Solar system

Abstract:

Titan is renowned for its complex atmosphere, where ongoing photochemistry leads to a rich mixture of organic molecules. Beginning with the splitting of methane by sunlight and other energetic particles, multi-carbon molecules are built up by successive addition of CxHy radicals and ions to one another. This process leads to the formation of ever-larger  molecules and eventually particulates, that sediment out on the surface. Our experimental knowledge of the molecular inventory comes from two techniques: direct sampling mass spectrometry, and remote sensing.  While the former has shown the presence of species at a very wide range of masses from 1-100+ Da, their structure and even stoichiometry is poorly known. In this respect, remote sensing spectroscopy is more robust, providing definitive detections of individual molecular types via unique patterns of IR and sub-millimeter energy transitions, however for a more limited range of species. Currently, 25 species have been definitively identified by remote sensing, ranging in size from H2 to benzene (C6H6). These include 12 hydrocarbons, with the rest a mixture of diatomics, nitriles and small oxygen compounds (H2O, CO, CO2). With direct sampling currently impossible before the Dragonfly mission returns a spacecraft to Titan in 2034, astronomers have been pushing forward with chemical identifications using a range of ground and space-based observatories. We report here on recent attempts to identify new C3 and C4 hydrocarbons in Titan’s atmosphere using the high-resolution (R~100000) TEXES spectrometer at the Infrared Telescope Facility (IRTF) – see examples in Fig. 1. Associated laboratory spectroscopy work is ongoing at the Jet Propulsion Laboratory (JPL) using a Bruker FTS spectrometer to identify the positions and intensities of the strongest gas bands, to assist with targeting the telescope searches, and interpretation of the data.  Identifications of new, heavy molecular species are urgently needed to constrain photochemical and dynamical models, and make advances in our understanding of the workings of Titan’s atmosphere, and its potential for astrobiology. Such work is also important for planning data collection and analysis from the upcoming NASA Dragonfly mission, where a sensitive mass spectrometer will assess the composition of surface materials and their relation to the atmospheric constituents, as well as Titan atmospheric data from other telescopes such as ALMA and JWST.Figure 1: Examples of currently undetected molecules in Titan's atmosphere: isomers of C4H8 and C4H10. We report on ongoing searches for these species with IRTF/TEXES.

Abstract:

Over four decades have elapsed since the last in situ spectrophotometric observations of the Venusian atmosphere, specifically from the Venera 11 (1978) and Venera 13 and 14 (1982) missions. These missions recorded spectral data during their descent from approximately 62 km to the surface. Unfortunately, the original data were lost; however, a portion has been reconstructed by digitising the graphical outputs that were generated during the initial data processing phase of each of the three missions [1]. This reconstructed data is crucial as it remains the sole set of in situ spectrophotometric observations of Venus’s atmosphere and is likely to be so for the foreseeable future.While re-analysing the reconstructed Venera datasets, we identified several artefacts, errors and sources of noise, necessitating the implementation of some corrections and validation checks to isolate the most unaffected part of the reconstructed data. Then, using NEMESIS, a radiative transfer and retrieval tool [2], we conducted a series of retrievals to simultaneously fit the downward-going spectra at all altitudes. During this process, several parameters were retrieved. The first set of retrievals focused on the structure of the main cloud deck (MCD), which includes the cloud base altitude and abundance profiles of all four cloud modes. Previous corrections that were used to account for the effect of the unknown UV absorber did not result in good fits with the spectra shortward of 0.6 µm. Hence, we derived a new correction by retrieving the imaginary refractive index spectra of the Mode 1 particles.In the next phase, the MCD retrievals were used to update the model atmospheres for each of the missions. Then, the H2O volume mixing ratio profiles were retrieved and compared with the previous retrievals using the same data by [1] along with other remote sensing observations. The final retrieval phase concentrated on characterising particulate matter in the deep atmosphere. In [3], we outlined a methodology for retrieving a near-surface particulate layer using the reconstructed Venera 13 dataset. In this new work, we apply this methodology to encompass the Venera 11 and 14 datasets and compare the retrievals from the three datasets.This research thus provides a comprehensive overview of three distinct retrievals: 1) main cloud deck, 2) H2O, and 3) near-surface particulates using the reconstructed spectrophotometric data of Venera 11, 13, and 14.References: [1] Ignatiev, N. I., Moroz, V. I., Moshkin, B. E., Ekonomov, A. P., Gnedykh, V. I., Grigor’ev, A. V., and Khatuntsev, I. V. Cosmic Research 35(1), 1–14 (1997).[2] Irwin, P. G., Teanby, N. A., de Kok, R., Fletcher, L. N., Howett, C. J., Tsang, C. C., Wilson, C. F., Calcutt, S. B., Nixon, C. A., and Parrish, P. D. Journal of Quantitative Spectroscopy and Radiative Transfer 109(6), 1136–1150 (2008).[3] Kulkarni, S. V., Irwin, P. G. J., Wilson, C. F., & Ignatiev, N. I. Journal of Geophysical Research: Planets, 130, e2024JE008728, (2025).

Abstract:

We present a re-analysis of archival data-cubes of Neptune obtained with the GEMINI Near-Infrared Integral Field Spectrometer (NIFS), aiming to refine constraints on the abundance of hydrogen sulphide (H₂S) in Neptune's atmosphere. To enhance spatial and spectral fidelity, we employ a modified CLEAN algorithm that effectively deconvolves the data while conserving flux. To mitigate observational and instrumental artifacts, we utilize Singular Spectrum Analysis (SSA) on single-wavelength images and apply Principal Component Analysis (PCA) across the full data-cube to suppress both random and systematic noise. Spectral retrievals are conducted using ArchNemesis, an optimal estimation inverse modeling tool. We retrieve vertical profiles at individual locations, and use Minnaert-corrected reflectivity functions across latitude bands to investigate latitudinal variability. Using the deconvolution and data analysis techniques, we are able to extract more scientific utility from legacy datasets and describe a template that can be repeated for similar datasets.

Abstract:

Introduction: Enceladus Thermal Mapper (ETM) is an Oxford-built high-heritage instrument that is being developed for outer solar system operations. ETM is based upon the design of Lunar Thermal Mapper (LTM, launched on Lunar Trailblazer, Fig. 1). It has a strong heritage story, including MIRMIS (on Comet Interceptor), Compact Modular Sounder (on TechDemoSat-1) and filters shared with Lunar Diviner (on Lunar Reconnaissance Orbiter). ETM is a miniaturized thermal infrared multispectral imager, with space for 15 spectral channels (bandpasses) that can be tailored to the mission requirements. It consists of a five-mirror telescope and optical system and an uncooled microbolometer detector array. Real-time calibration is achieved using a motorized mirror to point to an onboard blackbody target and empty space. ETM has an IFOV of 35 mm, so assuming a 100 30 km orbit it will have a spatial resolution of 40 to 70 m/pixel and a swath width of 14 - 27 km. ETM Updates: Through UK Space Agency funding we have developed three areas of ETM: its filter profile, radiation tolerance and sensitivity to Enceladus-like surfaces. Filters: ETM is a push broom thermal mapper, which works by the detector being swept over a surface. Each of the detector’s 15 channels is made up 16 rows, which are coadded to increase the signal to noise. A recently completed preliminary study has updated ETM’s bandpasses to include filters between 6.25 mm and 200 mm to enable it to detect Enceladus’ polar winter (170 K). Depending on the mission goals not all channels need to be utilised to achieve this, making some available for additional studies (e.g. searching for salt). Radiation: The radiation environments of Enceladus are vastly different to those of the Moon. Recent radiation testing and analysis showed that the majority of ETM’s existing design is already highly radiation tolerant. With some additional shielding and one component change all parts can reach the radiation hardness required to operate in the Saturn-system. The additional shielding may be provided by the spacecraft structure, depending on the adopted design. Sensitivity: ETM’s sensitivity to cryogenic surfaces is currently predicted through a well-characterised model. However, as part of the LTM calibration campaign we plan to directly measure its sensitivity to

Abstract:

Introduction: Mimas is a small moon of Saturn with a heavily cratered surface and high eccentricity, suggesting an inactive past. It was, thus, surprising when Cassini measurements of Mimas’ libration and tracking of its pericenter precession revealed that Mimas maintains an ocean under an ice shell 20-30 km thick [5,12]. Subsequent investigations into how an ocean-bearing Mimas could have avoided developing tidally-driven fractures [7], its tidal heating budget  [8], constraints on shell thickness from the formation of the Herschel impact basin [3], and thermal-orbital evolution models [5,9] all point to a young ocean that has emerged within the past 10-15 Myr. These results suggest that Mimas may possess the youngest ocean in the Solar System, making it an important target for understanding the early stages of ocean development – such as for Enceladus’ ocean – and the habitability of ocean worlds through time. Uranus’ moon, Miranda, may also have developed an ocean relatively late in its history (e.g., [2]); understanding the evolutionary and geophysical processes at Mimas may help prepare us for a future mission to the Uranian system.Here, we expand upon past work [8,9], in which we relied on globally-averaged tidal heating and constant surface temperature, to develop a map of plausible ice shell thicknesses and surface heat flows on Mimas assuming a present-day ocean. Our goals are to determine the variability in heat flow and ice shell thickness that result from spatial differences in surface temperature and strength of the tide and to quantify requirements that would enable ocean detection via heat flow measurements. Our results also provide estimates of tidal power, which affect the circularization timescale and ocean lifetime within thermal-orbital evolution models.Methods: We utilize the numerical toolkit MATH [13] to compute tidal heating within Mimas’ ice shell and identify the equilibrium ice shell thickness and surface heat flux at a suite of locations across Mimas. These calculations depend on the surface temperature and the basal heat flux. Here, we develop a surface temperature map based on models of solar insolation, and informed by Cassini measurements (e.g., [4]), to obtain robust temperatures. We then vary the basal heat flux across a range that encompasses minimal heating from only radiogenic decay to high heat fluxes associated with dissipation in Mimas’ rocky interior. We use the inferred ice shell thickness of 20-30 km [5,12] to determine the basal heating cases that provide consistent results. From these maps, we can deduce the precision needed to use heat flow measurements to differentiate between a fully frozen Mimas, which likely produces endogenic heat flows of ~1 to several mW/m2 (e.g., [9]), and an ocean-bearing Mimas.The ice shell thickness maps can also be used to compute the tidal dissipation associated with Mimas’ present-day orbit and interior structure. We will input these values into the numerical toolkit PISTES [11] to assess the extent to which Mimas’ ice shell evolution can occur over a longer timescale and/or begin at a higher eccentricity than in past models. These results are particularly important for understanding how Mimas came to possess an ocean. While we expect that Mimas’ ocean emerged due to a recent eccentricity-pumping event that increased its eccentricity to the point of melting, the cause and details are not well-understood. A gap in Saturn’s rings, known as the Cassini Division, appears to record Mimas’ phase of inward migration and increasing eccentricity [1,6]. However, models of this process require Mimas to reach a much higher eccentricity than the thermal-orbital evolution models predict; Mimas’ entire ice shell would have melted in that case, which is inconsistent with its geologic record (see discussion in [5]). In addition, the timescales for Mimas’ subsequent outward migration are in conflict. These discrepancies motivate further investigation into Mimas’ thermal-orbital evolution to determine whether the initial conditions and lifetime of the ocean can be extended.Anticipated results: In Figure 1, we show maps of surface heat flow and ice shell thickness for Europa, assuming different basal heating values [10], which we created using the same tools and approach we are now applying to Mimas. We will present similar maps of ice shell thickness and heat flow across Mimas at its present-day eccentricity that are consistent with the inferred average ice shell thickness. We will also present the precision required for future heat flow measurements to detect the ocean and constrain the thickness of the ice shell, which we will compare to our recent Europa results. Finally, we will present revised thermal-orbital evolution models that account for differences in tidal dissipation between the globally-averaged and spatially-variable models of Mimas and discuss the implications of our findings on the development and age of Mimas’ ocean.Figure 1: We show equilibrium ice shell thicknesses (left) and surface heat flows (right) for Europa assuming different values of the basal heat flux (rows) and applying surface temperatures from model fits to Galileo data (see [10]). Variations in tidal strength exert a strong control on the pattern of heat flow while surface temperature creates deviations in the shell thickness map from the purely tidal pattern. We are conducting a similar investigation of Mimas to better understand the current state of its ocean and ice shell, develop measurement requirements, and explore implications for the ocean’s evolution.References: [1] Baillié, K., et al. (2019) MNRAS 486, p. 2933-2946. [2] Beddingfield, C.B. et al. (2022) PSJ 3, 174. [3] Denton, C.A., and A.R. Rhoden (2022) GRL 49, e2022GL100516. [4] Howett, C. J. A., et al. (2020) Icarus 348. [5] Lainey, V., et al. (2024) Nature 626, p. 280 – 282. [6] Noyelles, B., et al. (2019) MNRAS 486, p. 2947–2963. [7] Rhoden, A.R., et al. (2017) JGR – Planets 122, p. 400-410. [8] Rhoden, A. R., & Walker, M. E. (2022) Icarus 376. [9] Rhoden, A. R., et al. (2024a) EPSL 635. [10] Rhoden et al. (2024b) AGU, Abs P23E-3117 [11] Rudolph, M. L., et al. (2022) GRL 49.[12] Tajeddine, R., et al. (2014) Science 346, p. 322–324.  [13] Walker, M. E., & Rhoden, A. R. (2022) PSJ 3, 149.