Exoplanet atmospheres

Abstract:

IntroductionSaturn’s largest moon, Titan, has a dense atmosphere comprised mostly of nitrogen and methane. The photolysis and ionization of these major componentsleads to complex chemical reactions, which create substantial diversity among Titan’s minor atmospheric constituents. Remote sensing and molecular  pectroscopy historically have been a critical tool for detecting trace gases in Titan’s atmosphere and help corroborate predictions of Titan’s atmospheric composition from photochemical models. Following the Voyager and Cassini missions, which provided a wealth of spectroscopic studies of Titan’s  atmosphere, ground-based measurements have been useful for detecting elusive trace gases. The Echelon-Cross-Echelle Spectrometer (EXES) is a high-resolution (R ∼ 90, 000) mid-infrared spectrometer that was previously operated aboard NASA’s Stratospheric Observatory For Infrared Astronomy (SOFIA)(1 ). EXES benefited from the high altitude flights during the SOFIA mission to make observations above the bulk of the atmosphere to avoid strong telluric absorption lines that inhibit ground based mid-IR spectrometers such as its sister instrument TEXES.Here we present EXES observations of Titan which were made in an attempt to detect two trace gases, triacetylene (C6H2) and dicyanoacetylene (C4N2). C6H2 is an important polyyne and is predicted to form readily from the addition of the ethynyl (C2H) radical with diacetylene (C4H2). It remains yet tobe detected, though, and the previous upper limit study was limited by the lower spectral resolution of Voyager’s IRIS (R ∼ 145)(2 ). Delpech et al. 1994 derived an upper-limit of 6 × 10−11 which would be detectable by EXES.Gas-phase C4N2 formation is primarily completed through C3N addition to HCN or, alternatively, CN addition to HC3N(3 ). The ice-phase C4N2, which is formed through solid-state photochemical reactions on the surface of HC3N ice grains, has been detected in spectra measured by Voyager’s IRIS and CIRSduring the Cassini mission (4, 5 ), yet C4N2 in the gas-phase remains elusive to spectroscopic detections. Again, previous studies of the gas-phase upper limits (3σ = 1.53 × 10−9) were performed using spectra collected by CIRS (R ∼ 1240) which has a resolving power significantly lower than EXES(6 ). The high-resolution of EXES will help improve on the upper limits of both of these species and allow for an updated comparison to photochemical model predictions of their vertical profiles in Titan’s atmosphere.Observations and ModelingMid-infrared observations of Titan were made in June of 2021, using EXES. These observations aim to detect the ν11 out-of-plane bending mode of C6H2 at 621 cm−1 and the perpendicular ν9 stretch of the gasphase C4N2 at 472 cm−1. Figure 1 shows a small portion of the EXES spectrum measured at the 621 cm−1 spectral setting. In this region there are strong emission features from diacetylene (C4H2) and propyne (C3H4) which must be fit before analyzing the C6H2 upper limits. Highlighted in the blue box is the region where the ν11 vibrational mode for C6H2 should be present.To model the collected spectra, we use the arch-NEMESIS radiative transfer package which is a new Python implementation of the NEMESIS radiative transfer code (7, 8 ). The radiative transfer modeling of the measured spectra occurs in two steps. The initial step is to retrieve the atmospheric profiles of the aerosols and known gases using the archNEMESIS optimal estimation algorithm. For the 621 cm−1 spectral setting, the vertical profiles of C4H2, C3H4, and aerosol continuum are retrieved, however, at the 472 cm−1 region, there are no emission features to fit and just the continuum level is retrieved by adjusting the aerosol profile. For both spectral regions, we use a temperature profile and initial gas profiles defined in Vuitton et al. 2019 photochemical model (3 ). The quality of each retrieval is determined by a goodnessof-fit metric (χ2) which compares the residual of the modeled spectrum to the noise of the measurement. Following the retrieval, we derive the upper limits by building forward models of the spectral regions where the abundance of each target species is iteratively increased and a subsequent χ2 is determined. We then take the difference, Δχ2, between the retrieved and updated forward model χ2 to find where the abundance causes significant deviation from the retrieved spectrum. Step-profiles, which have a cutoff altitude and constant abundance above this cutoff, were used to determine the upper-limits for each species. This method has been applied for many different upper limits studies of gases predicted in Titan’s atmosphere (9, 10 ).ResultsBased on these observations, C6H2 and gas-phase C4N2 remain undetected and therefore, we derive the upper limits to their atmospheric abundance. We improve upon the upper limits of C6H2 and C4N2 by an order of magnitude for both species. Figure 2 shows Δχ2 increase sharply with increased abundance for both C6H2 and C4N2. For C6H2 the 3σ upper limit (Δχ2 = 9) is on the order of 10−11 and for C4N2, 10−10. These new upper limits improve on the previously derived upper limits by an order of magnitude for each target species. More work is still being done to precisely determine the upper limits and compare these values to the current photochemical model predictions of their abundance. The values of the 1σ, 2σ, and 3σ upper limits for each species will be reported in the presentation. The upper limits derived improved upon the previous upper limits by an order of magnitude and we are currently working on comparing these upper limits to photochemical models of Titan’s atmospheric composition to build a better understanding of the chemical pathways in Titan’s atmosphere which will also be discussed in the presentation. AcknowledgmentsThe material is based upon work supported by NASA under award number 80GSFC24M0006.References1. Richter et al., 20182. Delpech et al., 19943. Vuitton et al., 20194. Samuelson et al, 19975. Anderson et al, 20166. Jolly et al., 20157. Alday et al, 20258. Irwin et al., 20089. Nixon et al., 201010. Teanby et al., 2013

Abstract:

This talk will cover the state of the art in whole-planet subNeptune modelling, and needs for the future.  Inferences about the composition of the deep envelope can be made on the basis of the way chemical transformations in the deep envelope may be evidenced in the observable atmosphere, such as has been attempted, for example, regarding the presence or absence of NH3 in the observable atmospheres of subNeptunes.  Such inferences require an understanding not only of deep envelope chemistry, but also of vertical mixing processes. The mixing process engages a number of poorly understood phenomena, such as mixing rates through stably stratified (nonconvective) internal radiative layers.  The occurrence of such radiative layers can be induced by compositional suppression of convection (e.g. due to high molecular weight H2O in an H2-rich atmosphere). We will review our modelling studies regarding this phenomenon.  Typically, the envelope-silicate interface is hot enough that the interface takes the form of a magma ocean, so compositional interchange with the magma ocean becomes crucial. This exchange includes rock vapours as well as lower molecular weight volatiles.  Our work on magma ocean exchanges will be reviewed. We highlight the importance of mineral physics experiments and molecular dynamics to provide crucially needed (and largely absent) thermodynamic parameters, particularly at high pressure.  At sufficiently high temperatures, silicate itself can become supercritical so that the distinction between silicate melt and silicate vapour disappears and the silicate substance becomes completely miscible with the lower molecular weight envelope.  Modeling and experiment regarding this novel and largely unexplored regime is particularly needed.

Abstract:

Introduction: This presentation will describe the Modular Infrared Molecules and Ices Sensor currently in final assembly and test at the University of Oxford, UK and VTT Finland for ESA’s upcoming Comet interceptor mission.The Comet Interceptor mission: The Comet Interceptor mission [1] was selected by ESA as the first of its new “F” class of missions in June 2019 and adopted in June 2022.  Comet Interceptor (CI) aims to be the first mission to visit a long period comet, preferably, a Dynamically New Comet (DNC), a subset of long-period comets that originate in the Oort cloud and may preserve some of the most primitive material from early in our Solar System’s history. CI is scheduled to launch to the Earth-Sun L2 point with ESA’s ARIEL [2] mission in ~2029 where it will wait for a suitable DNC target.The CI mission is comprised of three spacecraft.  Spacecraft A will pass by the target nucleus at ~1000 km to mitigate against hazards caused by dust due to the wide range of possible encounter velocities (e.g. 10 – 70 km/s).  As well as acting as a science platform, Spacecraft A will deploy and provide a communications hub for two smaller spacecrafts, B1 (supplied by the Japanese space agency JAXA) and B2 that will perform closer approaches to the nucleus.  Spacecrafts B1 and B2 will make higher risk/higher return measurements but with the increased probability that they will not survive the whole encounter.The MIRMIS Instrument: The Modular InfraRed Molecules and Ices sensor (MIRMIS, Figure 1) instrument is part of the CI Spacecraft A scientific payload.  The MIRMIS consortium includes hardware contributions from Finland (VTT Finland) and the UK (University of Oxford) with members of the instrument team from the Universities of Helsinki, Lyon, NASA’s Goddard Space Flight Center, and Southwest Research Institute.MIRMIS will map the spatial distribution of temperatures, ices, minerals and gases in the nucleus and coma of the comet using covering a spectral range of 0.9 to 25 microns.  An imaging Fabry-Perot interferometer will provide maps of composition at a scale of ~180 m at closest approach from 0.9 to 1.7 microns.  A Fabry-Perot point spectrometer will make observations of the coma and nucleus at wavelengths from 2.5 to 5 microns and finally a thermal imager will map the temperature and composition of the nucleus at a spatial resolution of 260 m using a series of multi-spectral filters from 6 to 25 microns.  Figure 1: (Top) The MIRMIS instrument for ESA’s Comet Interceptor mission. (Bottom) The MIRMIS Structural Thermal model under test at University of Oxford.The MIRMIS instrument is compact (548.5 x 282.0 x 126.8 mm) and low mass (

Abstract:

Abstract Limb-resolved transmission spectroscopy has the potential to transform our understanding of exoplanetary atmospheres. By separately measuring the transmission spectra of the evening and morning limbs, these atmospheric regions can be individually characterized, shedding light into the global distribution and transport of key atmospheric properties from transit observations alone. In this work, we follow up the recent detection of limb asymmetry on the exoplanet WASP-107 b by reanalyzing literature observations of WASP-107 b using all of James Webb Space Telescope’s science instruments (Near Infrared Imager and Slitless Spectrograph (NIRISS), Near-Infrared Camera, Near Infrared Spectrograph (NIRSpec), and Mid-Infrared Instrument) to measure its limb transmission spectra from ∼1 to 12 μm. We confirm the evening–morning temperature difference inferred previously and find that it is qualitatively consistent with predictions from global circulation models. We find evidence for evening–morning variation in SO2 and CO2 abundance, and significant cloud coverage only on WASP-107 b’s morning limb. We find that the NIRISS and NIRSpec observations are potentially contaminated by occulted starspots, which we leverage to investigate stellar contamination’s impact on limb asymmetry measurements. We find that starspot crossings can significantly bias the inferred evening and morning transmission spectra depending on when they occur during the transit, and develop a simple correction model which successfully brings these instruments’ spectra into agreement with the uncontaminated observations.

Abstract:

Space weather events in exoplanetary environments sourced from transient host star emissions, including stellar flares, coronal mass ejections, and stellar proton events, can substantially influence a planet's habitability and atmospheric evolution history. These time-dependent events may also affect our ability to measure and interpret its properties by modulating reservoirs of key chemical compounds and changing the atmosphere’s brightness temperature. The majority of previous work focusing on photochemical effects, ground-level UV dosages, and consequences on observed spectra. Here, using three-dimensional general circulation models with interactive photochemistry, we simulate the climate and chemical impacts of stellar energetic particle events and periodic enhancements of UV photons. We use statistical methods to examine their effects on synchronously rotating TRAPPIST-1e-like planets on a range of spatiotemporal scales. We find that abrupt thermospheric cooling is associated with radiative cooling of NO and CO2, and middle-to-lower atmospheric warming is associated with elevated infrared absorbers such as N2O and H2O. In certain regimes, in particular for climates around moderately active stars, atmospheric temperature changes are strongly affected by O3 variability. Cumulative effects are largely determined by the flare frequency and the instantaneous effects are dependent on the flare’s spectral shape and energy. In addition to effects on planetary climate and atmospheric chemistry, we find that intense flares can energize the middle atmosphere, causing enhancements in wind velocities up to 40 m s−1 in substellar nightsides between 30 and 50 km in altitude. Our results suggest that successive, more energetic eruptive events from younger stars may be a pivotal factor in determining the atmosphere dynamics of their planets.