogen rain clouds and global methane oceans has been discussed widely. From specific features of radio occultation and other Voyager results, however, it is concluded that nitrogen does not condense on Titan and that Titan has neither global methane oceans nor a global cloud of liquid methane droplets. Certain results indirectly support the conjecture that methane does not condense at any location. However, other considerations favor a methane ice haze high in the troposphere, and liquid and solid methane might exist on the surface and as low clouds at polar latitudes.

Infrared observations of the saturnian system from voyager 1

During the passage of Voyager 1 through the Saturn system, the infrared instrument acquired spectral and radiometric data on Saturn, the rings, and Titan and other satellites. Infrared spectra of Saturn indicate the presence of H(2), CH(4), NH(3), PH(3), C(2)H(2), C(2)H(6), and possibly C(3)H(4) and C(3)H(8). A hydrogen mole fraction of 0.94 is inferred with an uncertainty of a few percent, implying a depletion of helium in the atmosphere of Saturn relative to that of Jupiter. The atmospheric thermal structure of Saturn shows hemisphere asymmetries that are consistent with a response to the seasonally varying insolation. Extensive small-scale latitudinal structure is also observed. On Titan, positive identifications of infrared spectral features are made for CH(4), C(2)H(2), C(2)H(4), C(2)H(6), and HCN; tentative identifications are made for C(3)H(4) and C(3)H(8). The infrared continuum opacity on Titan appears to be quite small between 500 and 600 cm(-1), implying that the solid surface is a major contributor to the observed emission over this spectral range; between 500 and 200 cm(-1) theopacity increases with decreasing wave number, attaining an optical thickness in excess of 2 at 200 cm(-1). Temperatures near the 1-millibar level are independent of longitude and local time but show a decrease of approximately 20 K between the equator and north pole, which suggests a seasonally dependent cyclostrophic zonal flow in the stratosphere of approximately 100 meters per second. Measurements of the C ring of Saturn yield a temperature of 85 +/- 1 K and an infrared optical depth of 0.09 +/- 0.01. Radiometer observations of sunlight transmitted through the ring system indicate an optical depth of 10(-1.3 +/-0.3) for the Cassini division. A phase integral of 1.02 +/- 0.06 is inferred for Rhea, which agrees with values for other icy bodies in the solar system. Rhea eclipse observations indicate the presence of surface materials with both high and low thermal inertias, the former most likely a blocky component and the latter a frost.

Theoretical study of HCN(+) + C2H2 reaction

A detailed theoretical investigation for the ion-molecule reaction of HCN (+) with C 2H 2 is performed at the B3LYP/6-311G(d,p) and CCSD(T)/6-311++G(3df,2pd) (single-point) levels. Possible energetically allowed reaction pathways leading to various low-lying dissociation products are probed. It is shown that eight dissociation products P 1 (H 2C 3N (+)+H), P 2 (CN+C 2H 3 (+)), P 3 (HC 3N (+)+H 2), P 4 (HCCCNH (+)+H), P 5 (H 2NCCC (+)+H), P 6 (HCNCCH (+)+H), P 7 (C 2H 2 (+)+HCN), and P 8 (C 2H 2 (+)+HNC) are both thermodynamically and kinetically accessible. Among the eight dissociation products, P 1 is the most abundant product. P 7 and P 3 are the second and third feasible products but much less competitive than P 1 , followed by the almost negligible product P 2 . Other products, P 4 (HCCCNH (+)+H), P 5 (HCNCCH (+)+H), P 6 (H 2NCCC (+)+H), and P 8 (C 2H 2 (+)+HNC) may become feasible at high temperatures. Because the intermediates and transition states involved in the reaction HCN (+) + C 2H 2 are all lower than the reactant in energy, the title reaction is expected to be rapid, as is consistent with the measured large rate constant at room temperature. The present calculation results may provide a useful guide for understanding the mechanism of HCN (+) toward other pi-bonded molecules.

Fluorescence excitation spectra of the bΠu1, b′Σu+1, cnΠu1, and cn′Σu+1 states of N2 in the 80–100nm region

Fluorescence excitation spectra produced through photoexcitation of N(2) using synchrotron radiation in the spectral region between 80 and 100 nm have been studied. Two broadband detectors were employed to simultaneously monitor fluorescence in the 115-320 nm and 300-700 nm regions, respectively. The peaks in the vacuum ultraviolet fluorescence excitation spectra are found to correspond to excitation of absorption transitions from the ground electronic state to the b (1)Pi(u), b(') (1)Sigma(u) (+), c(n) (1)Pi(u) (with n=4-8), c(n) (') (1)Sigma(u) (+) (with n=5-9), and c(4) (')(v('))(1)Sigma(u) (+) (with v(')=0-8) states of N(2). The relative fluorescence production cross sections for the observed peaks are determined. No fluorescence has been produced through excitation of the most dominating absorption features of the b-X transition except for the (1,0), (5,0), (6,0), and (7,0) bands, in excellent agreement with recent lifetime measurements and theoretical calculations. Fluorescence peaks, which correlate with the long vibrational progressions of the c(4) (') (1)Sigma(u) (+) (with v(')=0-8) and the b(') (1)Sigma(u) (+) (with v(') up to 19), have been observed. The present results provide important information for further unraveling of complicated and intriguing interactions among the excited electronic states of N(2). Furthermore, solar photon excitation of N(2) leading to the production of c(4) (')(0) may provide useful data required for evaluating and analyzing dayglow models relevant to the interpretation of c(4) (')(0) in the atmospheres of Earth, Jupiter, Saturn, Titan, and Triton.

Discovery of an aurora on Mars

In the high-latitude regions of Earth, aurorae are the often-spectacular visual manifestation of the interaction between electrically charged particles (electrons, protons or ions) with the neutral upper atmosphere, as they precipitate along magnetic field lines. More generally, auroral emissions in planetary atmospheres "are those that result from the impact of particles other than photoelectrons" (ref. 1). Auroral activity has been found on all four giant planets possessing a magnetic field (Jupiter, Saturn, Uranus and Neptune), as well as on Venus, which has no magnetic field. On the nightside of Venus, atomic O emissions at 130.4 nm and 135.6 nm appear in bright patches of varying sizes and intensities, which are believed to be produced by electrons with energy <300 eV (ref. 7). Here we report the discovery of an aurora in the martian atmosphere, using the ultraviolet spectrometer SPICAM on board Mars Express. It corresponds to a distinct type of aurora not seen before in the Solar System: it is unlike aurorae at Earth and the giant planets, which lie at the foot of the intrinsic magnetic field lines near the magnetic poles, and unlike venusian auroras, which are diffuse, sometimes spreading over the entire disk. Instead, the martian aurora is a highly concentrated and localized emission controlled by magnetic field anomalies in the martian crust.

Ultraviolet Imaging Spectroscopy Shows an Active Saturnian System

Neutral oxygen in the saturnian system shows variability, and the total number of oxygen atoms peaks at 4 x 10(34). Saturn's aurora brightens in response to solar-wind forcing, and the auroral spectrum resembles Jupiter's. Phoebe's surface shows variable water-ice content, and the data indicate it originated in the outer solar system. Saturn's rings also show variable water abundance, with the purest ice in the outermost A ring. This radial variation is consistent with initially pure water ice bombarded by meteors, but smaller radial structures may indicate collisional transport and recent renewal events in the past 10(7) to 10(8) years.

Morphological differences between Saturn's ultraviolet aurorae and those of Earth and Jupiter

It has often been stated that Saturn's magnetosphere and aurorae are intermediate between those of Earth, where the dominant processes are solar wind driven, and those of Jupiter, where processes are driven by a large source of internal plasma. But this view is based on information about Saturn that is far inferior to what is now available. Here we report ultraviolet images of Saturn, which, when combined with simultaneous Cassini measurements of the solar wind and Saturn kilometric radio emission, demonstrate that its aurorae differ morphologically from those of both Earth and Jupiter. Saturn's auroral emissions vary slowly; some features appear in partial corotation whereas others are fixed to the solar wind direction; the auroral oval shifts quickly in latitude; and the aurora is often not centred on the magnetic pole nor closed on itself. In response to a large increase in solar wind dynamic pressure Saturn's aurora brightened dramatically, the brightest auroral emissions moved to higher latitudes, and the dawn side polar regions were filled with intense emissions. The brightening is reminiscent of terrestrial aurorae, but the other two variations are not. Rather than being intermediate between the Earth and Jupiter, Saturn's auroral emissions behave fundamentally differently from those at the other planets.

An interplanetary shock traced by planetary auroral storms from the Sun to Saturn

A relationship between solar activity and aurorae on Earth was postulated long before space probes directly detected plasma propagating outwards from the Sun. Violent solar eruption events trigger interplanetary shocks that compress Earth's magnetosphere, leading to increased energetic particle precipitation into the ionosphere and subsequent auroral storms. Monitoring shocks is now part of the 'Space Weather' forecast programme aimed at predicting solar-activity-related environmental hazards. The outer planets also experience aurorae, and here we report the discovery of a strong transient polar emission on Saturn, tentatively attributed to the passage of an interplanetary shock--and ultimately to a series of solar coronal mass ejection (CME) events. We could trace the shock-triggered events from Earth, where auroral storms were recorded, to Jupiter, where the auroral activity was strongly enhanced, and to Saturn, where it activated the unusual polar source. This establishes that shocks retain their properties and their ability to trigger planetary auroral activity throughout the Solar System. Our results also reveal differences in the planetary auroral responses on the passing shock, especially in their latitudinal and local time dependences.