COMPOSITIONAL MEASUREMENTS OF SATURN'S UPPER ATMOSPHERE AND RINGS FROM CASSINI INMS

In September 2017, the Cassini-Huygens mission to the Saturn system came to an end as the spacecraft intentionally entered the planet’s atmosphere. Prior to entry, the spacecraft executed a series of 22 highly inclined orbits, the Grand Finale orbits, through the previously unexplored region between Saturn and its innermost D ring, yielding the first in situ measurements of the planet’s upper atmosphere and ring system. During these orbits, measurements from the Ion and Neutral Mass Spectrometer (INMS) revealed that the composition of Saturn’s thermosphere is intricately connected to Saturn’s D ring and much more chemically complex than previously believed. These measurements enable the investigation of the composition of the upper atmosphere and rings, the thermal structure and energetics of the upper atmosphere, and the transfer of material from the rings to the atmosphere. In this thesis, we provide an in-depth compositional analysis of the mass spectra returned from INMS during Cassini’s deepest Grand Finale orbits into Saturn’s atmosphere. This includes four orbits that measured the isothermal region of Saturn’s thermosphere (orbits 288, 290, 291, and 292) and atmospheric entry (orbit 293), which probed approximately 200 km deeper than the other orbits and detected an increase in temperature in Saturn’s thermosphere. Signal returned from the instrument includes native Saturn species, as expected, as well as a significant amount of signal attributed to ices and higher mass organics believed to be flowing into Saturn’s atmosphere from the rings. We identify species present in the spectra using a mass spectral deconvolution algorithm specifically designed to handle unit resolution spaceflight mass spectrometry data when limited calibration data is available. The retrieved mixing ratio and density profiles suggest that many species exhibit behavior indicative of an external source that is likely Saturn’s innermost D ring, and that this ring material heavily influences Saturn’s thermospheric composition. We use a 1-D diffusion model to analyze the distribution of species and calculate the downward external flux and mass deposition rates of ring volatile species into Saturn’s atmosphere. During these observations ring material was being deposited into Saturn’s equatorial region at a rate on the order of 104 kg/s. An influx of such magnitude would deplete the D ring on the order of thousands of years, leading to the speculation that the influx must be caused by a transient phenomenon that could be a consequence of recent perturbations in the region. This influx of material could have far reaching implications on the energetics, dynamics, and temperature structure in this region and could influence haze and cloud production in Saturn’s atmosphere. These analyses are vital to improve our understanding of the interactions between Saturn and its rings, and the results are critical to advance photochemical modeling efforts of Saturn’s upper atmosphere

Titan Atmospheric Chemistry Revealed by Low-temperature N2-CH4 Plasma Discharge Experiments

Chemistry in Titan's N2-CH4 atmosphere produces complex organic aerosols. The chemical processes and the resulting organic compounds are still far from understood, although extensive observations, laboratory, and theoretical simulations have greatly improved physical and chemical constraints on Titan's atmosphere. Here, we conduct a series of Titan atmosphere simulation experiments with N2-CH4 gas mixtures and investigate the effect of initial CH4 ratio, pressure, and flow rate on the production rates and composition of the gas and solid products at a Titan relevant temperature (100 K) for the first time. We find that the production rate of the gas and solid products increases with increasing CH4 ratio. The nitrogen-containing species have much higher yield than hydrocarbons in the gas products, and the N-to-C ratio of the solid products appears to be the highest compared to previous plasma simulations with the same CH4 ratio. The greater degree of nitrogen incorporation in the low temperature simulation experiments suggests temperature may play an important role in nitrogen incorporation in Titan's cold atmosphere. We also find that H2 is the dominant gas product and serves as an indicator of the production rate of new organic molecules in the experiment, and that CH2NH may greatly contribute to the incorporation of both carbon and nitrogen into the solid particles. The pressure and flow rate affect the amount of time of the gas mixture exposed to the energy source and therefore impact the N2-CH4 chemistry initiated by the plasma discharge, emphasizing the influence of the energy flux in Titan atmospheric chemistry.Comment: Accepted in ACS Earth and Space Chemistry, 6 figure

Probing Titan's Complex Atmospheric Chemistry Using the Atacama Large Millimeter/Submillimeter Array

Titan is Saturn's largest moon, with a thick (1.45 bar) atmosphere composed primarily of molecular nitrogen and methane. Atmospheric photochemistry results in the production of a wide range of complex organic molecules, including hydrocarbons, nitriles, aromatics and other species of possible pre-biotic relevance. Titan's carbon-rich atmosphere may be analogous to that of primitive terrestrial planets throughout the universe, yet its origin, evolution and complete chemical inventory are not well understood. Here we present spatially-resolved maps of emission from C2H5CN, HNC, HC3N, CH3CN and CH3CCH in Titan's atmosphere, observed using the Atacama Large Millimeter/submillimeter Array (ALMA) in 2012-2013. These data show previously-undetected spatial structures for the observed species and provide the first spectroscopic detection of C2H5CN on Titan. Our maps show spatially resolved peaks in Titan's northern and southern hemispheres, consistent with photochemical production and transport in the upper atmosphere followed by subsidence over the poles. The HNC emission peaks are offset from the polar axis, indicating that Titan's mesosphere may be more longitudinally variable than previously thought

Detection of Cyclopropenylidene on Titan with ALMA

We report the first detection on Titan of the small cyclic molecule cyclopropenylidene (c-C3H2) from high-sensitivity spectroscopic observations made with the Atacama Large Millimeter/submillimeter Array. Multiple lines of cyclopropenylidene were detected in two separate data sets: ~251 GHz in 2016 (Band 6) and ~352 GHz in 2017 (Band 7). Modeling of these emissions indicates abundances of 0.50 ± 0.14 ppb (2016) and 0.28 ± 0.08 (2017) for a 350 km step model, which may either signify a decrease in abundance, or a mean value of 0.33 ± 0.07 ppb. Inferred column abundances are (3–5) × 1012 cm−2 in 2016 and (1–2) × 1012 cm−2 in 2017, similar to photochemical model predictions. Previously the C3H${}_{3}^{+}$ ion has been measured in Titan's ionosphere by Cassini's Ion and Neutral Mass Spectrometer (INMS), but the neutral (unprotonated) species has not been detected until now, and aromatic versus aliphatic structure could not be determined by the INMS. Our work therefore represents the first unambiguous detection of cyclopropenylidene, the second known cyclic molecule in Titan's atmosphere along with benzene (C6H6) and the first time this molecule has been detected in a planetary atmosphere. We also searched for the N-heterocycle molecules pyridine and pyrimidine finding nondetections in both cases, and determining 2σ upper limits of 1.15 ppb (c-C5H5N) and 0.85 ppb (c-C4H4N2) for uniform abundances above 300 km. These new results on cyclic molecules provide fresh constraints on photochemical pathways in Titan's atmosphere, and will require new modeling and experimental work to fully understand the implications for complex molecule formation

Saturn's atmospheric response to the large influx of ring material inferred from Cassini INMS measurements

During the Grand Finale stage of the Cassini mission, organic-rich ring material was discovered to be flowing into Saturn's equatorial upper atmosphere at a surprisingly large rate. Through a series of photochemical models, we have examined the consequences of this ring material on the chemistry of Saturn's neutral and ionized atmosphere. We find that if a substantial fraction of this material enters the atmosphere as vapor or becomes vaporized as the solid ring particles ablate upon atmospheric entry, then the ring-derived vapor would strongly affect the composition of Saturn's ionosphere and neutral stratosphere. Our surveys of Cassini infrared and ultraviolet remote-sensing data from the final few years of the mission, however, reveal none of these predicted chemical consequences. We therefore conclude that either (1) the inferred ring influx represents an anomalous, transient situation that was triggered by some recent dynamical event in the ring system that occurred a few months to a few tens of years before the 2017 end of the Cassini mission, or (2) a large fraction of the incoming material must have been entering the atmosphere as small dust particles less than ~100 nm in radius, rather than as vapor or as large particles that are likely to ablate. Future observations or upper limits for stratospheric neutral species such as HC$_3$N, HCN, and CO$_2$ at infrared wavelengths could shed light on the origin, timing, magnitude, and nature of a possible vapor-rich ring-inflow event.Comment: accepted in Icaru

Volatiles in Titan's lower atmosphere: Reinterpretation of Huygens-GCMS data

International audienceUsing a novel approach, we reanalyzed data acquired by the GCMS instrument aboard Huygens during its descent to Titan. We were able to retrieve concentration of several key volatile species in Titan's troposphere and their vertical profiles

The CH₄structure in Titan's upper atmosphere revisited

International audienc

Seasonal variation of trace species in Titan's ionosphere

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The CH₄structure in Titan's upper atmosphere revisited

International audienc