arXiv subjects

Conor A. Nixon

Publications and source records attributed to Conor A. Nixon.

At least 19 recordsLinked to original sources

arXiv2026

Temporal Variability of Titan's Middle-Atmospheric Zonal Winds from Southern Fall to Late Winter (2016-2023)

Previous ALMA observations have revealed unexpectedly strong and rapid variations in Titan's high-altitude equatorial zonal winds, which present a challenge to our understanding of this moon's atmospheric dynamics. Here we report further measurements of Titan's zonal wind field at middle atmospheric altitudes $\approx200$-490 km, based on ALMA observations of spatially and spectrally resolved CH$_3$CN emission between 2016-2023 (corresponding to a solar longitude $L_s=81.6^{\circ}$-$156.4^{\circ}$, spanning Titan's late southern autumn to late winter). These observations indicate substantial, ongoing, rapid temporal variability of Titan's zonal winds, on timescales as short as $\approx1$ Earth month ($0.9^{\circ}$ in $L_s$). The strongest variability is at (near-)equatorial latitudes where the zonal winds are fastest (up to $240\pm26$ ms$^{-1}$), with repeated $\sim40$ ms$^{-1}$ fluctuations occurring between $L_s=146.0^{\circ}$-$156.4^{\circ}$. The observed longer-term trends are qualitatively well-reproduced by two independent, state-of-the-art general circulation models (GCMs), which match the observed decrease in zonal wind speeds over a broad range of latitudes between $L_s=81.6^{\circ}$-$146.0^{\circ}$, followed by a more moderate increase between $L_s=146.0^{\circ}$-$156.4^{\circ}$. The wind speeds at mid-to-high latitudes ($\sim-45^{\circ}$ south) are also typically reasonably well reproduced by the GCMs (differing by $\lesssim30$%). However, the equatorial wind speeds are up to a factor of $\sim2$ faster than model predictions (with the greatest discrepancy around solstice; $L_s\approx90^{\circ}$). Rapid temporal variability of the retrieved zonal winds indicates the presence of strong atmospheric instabilities that are not well reproduced by models, suggesting a need for future GCM improvements.

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arXiv2026

Titan's Resources and their Utilization

Saturn's moon Titan is a unique environment in the solar system. It is the only moon with an atmosphere, composed primarily of the gases N2 and CH4. It is also the only world to have abundant surface hydrocarbons CxHy, which are found as both liquids (seas, lakes) and solids (dunes). Meanwhile, oxygen is also readily accessible in the form of crustal water. This combination of abundant reduced carbon, along with available nitrogen and oxygen makes Titan an enticing world rich in resources that can be readily used to make food, fuel, building materials and more - potentially mission-enabling for long-duration voyages or habitats in the outer solar system. At the same time Titan, as an icy moon, is likely to be depleted at the surface in heavier elements including metals, which must therefore be found and brought from elsewhere. In this article we describe both the available resources on Titan, and also their potential uses. We compare and contrast the resource availability and potential in-situ utilization (ISRU) with other destinations suggested for human habitation such as the Moon and Mars. We conclude by discussing what future work will be important to further characterize Titan's resources, and to develop technologies for their utilization.

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arXiv2026

Foundations for Discovery: A Coordinated Fleet Approach to NASA Astrophysics

This white paper presents an analysis of Astro2020 science priorities and NASA's future astrophysics mission architecture, advocating for a coordinated fleet of \$1--2B missions, smaller than typical Flagship observatories, but strategically designed to complement them, i.e. a ``Next Generation Great Observatories" program. The study addresses opportunities in current mission planning, design, and implementation and proposes a strategic approach to maximize scientific return on investment while strengthening partnerships across NASA divisions, other government organizations, universities, and industry.

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arXiv2026

Detection of propadiene (CH$_2$CCH$_2$), propene (C$_3$H$_6$) and non-detection of propane (C$_3$H$_8$) in Jupiter's northern polar stratosphere

We report the first detection of stratospheric propadiene (CH$_2$CCH$_2$) and propene (C$_3$H$_6$) at Jupiter's mid-to-high northern latitudes using IRTF-TEXES measurements recorded on March 5-6, 2025. Using radiative transfer software to quantitatively test for the presence of propadiene and propene, we report a $>$12-$σ$ detection of propadiene and a $>$17-$σ$ detection of propene inside Jupiter's northern auroral region (henceforth 'NAR'), where the species are most concentrated. For example, at 62$^\circ$N inside Jupiter's NAR, we derive a 1-mbar propadiene abundance of 2.0 $\pm$ 0.2 ppbv, which is 40 $\pm$ 3 higher than abundances predicted by the Moses & Poppe (2017) photochemical model (henceforth 'MP17'), and significantly higher than the 1.2-ppbv upper limit abundance derived at 42$^\circ$N (the lowest latitude sampled by the observations). Similarly, we derive a 1-mbar propene abundance of 8.1 $\pm$ 0.5 ppbv at 62$^\circ$N inside Jupiter's NAR, which is 28 $\pm$ 2 higher than the MP17 predicted abundance and higher than the 6-ppbv 1-mbar upper limit abundance derived at 42$^\circ$N. The fact that propadiene and propene are most enriched inside Jupiter's NAR strongly suggests that perturbations to the chemistry by auroral-related heating and exogenous ions/electrons are responsible for their significant enrichment. Spectral features of propane (C$_3$H$_8$) were not detected at any of the locations sampled by the data: 3-$σ$ upper limits of 10 ppbv were derived at the 10-mbar level at 62$^\circ$N inside Jupiter's NAR. The non-detection of propane could, in part, be explained by the vertical sensitivity of its spectral features to deeper pressures, where there is negligible auroral-related heating. The results of this work advocate for development of ion-neutral chemistry models of Jupiter's polar stratosphere.

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arXiv2026

Terrestrial Analogs to Titan for Geophysical Research

Saturn's moon Titan exhibits remarkable parallels to the Earth in many geophysical and geological processes not found elsewhere in the solar system at the present day. These include a nitrogen atmosphere with a condensible gas - methane - replacing the Earth's water, leading to an active meteorology with rainfall and surface manifestations including rivers, lakes and seas, and the dissolution of karstic terrain. Other phenomena such as craters, dunes, and tectonic features are found elsewhere - e.g. on Mars and Venus - but their continuing alteration by pluvial, fluvial and lacustrine processes can be studied only on Earth and Titan. Meanwhile Titan also hosts an interior liquid water ocean with similarities to the Earth as well as to ocean worlds such as Europa and Enceladus. Our focus in this review paper is twofold: to describe the geophysical and geological parallels between Earth and Titan, and to evaluate the yet-underexploited possibilities for field analog research to gain new knowledge about these processes. To date, Titan's much colder temperature and different atmospheric and crustal materials have led to a skepticism that useful analogs can be found on Earth. Our conclusion, however, is that a much larger range of useful analog field work is possible and this work will substantially enhance our knowledge of both worlds. Such investigation will supplement the existing sparse data for Titan returned by space missions, will greatly enhance our understanding of such datasets, and will help to provide science impetus and goals for future missions.

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arXiv2025

Exploring Giant Planet Atmospheres with Habitable Worlds Observatory

Visible and ultraviolet imaging and spectroscopy of Solar System giant planets can set the paradigm for the atmospheric, ionospheric, and magnetospheric processes shaping the diversity of giant exoplanets, brown dwarfs, and their interactions with stellar hosts. Spectra of their molecular absorptions, aerosol scattering, airglow, and auroral emissions can reveal these dynamic atmospheres in three dimensions. Given that giant planets are extended, bright, moving, and rotating objects, with extreme dynamic range and highly variable appearances, they impose specific mission and instrumentation requirements on future large space-based optical/UV observatories like the proposed Habitable Worlds Observatory (HWO). We advocate that HWO must have the capability to track non-sidereal targets like the giant planets and their satellites; should be able to view auroras and atmospheres without saturation (e.g., through the use of filters or fast read-out modes); and with a high dynamic range to explore faint objects near bright discs. HWO should enable spatially-resolved spectroscopy from $\sim80$ nm to $\sim900$ nm, capturing H$_2$ Lyman and Werner band series and H Lyman-$α$ in the far-UV; molecular absorptions and scattering in the mid-UV/visible; and deep hydrogen/methane absorptions in the 800-900 nm for cloud characterisation and CH$_4$ mapping. Imaging should enable time-resolved observations, from seconds to create auroral movies, to hours for cloud tracking and winds, to months and years for atmosphere/ionosphere variability. We advocate that an imager should have sufficient field of view to capture Jupiter ($>50$\arcsec), and that UV/visible integral field spectrographs be considered with both narrow ($3$\arcsec) and wide ($>10$\arcsec) field capabilities to provide efficient mapping of atmospheres and auroras. [Abbr]

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arXiv2025

Foundation Models for Astrobiology: Paper I -- Workshop and Overview

Advances in machine learning over the past decade have resulted in a proliferation of algorithmic applications for encoding, characterizing, and acting on complex data that may contain many high dimensional features. Recently, the emergence of deep-learning models trained across very large datasets has created a new paradigm for machine learning in the form of Foundation Models. Foundation Models are programs trained on very large and broad datasets with an extensive number of parameters. Once built, these powerful, and flexible, models can be utilized in less resource-intensive ways to build many different, downstream applications that can integrate previously disparate, multimodal data. The development of these applications can be done rapidly and with a much lower demand for machine learning expertise. And the necessary infrastructure and models themselves are already being established within agencies such as NASA and ESA. At NASA this work is across several divisions of the Science Mission Directorate including the NASA Goddard and INDUS Large Language Models and the Prithvi Geospatial Foundation Model. And ESA initiatives to bring Foundation Models to Earth observations has led to the development of TerraMind. A workshop was held by the NASA Ames Research Center and the SETI Institute, in February 2025, to investigate the potential of Foundation Models for astrobiological research and to determine what steps would be needed to build and utilize such a model or models. This paper shares the findings and recommendations of that workshop, and describes clear near-term, and future opportunities in the development of a Foundation Model (or Models) for astrobiology applications. These applications would include a biosignature, or life characterization, task, a mission development and operations task, and a natural language task for integrating and supporting astrobiology research needs.

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arXiv2025

Using SOFIA's EXES to search for C$_6$H$_2$ and C$_4$N$_2$ in Titan's atmosphere

In Titan's atmosphere, the chemistry of small hydrocarbons and nitriles represent an important link from molecular species to the ubiquitous organic haze that gives Titan its characteristic yellow color. Here we present a new search for two previously undetected molecules, triacetylene (C$_{6}$H$_{2}$) and the gas phase dicyanoacetylene (C$_{4}$N$_{2}$), using the Echelon-Cross-Echelle Spectrograph (EXES) instrument aboard the SOFIA (Stratospheric Observatory For Infrared Astronomy) aircraft. We do not detect these two molecules but determine upper limits for their mixing ratios and column abundances. We find the $3σ$ upper limits on the uniform volume mixing ratio (VMR) above 100 km for C$_{6}$H$_{2}$ to be $4.3\times10^{-11}$ which is lower than the photochemical model predictions. This new upper limit suggests that the growth of linear molecules is inhibited. We also put a strict upper limit on the uniform VMR for gas phase C$_{4}$N$_{2}$ above 125 km to be $1.0\times10^{-10}$. This upper limit is well below the saturation mixing ratio at this altitude for C$_{4}$N$_{2}$ and greatly limits the feasibility of C$_{4}$N$_{2}$ forming ice from condensation.

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arXiv2025

The Discovery of 25 um Interstellar Methanol

We present the first astrophysical detection of methanol (CH3OH) in the torsional band near 25 um. Using high resolution mid-infrared (MIR) spectroscopy, we identified over seventy gas-phase CH3OH absorption lines between 20 and 28 um towards the massive protostar NGC 7538 IRS 1 with SOFIA/EXES. We derive a temperature of 180 K and a total column density of 2 x 10^17 cm-2, comparable to sub-mm measurements. Complementary analysis of acetylene (C2H2) absorption lines is also included. Both CH3OH and C2H2 reveal an unresolved second velocity component. These MIR absorption lines likely probe the molecular material in two edge-on disks, supporting the scenario that NGC 7538 IRS 1 consists of multiple protostars. We provide an updated line list for the torsional band of CH3OH, which was generated from lab work and model calculations. This discovery and the updated line list will enable the search for CH3OH in JWST/MIRI spectra.

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arXiv2025

The Atmosphere of Titan in Late Northern Summer from JWST and Keck Observations

Saturn's moon Titan undergoes a long annual cycle of 29.45 Earth years. Titan's northern winter and spring were investigated in detail by the Cassini-Huygens spacecraft (2004-2017), but the northern summer season remains sparsely studied. Here we present new observations from the James Webb Space Telescope (JWST) and Keck II telescope made in 2022 and 2023 during Titan's late northern summer. Using JWST's mid-infrared instrument, we spectroscopically detected the methyl radical, the primary product of methane break-up and key to the formation of ethane and heavier molecules. Using the near-infrared spectrograph onboard JWST, we detected several non-local thermodynamic equilibrium CO and CO2 emission bands, which allowed us to measure these species over a wide altitude range. Lastly, using the near-infrared camera onboard JWST and Keck II, we imaged northern hemisphere tropospheric clouds evolving in altitude, which provided new insights and constraints on seasonal convection patterns. These observations pave the way for new observations and modelling of Titan's climate and meteorology as it progresses through the northern fall equinox, when its atmosphere is expected to show notable seasonal changes.

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arXiv2025

Internal Heat and Energy Imbalance of Uranus

With its extreme axial tilt, radiant energy budget and internal heat of Uranus remain among the most intriguing mysteries of our Solar System. Here, we present the global radiant energy budget spanning a complete orbital period, revealing significant seasonal variations driven primarily by the highly variable solar flux. Despite these fluctuations, emitted thermal power consistently exceeds absorbed solar power, indicating a net energy loss and ongoing global cooling. Based on the seasonal variations of radiant energy budget, we determine a statistically significant internal heat flux. This finding resolves a long-standing debate over whether Uranus possesses internal heat. We also examine the energy budget of the weather layer by combining the internal heat with the radiant energies, revealing significant energy imbalances at both global and hemispheric scales. These global and hemispheric imbalances should be considered in theoretical and numerical models. The Uranus flagship mission, as recommended by the recent survey, will provide crucial observations to address more unresolved questions and advance our understanding of this enigmatic ice giant.

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arXiv2025

Open Questions and Future Directions in Titan Science

In this chapter we attempt to distill the very large number of possible future inquiries of Titan into a relatively concise list of twenty high level questions - each of which of would necessarily entail a multitude of more specific investigations and studies. While this list does not encompass all possible open questions, and is divided into topics according to our preference and not in any way uniquely, we believe that it does however span a wide range of the most intriguing topics about Titan, and may form some sort of guide especially for those embarking into Titan studies for the first time. At the end of this chapter we return to explore how these four techniques may be used to answer the large, high-level open questions in Titan science.

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arXiv2024

The Polar Stratosphere of Jupiter

Observations of the Jovian upper atmosphere at high latitudes in the UV, IR and mm/sub-mm all indicate that the chemical distributions and thermal structure are broadly influenced by auroral particle precipitations. Mid-IR and UV observations have shown that several light hydrocarbons (up to 6 carbon atoms) have altered abundances near Jupiter's main auroral ovals. Ion-neutral reactions influence the hydrocarbon chemistry, with light hydrocarbons produced in the upper stratosphere, and heavier hydrocarbons as well as aerosols produced in the lower stratosphere. One consequence of the magnetosphere-ionosphere coupling is the existence of ionospheric jets that propagate into the neutral middle stratosphere, likely acting as a dynamical barrier to the aurora-produced species. As the ionospheric jets and the background atmosphere do not co-rotate at the same rate, this creates a complex system where chemistry and dynamics are intertwined. The ion-neutral reactions produce species with a spatial distribution following the SIII longitude system in the upper stratosphere. As these species sediment down to the lower stratosphere, and because of the progressive dynamical decoupling between the ionospheric flows and the background atmosphere, the spatial distribution of the auroral-related species progressively follows a zonal distribution with increasing pressures that ultimately produces a system of polar and subpolar hazes that extends down to the bottom of the stratosphere. This paper reviews the most recent work addressing different aspects of this environment.

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arXiv2024

Observations of Titan's Stratosphere During Northern Summer: Temperatures, CH3CN and CH3D Abundances

Titan's atmospheric composition and dynamical state have previously been studied over numerous epochs by both ground- and space-based facilities. However, stratospheric measurements remain sparse during Titan's northern summer and fall. The lack of seasonal symmetry in observations of Titan's temperature field and chemical abundances raises questions about the nature of the middle atmosphere's meridional circulation and evolution over Titan's 29-yr seasonal cycle that can only be answered through long-term monitoring campaigns. Here, we present maps of Titan's stratospheric temperature, acetonitrile (or methyl cyanide; CH$_3$CN), and monodeuterated methane (CH$_3$D) abundances following Titan's northern summer solstice obtained with Band 9 ($\sim0.43$ mm) ALMA observations. We find that increasing temperatures towards high-southern latitudes, currently in winter, resemble those observed during Titan's northern winter by the Cassini mission. Acetonitrile abundances have changed significantly since previous (sub)millimeter observations, and we find that the species is now highly concentrated at high-southern latitudes. The stratospheric CH$_3$D content is found to range between 4-8 ppm in these observations, and we infer the CH$_4$ abundance to vary between $\sim0.9-1.6\%$ through conversion with previously measured D/H values. A global value of CH$_4=1.15\%$ was retrieved, lending further evidence to the temporal and spatial variability of Titan's stratospheric methane when compared with previous measurements. Additional observations are required to determine the cause and magnitude of stratospheric enhancements in methane during these poorly understood seasons on Titan.

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arXiv2024

The Composition and Chemistry of Titan's Atmosphere

In this article 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 (ALMA), 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 paper 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 paper is devoted to the gaps in our present knowledge of Titan's chemical composition and how further work may fill those gaps.

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arXiv2023

Spitzer IRS Observations of Titan as a Precursor to JWST MIRI Observations

In this work we present, for the first time, infrared spectra of Titan from the Spitzer Space Telescope ($2004-2009$). The data are from both the short wavelength-low resolution (SL, $5.13-14.29\mathrm{μm}, R\sim60-127$) and short wavelength-high resolution channels (SH, $9.89 - 19.51\mathrm{μm}, R\sim600$) showing the emissions of CH$_{4}$, C$_{2}$H$_{2}$, C$_{2}$H$_{4}$, C$_{2}$H$_{6}$, C$_{3}$H$_{4}$, C$_{3}$H$_{6}$, C$_{3}$H$_{8}$, C$_{4}$H$_{2}$, HCN, HC$_{3}$N, and CO$_{2}$. We compare the results obtained for Titan from Spitzer to those of the Cassini Composite Infrared Spectrometer (CIRS) for the same time period, focusing on the $16.35-19.35\mathrm{μm}$ wavelength range observed by the SH channel but impacted by higher noise levels in CIRS observations. We use the SH data to provide estimated haze extinction cross-sections for the $16.67-17.54\mathrm{μm}$ range that are missing in previous studies. We conclude by identifying spectral features in the $16.35-19.35\mathrm{μm}$ wavelength range, including two prominent emission features at 16.39 and $17.35\mathrm{μm}$, that could be analyzed further through upcoming James Webb Space Telescope Cycle 1 observations with the Mid-Infrared Instrument ($5.0-28.3\mathrm{μm}, R\sim1500-3500$). We also highlight gaps in current spectroscopic knowledge of molecular bands, including candidate trace species such as C$_{60}$ and detected trace species such as C$_{3}$H$_{6}$, that could be addressed by theoretical and laboratory study.

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arXiv2021

Science goals and new mission concepts for future exploration of Titan's atmosphere geology and habitability: Titan POlar Scout/orbitEr and In situ lake lander and DrONe explorer (POSEIDON)

In response to ESA Voyage 2050 announcement of opportunity, we propose an ambitious L-class mission to explore one of the most exciting bodies in the Solar System, Saturn largest moon Titan. Titan, a "world with two oceans", is an organic-rich body with interior-surface-atmosphere interactions that are comparable in complexity to the Earth. Titan is also one of the few places in the Solar System with habitability potential. Titan remarkable nature was only partly revealed by the Cassini-Huygens mission and still holds mysteries requiring a complete exploration using a variety of vehicles and instruments. The proposed mission concept POSEIDON (Titan POlar Scout/orbitEr and In situ lake lander DrONe explorer) would perform joint orbital and in situ investigations of Titan. It is designed to build on and exceed the scope and scientific/technological accomplishments of Cassini-Huygens, exploring Titan in ways that were not previously possible, in particular through full close-up and in situ coverage over long periods of time. In the proposed mission architecture, POSEIDON consists of two major elements: a spacecraft with a large set of instruments that would orbit Titan, preferably in a low-eccentricity polar orbit, and a suite of in situ investigation components, i.e. a lake lander, a "heavy" drone (possibly amphibious) and/or a fleet of mini-drones, dedicated to the exploration of the polar regions. The ideal arrival time at Titan would be slightly before the next northern Spring equinox (2039), as equinoxes are the most active periods to monitor still largely unknown atmospheric and surface seasonal changes. The exploration of Titan northern latitudes with an orbiter and in situ element(s) would be highly complementary with the upcoming NASA New Frontiers Dragonfly mission that will provide in situ exploration of Titan equatorial regions in the mid-2030s.

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arXiv2020

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/sub-millimeter Array (ALMA). Multiple lines of cyclopropenylidene were detected in two separate datasets: ~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)E12 cm-2 in 2016 and (1-2)E12 cm-2 in 2017, similar to photochemical model predictions. Previously the C3H3+ 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 non-detections in both cases, and determining 2-σ 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.

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