arXiv subjects

Nicholas A. Teanby

Publications and source records attributed to Nicholas A. Teanby.

At least 19 recordsLinked to original sources

arXiv2026

Disk-averaged spectrum generation for spherically symmetric planets and moons

Here I present a simple and efficient method for generating a disk-averaged planetary spectrum. The technique uses a weighted average of component spectra generated on an irregular grid of offsets from the planet center. This accounts for variations in center to limb brightness and emission from beyond the limb, which is important for bodies with extended atmospheres such as Saturn's moon Titan. The method assumes spherically symmetric emission and requires a grid of component spectra fine enough to faithfully capture radial radiance variations. An example application to Titan's 7.7 $μ$m methane emission demonstrates the method.

astro-ph.EP↗

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.

astro-ph.EP↗

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.

astro-ph.EP↗

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.

astro-ph.EP↗

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.

astro-ph.EP↗

arXiv2024

Atacama Large Aperture Submillimeter Telescope (AtLAST) Science: Planetary and Cometary Atmospheres

The study of planets and small bodies within our Solar System is fundamental for understanding the formation and evolution the Earth and other planets. Compositional and meteorological studies of the giant planets provide a foundation for understanding the nature of the most commonly observed exoplanets, while spectroscopic observations of the atmospheres of terrestrial planets, moons, and comets provide insights into the past and present-day habitability of planetary environments, and the availability of the chemical ingredients for life. While prior and existing (sub)millimeter observations have led to major advances in these areas, progress is hindered by limitations in the dynamic range, spatial and temporal coverage, as well as sensitivity of existing telescopes and interferometers. Here, we summarize some of the key planetary science use cases that factor into the design of the Atacama Large Aperture Submillimeter Telescope (AtLAST), a proposed 50-m class single dish facility: (1) to more fully characterize planetary wind fields and atmospheric thermal structures, (2) to measure the compositions of icy moon atmospheres and plumes, (3) to obtain detections of new, astrobiologically relevant gases and perform isotopic surveys of comets, and (4) to perform synergistic, temporally-resolved measurements in support of dedicated interplanetary space missions. The improved spatial coverage (several arcminutes), resolution ($\sim1.2''-12''$), bandwidth (several tens of GHz), dynamic range ($\sim10^5$) and sensitivity ($\sim1$ mK km s$^{-1}$) required by these science cases would enable new insights into the chemistry and physics of planetary environments, the origins of prebiotic molecules and the habitability of planetary systems in general.

astro-ph.EP↗

arXiv2023

Latitudinal variations in methane abundance, aerosol opacity and aerosol scattering efficiency in Neptune's atmosphere determined from VLT/MUSE

Spectral observations of Neptune made in 2019 with the MUSE instrument at the Very Large Telescope in Chile have been analysed to determine the spatial variation of aerosol scattering properties and methane abundance in Neptune's atmosphere. The darkening of the South Polar Wave (SPW) at $\sim$ 60$^\circ$S, and dark spots such as the Voyager 2 Great Dark Spot is concluded to be due to a spectrally-dependent darkening ($λ< 650$nm) of particles in a deep aerosol layer at $\sim$ 5 bar and presumed to be composed of a mixture of photochemically-generated haze and H$_2$S ice. We also note a regular latitudinal variation of reflectivity at wavelengths of very low methane absorption longer than $\sim$ 650 nm, with bright zones latitudinally separated by $\sim$ 25$^\circ$. This feature, similar to the spectral characteristics of a discrete deep bright spot DBS-2019 found in our data, is found to be consistent with a brightening of the particles in the same $\sim$5-bar aerosol layer at $λ> 650 $ nm. We find the properties of an overlying methane/haze aerosol layer at $\sim$ 2 bar are, to first-order, invariant with latitude, while variations in the opacity of an upper tropospheric haze layer reproduce the observed reflectivity at methane-absorbing wavelengths, with higher abundances found at the equator and also in a narrow `zone' at $80^\circ$S. Finally, we find the mean abundance of methane below its condensation level to be 6--7% at the equator reducing to $\sim$3% south of $\sim$25$^\circ$S, although the absolute abundances are model dependent.

astro-ph.EP↗

arXiv2023

Spectral determination of the colour and vertical structure of dark spots in Neptune's atmosphere

Previous observations of dark vortices in Neptune's atmosphere, such as Voyager-2's Great Dark Spot, have been made in only a few, broad-wavelength channels, which has hampered efforts to pinpoint their pressure level and what makes them dark. Here, we present Very Large Telescope (Chile) MUSE spectrometer observations of Hubble Space Telescope's NDS-2018 dark spot, made in 2019. These medium-resolution 475 - 933 nm reflection spectra allow us to show that dark spots are caused by a darkening at short wavelengths (< 700 nm) of a deep ~5-bar aerosol layer, which we suggest is the H$_2$S condensation layer. A deep bright spot, named DBS-2019, is also visible on the edge of NDS-2018, whose spectral signature is consistent with a brightening of the same 5-bar layer at longer wavelengths (> 700 nm). This bright feature is much deeper than previously studied dark spot companion clouds and may be connected with the circulation that generates and sustains such spots.

astro-ph.EP↗

arXiv2022

Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including Dark Spots

We present a reanalysis (using the Minnaert limb-darkening approximation) of visible/near-infrared (0.3 - 2.5 micron) observations of Uranus and Neptune made by several instruments. We find a common model of the vertical aerosol distribution that is consistent with the observed reflectivity spectra of both planets, consisting of: 1) a deep aerosol layer with a base pressure > 5-7 bar, assumed to be composed of a mixture of H2S ice and photochemical haze; 2) a layer of photochemical haze/ice, coincident with a layer of high static stability at the methane condensation level at 1-2 bar; and 3) an extended layer of photochemical haze, likely mostly of the same composition as the 1-2-bar layer, extending from this level up through to the stratosphere, where the photochemical haze particles are thought to be produced. For Neptune, we find that we also need to add a thin layer of micron-sized methane ice particles at ~0.2 bar to explain the enhanced reflection at longer methane-absorbing wavelengths. We suggest that methane condensing onto the haze particles at the base of the 1-2-bar aerosol layer forms ice/haze particles that grow very quickly to large size and immediately 'snow out' (as predicted by Carlson et al. 1988), re-evaporating at deeper levels to release their core haze particles to act as condensation nuclei for H2S ice formation. In addition, we find that the spectral characteristics of 'dark spots', such as the Voyager-2/ISS Great Dark Spot and the HST/WFC3 NDS-2018, are well modelled by a darkening or possibly clearing of the deep aerosol layer only.

astro-ph.EP↗

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.

astro-ph.EP↗

arXiv2020

Uranus' stratospheric HCl upper limit from Herschel/SPIRE

Herschel/SPIRE observations of Uranus are used to search for stratospheric hydrogen chloride (HCl) emission at 41.74 cm$^{-1}$. HCl was not detected and instead 3$σ$ upper limits were determined; $<$6.2 ppb ($<$2.0$\times$10$^{14}$ molecules/cm$^{2}$) for a 0.1 mbar step profile and $<$0.40 ppb ($<$1.2$\times$10$^{14}$ molecules/cm$^{2}$) for a 1 mbar step profile. HCl is expected to have an external source and these upper limits are consistent with abundances of other external species (CO, H$_2$O, CO$_2$) and a solar composition source.

astro-ph.EP↗

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.

astro-ph.EP↗

arXiv2019

Cassini Composite Infrared Spectrometer (CIRS) Observations of Titan 2004--2017

From 2004 to 2017, the Cassini spacecraft orbited Saturn, completing 127 close flybys of its largest moon, Titan. Cassini's Composite Infrared Spectrometer (CIRS), one of 12 instruments carried on board, profiled Titan in the thermal infrared (7-1000 microns) throughout the entire 13-year mission. CIRS observed on both targeted encounters (flybys) and more distant opportunities, collecting 8.4 million spectra from 837 individual Titan observations over 3633 hours. Observations of multiple types were made throughout the mission, building up a vast mosaic picture of Titan's atmospheric state across spatial and temporal domains. This paper provides a guide to these observations, describing each type and chronicling its occurrences and global-seasonal coverage. The purpose is to provide a resource for future users of the CIRS data set, as well as those seeking to put existing CIRS publications into the overall context of the mission, and to facilitate future inter-comparison of CIRS results with those of other Cassini instruments, and ground-based observations.

astro-ph.EP↗

arXiv2019

Latitudinal variation in the abundance of methane (CH4) above the clouds in Neptune's atmosphere from VLT/MUSE Narrow Field Mode Observations

Observations of Neptune, made in 2018 using the new Narrow Field Adaptive Optics mode of the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT) from 0.48 - 0.93 micron, are analysed here to determine the latitudinal and vertical distribution of cloud opacity and methane abundance in Neptune's observable troposphere (0.1 - ~3 bar). Previous observations at these wavelengths in 2003 by HST/STIS (Karkoschka and Tomasko 2011, Icarus 205, 674-694) found that the mole fraction of methane above the cloud tops (at ~2 bar) varied from ~4% at equatorial latitudes to ~2% at southern polar latitudes, by comparing the observed reflectivity at wavelengths near 825 nm controlled primarily by either methane absorption or H2-H2/H2-He collision-induced absorption. We find a similar variation in cloud-top methane abundance in 2018, which suggests that this depletion of methane towards Neptune's pole is potentially a long-lived feature, indicative of long-term upwelling at mid-equatorial latitudes and subsidence near the poles. By analysing these MUSE observations along the central meridian with a retrieval model, we demonstrate that a broad boundary between the nominal and depleted methane abundances occurs at between 20 - 40S. We also find a small depletion of methane near the equator, perhaps indicating subsidence there, and a local enhancement near 60 - 70S, which we suggest may be associated with South Polar Features (SPFs) seen in Neptune's atmosphere at these latitudes. Finally, by the use of both a reflectivity analysis and a principal component analysis, we demonstrate that this depletion of methane towards the pole is apparent at all locations on Neptune's disc, and not just along its central meridian.

astro-ph.EP↗

arXiv2018

Probable detection of hydrogen sulphide (H$_2$S) in Neptune's atmosphere

Recent analysis of Gemini-North/NIFS H-band (1.45 - 1.8 $μ$m) observations of Uranus, recorded in 2010, with recently updated line data has revealed the spectral signature of hydrogen sulphide (H$_2$S) in Uranus's atmosphere (Irwin et al., 2018). Here, we extend this analysis to Gemini-North/NIFS observations of Neptune recorded in 2009 and find a similar detection of H$_2$S spectral absorption features in the 1.57 - 1.58 $μ$m range, albeit slightly less evident, and retrieve a mole fraction of $\sim1-3$ ppm at the cloud tops. We find a much clearer detection (and much higher retrieved column abundance above the clouds) at southern polar latitudes compared with equatorial latitudes, which suggests a higher relative humidity of H$_2$S here. We find our retrieved H$_2$S abundances are most consistent with atmospheric models that have reduced methane abundance near Neptune's south pole, consistent with HST/STIS determinations (Karkoschka and Tomasko, 2011). We also conducted a Principal Component Analysis (PCA) of the Neptune and Uranus data and found that in the 1.57 - 1.60 $μ$m range, some of the Empirical Orthogonal Functions (EOFs) mapped closely to physically significant quantities, with one being strongly correlated with the modelled H$_2$S signal and clearly mapping the spatial dependence of its spectral detectability. Just as for Uranus, the detection of H$_2$S at the cloud tops constrains the deep bulk sulphur/nitrogen abundance to exceed unity (i.e. $ > 4.4 - 5.0$ times the solar value) in Neptune's bulk atmosphere, provided that ammonia is not sequestered at great depths, and places a lower limit on its mole fraction below the observed cloud of (0.4 - 1.3) $\times 10^{-5}$. The detection of gaseous H$_2$S at these pressure levels adds to the weight of evidence that the principal constituent of the 2.5 - 3.5-bar cloud is likely to be H$_2$S ice.

astro-ph.EP↗

arXiv2016

ALMA Observations of HCN and its Isotopologues on Titan

We present sub-millimeter spectra of HCN isotopologues on Titan, derived from publicly available ALMA flux calibration observations of Titan taken in early 2014. We report the detection of a new HCN isotopologue on Titan, H$^{13}$C$^{15}$, and confirm an earlier report of detection of DCN. We model high signal-to-noise observations of HCN, H$^{13}$CN, HC$^{15}$N, DCN, and H$^{13}$C$^{15}$ to derive abundances and infer the following isotopic ratios: $^{12}$C/$^{13}$C = 89.8 $\pm$ 2.8, $^{14}$N/$^{15}$N = 72.3 $\pm$ 2.2, D/H = (2.5 $\pm$ 0.2)$\times$10$^{-4}$, and HCN/H$^{13}$C$^{15}$ = 5800 $\pm$ 270 (1$σ$ errors). The carbon and nitrogen ratios are consistent with and improve on the precision of previous results, confirming a factor of $\sim$2.3 elevation in $^{14}$N/$^{15}$N in HCN compared to N$_2$ and a lack of fractionation in $^{12}$C/$^{13}$C from the protosolar value. This is the first published measurement of D/H in a nitrile species on Titan, and we find evidence for a factor of $\sim$2 deuterium enrichment in hydrogen cyanide compared to methane. The isotopic ratios we derive may be used as constraints for future models to better understand the fractionation processes occurring in Titan's atmosphere.

astro-ph.EP↗

arXiv2016

Isotopic Ratios of Carbon and Oxygen in Titan's CO using ALMA

We report interferometric observations of carbon monoxide (CO) and its isotopologues in Titan's atmosphere using the Atacama Large Millimeter/submillimeter Array (ALMA). The following transitions were detected: CO (J = 1-0, 2-1, 3-2, 6-5), $^{13}$CO (J = 2-1, 3-2, 6-5), C$^{18}$O (J = 2-1, 3-2), and C$^{17}$O (J = 3-2). Molecular abundances and the vertical atmospheric temperature profile were derived by modeling the observed emission line profiles using NEMESIS, a line-by-line radiative transfer code. We present the first spectroscopic detection of $^{17}$O in the outer solar system with C$^{17}$O detected at > 8$σ$ confidence. The abundance of CO was determined to be 49.6 ${\pm}$ 1.8 ppm, assumed to be constant with altitude, with isotopic ratios $^{12}$C/$^{13}$C = 89.9 ${\pm}$ 3.4, $^{16}$O/$^{18}$O = 486 ${\pm}$ 22, and $^{16}$O/$^{17}$O = 2917${\pm}$359. The measurements of $^{12}$C/$^{13}$C and $^{16}$O/$^{18}$O ratios are the most precise values obtained in Titan's atmospheric CO to date. Our results are in good agreement with previous studies and suggest no significant deviations from standard terrestrial isotopic ratios.

astro-ph.EP↗

arXiv2015

Titan Science with the James Webb Space Telescope (JWST)

The James Webb Space Telescope (JWST), scheduled for launch in 2018, is the successor to the Hubble Space Telescope (HST) but with a significantly larger aperture (6.5 m) and advanced instrumentation focusing on infrared science (0.6-28.0 $μ$m ). In this paper we examine the potential for scientific investigation of Titan using JWST, primarily with three of the four instruments: NIRSpec, NIRCam and MIRI, noting that science with NIRISS will be complementary. Five core scientific themes are identified: (i) surface (ii) tropospheric clouds (iii) tropospheric gases (iv) stratospheric composition and (v) stratospheric hazes. We discuss each theme in depth, including the scientific purpose, capabilities and limitations of the instrument suite, and suggested observing schemes. We pay particular attention to saturation, which is a problem for all three instruments, but may be alleviated for NIRCam through use of selecting small sub-arrays of the detectors - sufficient to encompass Titan, but with significantly faster read-out times. We find that JWST has very significant potential for advancing Titan science, with a spectral resolution exceeding the Cassini instrument suite at near-infrared wavelengths, and a spatial resolution exceeding HST at the same wavelengths. In particular, JWST will be valuable for time-domain monitoring of Titan, given a five to ten year expected lifetime for the observatory, for example monitoring the seasonal appearance of clouds. JWST observations in the post-Cassini period will complement those of other large facilities such as HST, ALMA, SOFIA and next-generation ground-based telescopes (TMT, GMT, EELT).

astro-ph.EP↗