arXiv subjects

Ella Sciamma-O'Brien

Publications and source records attributed to Ella Sciamma-O'Brien.

12 recordsLinked to original sources

arXiv2026

A JWST Study of Stardust. II. Polycyclic Aromatic Hydrocarbons in Comet 81P/Wild 2 and Implications for Primitive Solar System Carbon

We report observations of comet 81P/Wild 2, target of the Stardust sample return mission, on UT 2023 March 20 and 24 at a heliocentric distance ($r_H$) of 1.85 au using the NIRSpec and MIRI integral field unit spectrographs on board the James Webb Space Telescope (JWST). This study is the first compositional comparison between JWST remote-sensing spectroscopy of a solar system object against terrestrial analysis of its returned samples. Recent work determined contributions of molecular emission from coma volatiles and thermal emission from the nucleus and coma dust grains to these spectra. Subtracting the molecular and thermal emission models produces a residual spectrum with a clear resonance near 3.37 $μ$m and multiple others spanning the $6 - 12$ $μ$m region. Analysis of these residuals provides strong evidence for the presence of polycyclic aromatic hydrocarbons (PAHs) in the coma of 81P/Wild 2. The resulting PAH populations reflect material incorporated into the nucleus of 81P/Wild 2 that was inherited from the diffuse interstellar medium, the interstellar medium, and the protoplanetary disk. The PAHs sensed by JWST are compared to those detected in laboratory analyses of the Stardust mission returned samples.

astro-ph.EP↗

arXiv2026

A Cross-Laboratory Comparison Study of Titan Haze Analogs: Surface Energy II

The organic haze particles on Titan play important roles in atmospheric cloud formation and aerosol-lake interactions. These processes are strongly influenced by the surface energy of the haze particles, which controls cohesion and wetting behavior. This study presents a comparative analysis of 32 laboratory-produced haze analog samples ("tholins") synthesized across three laboratories. Using contact angle measurements, we determine the total surface energy and its dispersive and polar components for all samples, systematically evaluating the effects of substrate choice, air exposure, initial N2/CH4 gas mixture, and experimental setup. We find that tholin samples exhibit minimal substrate dependence, whereas exposure to ambient air substantially modifies the surface chemistry, altering the balance between dispersive and polar components. Thus, future Titan-relevant surface property measurements may use any substrate but must keep samples pristine. Surface energies vary weakly across methane concentrations, from 1-10% CH4 in N2, indicating that Titan's hazes formed at different altitudes should exhibit broadly similar cohesiveness. In contrast, experimental conditions such as gas exposure time and energy source produce the dominant differences in surface energy, driven largely by variations in polar components. Despite these differences, tholin samples exhibit high dispersive components, implying that Titan's hazes should act as efficient cloud condensation nuclei for hydrocarbon clouds and should generally sink into Titan's lakes. Given observed ethane ice clouds, we conclude that cold plasma tholins may be better physical analogs for Titan's hazes than samples produced with far-ultraviolet irradiation, though intrinsic surface energies of UV tholins remain uncertain due to film thickness limitations.

astro-ph.EP↗

arXiv2026

A JWST Study of Stardust. I. Infrared Spectroscopy of Comet 81P/Wild 2 and Overall Composition

We report observations of comet 81P/Wild 2, target of the Stardust sample return mission, on UT 2023 March 20 and 24 at a heliocentric distance ($r_H$) of 1.85 au using the NIRSpec and MIRI integral field unit spectrographs on board the James Webb Space Telescope (JWST). This study is the first compositional comparison between JWST remote-sensing spectroscopy of a solar system object against terrestrial analysis of its returned samples. We securely detected molecular emission from H$_2$O, CH$_4$, C$_2$H$_6$, CH$_3$OH, CO, CO$_2$, $^{13}$CO$_2$, OCS, HCN, and CN and find molecular abundances consistent within $2σ$ with those reported during previous perihelion passages. The water ortho-to-para ratio was $2.76\pm0.05$, and the $^{12}$CO$_2$/$^{13}$CO$_2$ ratio was $85\pm4$. Thermal emission from the nucleus and dust was detected and modeled, providing an effective nucleus radius of $1.77\pm0.04$ km and a dust composition (relative mass fraction of the submicron grains) of $\sim36\%$ amorphous carbon, $\sim25\%$ amorphous Mg:Fe olivine, $\sim23\%$ Mg-rich crystalline olivine, and $\sim15\%$ amorphous Mg:Fe pyroxene. The crystalline mass fraction of the sub-micron grains in the coma was $0.362\pm0.003$. Comparison of the JWST-derived thermal model against the fine-grained materials in Stardust returned samples demonstrates complementarity between the missions, with each most sensitive to a different population of the coma dust grains.

astro-ph.EP↗

arXiv2026

Experimental Measurements of Benzene Ice Critical Saturation Ratios on Titan Tholins and Impact on the Microphysical Modeling of Titan's South Polar Benzene Cloud

The Cassini Composite Infrared Spectrometer (CIRS) revealed the presence of a benzene (C6H6) ice cloud in Titan's autumn south polar stratosphere, following the northern spring equinox in August 2009. This event increased the mixing ratio of benzene and raised the cloud top altitude. We present here new experimental measurements of the critical saturation (Scrit) of pure C6H6 ice at low temperature (from 13.8 at 138 K to 3.3 at 157 K), representative of Titan's atmospheric temperatures, when deposited onto Titan aerosol analogs (tholins) produced in the NASA Ames COsmic SImulation Chamber (COSmIC). Comparisons of Scrit values of benzene vapor deposition obtained on blank substrates versus on Titan tholins reveals the highly favorable role aerosols play as condensation nuclei. We have included these new Scrit measurements to calculate the nucleation contact parameter from m = 2.6e-4 T + 0.9494 in the Community Aerosol and Radiation Model for Atmospheres (CARMA) to investigate the variation in size and number density of C6H6 cloud particles as a function of altitude in Titan's southern polar atmosphere at 87 S. We discuss how these new temperature-dependent measurements impact the microphysical modeling and result in simulated benzene clouds forming about 20 km lower than seen in previous observations. A possible explanation for this discrepancy is that the cloud system was likely influenced by the co-condensation of more than one volatile gas species.

astro-ph.EP↗

arXiv2025

Refractive indices of photochemical haze analogs for Solar System and exoplanet applications : a cross-laboratory comparative study between the PAMPRE and COSmIC experimental set-ups

Previous observations of Titan, Pluto and Solar System gas giants along with recent observations of exoplanet atmospheres with the James Webb Space Telescope taught us that photochemical hazes are ubiquitous and form in a variety of temperature, gas composition and irradiation environments. Despite being crucial to understand their impact on observations and on the planetary radiative budget, the refractive indices of these haze particles are unknown and strongly influenced by changes in the gas phase chemistry. In this study, we perform a cross-laboratory investigation to assess the effect of the experimental set-up and gas composition on the refractive indices of Titan, Pluto and exoplanet haze analogs. We report new data in a broad spectral range from UV to far-IR (up to 200 microns) for future use in climate models and retrieval frameworks.

astro-ph.EP↗

arXiv2025

Modeling Low-Temperature Plasmas Simulating Titan's Atmosphere

In the study presented here, we model the gas phase chemistry induced by plasma discharge at low temperature (150 K) in the NASA Ames COSmIC Simulation Chamber (COSmIC) using a 1-dimensional multi-fluid plasma model named CO-PRISM (COSmIC Plasma Reactivity and Ionization Simulation Model). Our model incorporates an extensive chemical reaction network to simulate the neutral-neutral and ion-neutral reactions occurring in the COSmIC experiments when using N2-CH4-based gas mixtures relevant to Titan's atmosphere. Our reaction network now includes crucial reactions involving the first electronically-excited state of atomic nitrogen, recent electron collision cross-sections, and radical chemistry. In particular, we have investigated the influence of C2H2 on the gas phase polymeric growth and the elemental composition of the chemical products, and we have compared our findings to recently published solid phase analyses. The modeling results are consistent with experimental measurements of N2-CH4-C2H2 plasmas on COSmIC, showing the production of C6Hx intermediates and precursors of larger organics, as well as methanimine in small concentration. Our numerical results point to cationic pathways enabling efficient intermediate-sized and nitrogen-rich \ce{C2H2}-driven chemistry driving tholin production. Comparison of the modeled gas phase elemental composition with elemental composition of the solid phase samples produced in COSmIC reveal similar trends, with C/N increasing when C2H2 is present in the gas mixture. Finally, our results demonstrate the importance of such synergistic studies using low-temperature plasma chemistry experiments combined with modeling efforts to improve our understanding of cold planetary environments.

physics.plasm-ph↗

arXiv2023

Material Properties of Organic Liquids, Ices, and Hazes on Titan

Titan has a diverse range of materials in its atmosphere and on its surface: the simple organics that reside in various phases (gas, liquid, ice) and the solid complex refractory organics that form Titan's haze layers. These materials all actively participate in various physical processes on Titan, and many material properties are found to be important in shaping these processes. Future in-situ exploration on Titan would likely encounter a range of materials, and a comprehensive database to archive the material properties of all possible material candidates will be needed. Here we summarize several important material properties of the organic liquids, ices, and the refractory hazes on Titan that are available in the literature and/or that we have computed. These properties include thermodynamic properties (phase change points, sublimation and vaporization saturation vapor pressure, and latent heat), physical property (density), and surface properties (liquid surface tensions and solid surface energies). We have developed a new database to provide a repository for these data and make them available to the science community. These data can be used as inputs for various theoretical models to interpret current and future remote sensing and in-situ atmospheric and surface measurements on Titan. The material properties of the simple organics may also be applicable to giant planets and icy bodies in the outer solar system, interstellar medium, protoplanetary disks, and exoplanets.

astro-ph.EP↗

arXiv2021

A Cross-Laboratory Comparison Study of Titan Haze Analogs: Surface Energy

In Titan's nitrogen-methane atmosphere, photochemistry leads to the production of complex organic particles, forming Titan's thick haze layers. Laboratory-produced aerosol analogs, or "tholins", are produced in a number of laboratories; however, most previous studies have investigated analogs produced by only one laboratory rather than a systematic, comparative analysis. In this study, we performed a comparative study of an important material property, the surface energy, of seven tholin samples produced in three independent laboratories under a broad range of experimental conditions, and explored their commonalities and differences. All seven tholin samples are found to have high surface energies, and are therefore highly cohesive. Thus, if the surface sediments on Titan are similar to tholins, future missions such as Dragonfly will likely encounter sticky sediments. We also identified a commonality between all the tholin samples: a high dispersive (non-polar) surface energy component of at least 30 mJ/m2. This common property could be shared by the actual haze particles on Titan as well. Given that the most abundant species interacting with the haze on Titan (methane, ethane, and nitrogen) are non-polar in nature, the dispersive surface energy component of the haze particles could be a determinant factor in condensate-haze and haze-lake liquids interactions on Titan. With this common trait of tholin samples, we confirmed the findings of a previous study by Yu et al. (2020) that haze particles are likely good cloud condensation nuclei (CCN) for methane and ethane clouds and would likely be completely wetted by the hydrocarbon lakes on Titan.

astro-ph.IM↗

arXiv2021

Titan: Earth-like on the Outside, Ocean World on the Inside

Thanks to the Cassini-Huygens mission, Titan, the pale orange dot of Pioneer and Voyager encounters has been revealed to be a dynamic, hydrologically-shaped, organic-rich ocean world offering unparalleled opportunities to explore prebiotic chemistry. And while Cassini-Huygens revolutionized our understanding of each of the three layers of Titan--the atmosphere, the surface, and the interior--we are only beginning to hypothesize how these realms interact. In this paper, we summarize the current state of Titan knowledge and discuss how future exploration of Titan would address some of the next decade's most compelling planetary science questions. We also demonstrate why exploring Titan, both with and beyond the Dragonfly New Frontiers mission, is a necessary and complementary component of an Ocean Worlds Program that seeks to understand whether habitable environments exist elsewhere in our solar system.

astro-ph.EP↗

arXiv2020

Critical Laboratory Studies to Advance Planetary Science and Support Missions

Laboratory studies for planetary science and astrobiology aimat advancing our understanding of the Solar System through the promotion of theoretical and experimental research into the underlying processes that shape it. Laboratory studies (experimental and theoretical) are crucial to interpret observations and mission data, and are key incubators for new mission concepts as well as instrument development and calibration. They also play a vital role in determining habitability of Solar System bodies, enhancing our understanding of the origin of life, and in the search for signs of life beyond Earth, all critical elements of astrobiology. Here we present an overview of the planetary science areas where laboratory studies are critically needed, in particular in the next decade. These areas include planetary & satellites atmospheres, surfaces, and interiors, primitive bodies such as asteroids, meteorites, comets, and trans-Neptunian objects, and signs of life. Generating targeted experimental and theoretical laboratory data that are relevant for a better understanding of the physical, chemical, and biological processes occurring in these environments is crucial. For each area we present i) a brief overview of the state-of-the-art laboratory work, ii) the challenges to analyze and interpret data sets from missions and ground-based observations and to support mission and concept development, and iii) recommendations for high priority laboratory studies.

astro-ph.IM↗

arXiv2020

Ultraviolet-Based Science in the Solar System: Advances and Next Steps

We review the importance of recent UV observations of solar system targets and discuss the need for further measurements, instrumentation and laboratory work in the coming decade. In the past decade, numerous important advances have been made in solar system science using ultraviolet (UV) spectroscopic techniques. Formerly used nearly exclusively for studies of giant planet atmospheres, planetary exospheres and cometary emissions, UV imaging spectroscopy has recently been more widely applied. The geyser-like plume at Saturn's moon Enceladus was discovered in part as a result of UV stellar occultation observations, and this technique was used to characterize the plume and jets during the entire Cassini mission. Evidence for a similar style of activity has been found at Jupiter's moon Europa using Hubble Space Telescope (HST) UV emission and absorption imaging. At other moons and small bodies throughout the solar system, UV spectroscopy has been utilized to search for activity, probe surface composition, and delineate space weathering effects; UV photometric studies have been used to uncover regolith structure. Insights from UV imaging spectroscopy of solar system surfaces have been gained largely in the last 1-2 decades, including studies of surface composition, space weathering effects (e.g. radiolytic products) and volatiles on asteroids (e.g. [2][39][48][76][84]), the Moon (e.g. [30][46][49]), comet nuclei (e.g. [85]) and icy satellites (e.g. [38][41-44][45][47][65]). The UV is sensitive to some species, minor contaminants and grain sizes often not detected in other spectral regimes. In the coming decade, HST observations will likely come to an end. New infrastructure to bolster future UV studies is critically needed. These needs include both developmental work to help improve future UV observations and laboratory work to help interpret spacecraft data. UV instrumentation will be a critical tool on missions to a variety of targets in the coming decade, especially for the rapidly expanding application of UV reflectance investigations of atmosphereless bodies.

astro-ph.IM↗

arXiv2019

The Fundamental Vibrational Frequencies and Spectroscopic Constants of the Dicyanoamine Anion, NCNCN$^-$ (C$_2$N$_3^-$): Quantum Chemical Analysis for Astrophysical and Planetary Environments

Detecting anions in space has relied on a strong collaboration between theoretical and laboratory analyses to measure rotational spectra and spectroscopic constants to high accuracy. The advent of improved quantum chemical tools operating at higher accuracy and reduced computational cost is a crucial solution for the fundamental characterization of astrophysically-relevant anions and their detection in the interstellar medium and planetary atmospheres. In this context, we have turned towards the quantum chemical analysis of the penta-atomic dicyanoamine anion NCNCN$^-$ (C$_2$N$_3^-$), a structurally bent and polar compound. We have performed high-level coupled cluster theory quartic force field (QFF) computations of C$_2$N$_3^-$ satisfying both computational cost and accuracy conditions. We provide for the first time accurate spectroscopic constants and vibrational frequencies for this ion. In addition to exhibiting various Fermi resonances, C$_2$N$_3^-$ displays a bright $ν$2 (2130.9 cm$^{-1}$) and a less intense $ν$1 (2190.7 cm$^{-1}$) fundamental vibrational frequency, making for strong markers for future infrared observations < 5$μ$m. We have also determined near-IR overtone and combination bands of the bright fundamentals for which the 2$ν$2 at 4312.1 cm$^{-1}$ (2.319 $μ$m) is the best candidate. C$_2$N$_3^-$ could potentially exist and be detected in nitrogen-rich environments of the ISM such as IRC +10216 and other carbon-rich circumstellar envelopes, or in the atmosphere of Saturn's moon Titan, where advanced N-based reactions may lead to its formation.

astro-ph.EP↗