Given the forthcoming launch of the James Webb Space Telescope (JWST), which will allow observing exoplanet atmospheres with unprecedented signal-to-noise ratio, spectral coverage, and spatial ...resolution, the uncertainties in the atmosphere modeling used to interpret the data need to be assessed. As the first step, we compare three independent 1D radiative-convective models: ATMO, Exo-REM, and petitCODE. We identify differences in physical and chemical processes that are taken into account thanks to a benchmark protocol we have developed. We study the impact of these differences on the analysis of observable spectra. We show the importance of selecting carefully relevant molecular linelists to compute the atmospheric opacity. Indeed, differences between spectra calculated with Hitran and ExoMol exceed the expected uncertainties of future JWST observations. We also show the limits of the precision of the models due to uncertainties on alkali and molecule lineshape, which induce spectral effects that are also larger than the expected JWST uncertainties. We compare two chemical models, Exo-REM and Venot Chemical Code, which do not lead to significant differences in the emission or transmission spectra. We discuss the observational consequences of using equilibrium or out-of-equilibrium chemistry and the major impact of phosphine, detectable with the JWST. Each of the models has benefited from the benchmarking activity and has been updated. The protocol developed in this paper and the online results can constitute a test case for other models.
•Two-cell dynamics exists in Titan’s stratosphere from January 2010 to June 2011.•Reversal of the global dynamics occurs within 2years after northern spring equinox.•Timing of the dynamics changes ...agrees with General Circulation Model predictions.•An unexpected cooling, starting in 2012, is observed at the south pole above 350km.
We analyzed spectra acquired at the limb of Titan in the 2006–2013 period by the Cassini/Composite Infrared Spectrometer (CIRS) in order to monitor the seasonal evolution of the thermal, gas composition and aerosol spatial distributions. We are primarily interested here in the seasonal changes after the northern spring equinox and interpret our results in term of global circulation seasonal changes. Data cover the 600–1500cm−1 spectral range at a resolution of 0.5 or 15.5cm−1 and probe the 150–500km vertical range with a vertical resolution of about 30km. Retrievals of the limb spectra acquired at 15.5cm−1 resolution allowed us to derive eight global maps of temperature, aerosols and C2H2, C2H6 and HCN molecular mixing ratios between July 2009 and May 2013. In order to have a better understanding of the global changes taking place after the northern spring equinox, we analyzed 0.5cm−1 resolution limb spectra to infer the mixing ratio profiles of 10 molecules for some latitudes. These profiles are compared with CIRS observations performed during the northern winter. Our observations are compatible with the coexistence of two circulation cells upwelling at mid-latitudes and downwelling at both poles from at last January 2010 to at least June 2010. One year later, in June 2011, there are indications that the global circulation had reversed compared to the winter situation, with a single pole-to-pole cell upwelling at the north pole and downwelling at the south pole. Our observations show that in December 2011, this new pole-to-pole cell has settled with a downward velocity of 4.4mm/s at 450km above the south pole. Therefore, in about two years after the equinox, the global circulation observed during the northern winter has totally reversed, which is in agreement with the predictions of general circulation models. We observe a sudden unexpected temperature decrease above the south pole in February 2012, which is probably related to the strong enhancement of molecular gas in this region, acting as radiative coolers. In July and November 2012, we observe a detached haze layer located around 320–330km, which is comparable to the altitude of the detached haze layer observed by the Cassini Imaging Science Subsystem (ISS) in the UV.
Saturn's largest moon Titan has a substantial nitrogen-methane atmosphere, with strong seasonal effects, including formation of winter polar vortices. Following Titan's 2009 northern spring equinox, ...peak solar heating moved to the northern hemisphere, initiating south-polar subsidence and winter polar vortex formation. Throughout 2010-2011, strengthening subsidence produced a mesospheric hot-spot and caused extreme enrichment of photochemically produced trace gases. However, in 2012 unexpected and rapid mesospheric cooling was observed. Here we show extreme trace gas enrichment within the polar vortex dramatically increases mesospheric long-wave radiative cooling efficiency, causing unusually cold temperatures 2-6 years post-equinox. The long time-frame to reach a stable vortex configuration results from the high infrared opacity of Titan's trace gases and the relatively long atmospheric radiative time constant. Winter polar hot-spots have been observed on other planets, but detection of post-equinox cooling is so far unique to Titan.
We present simultaneous Hubble Space Telescope (HST) WFC3+Spitzer IRAC variability monitoring for the highly variable young (∼20 Myr) planetary-mass object PSO J318.5−22. Our simultaneous HST + ...Spitzer observations covered approximately two rotation periods with Spitzer and most of a rotation period with the HST. We derive a period of 8.6 0.1 hr from the Spitzer light curve. Combining this period with the measured for this object, we find an inclination of 56 2 8 1. We measure peak-to-trough variability amplitudes of 3.4% 0.1% for Spitzer Channel 2 and 4.4%-5.8% (typical 68% confidence errors of ∼0.3%) in the near-IR bands (1.07-1.67 m) covered by the WFC3 G141 prism-the mid-IR variability amplitude for PSO J318.5−22 is one of the highest variability amplitudes measured in the mid-IR for any brown dwarf or planetary-mass object. Additionally, we detect phase offsets ranging from 200° to 210° (typical error of ∼4°) between synthesized near-IR light curves and the Spitzer mid-IR light curve, likely indicating depth-dependent longitudinal atmospheric structure in this atmosphere. The detection of similar variability amplitudes in wide spectral bands relative to absorption features suggests that the driver of the variability may be inhomogeneous clouds (perhaps a patchy haze layer over thick clouds), as opposed to hot spots or compositional inhomogeneities at the top-of-atmosphere level.
The Cassini/Composite InfraRed Spectrometer (CIRS) instrument has been observing the middle atmosphere of Titan over almost half a Saturnian year. We used the CIRS dataset processed through the ...up-to-date calibration pipeline to characterize seasonal changes of temperature and abundance profiles in the middle atmosphere of Titan, from mid-northern winter to early northern summer all around the satellite. We used limb spectra from 590 to 1500 cm−1 at 0.5-cm−1 spectral resolution, which allows us to probe different altitudes. We averaged the limb spectra recorded during each flyby on a fixed altitude grid to increase the signal-to-noise ratio. These thermal infrared data were analyzed by means of a radiative transfer code coupled with an inversion algorithm, in order to retrieve vertical temperature and abundance profiles. These profiles cover an altitude range of approximately 100 to 600 km, at 10- or 40-km vertical resolution (depending on the observation). Strong changes in temperature and composition occur in both polar regions where a vortex is in place during the winter. At this season, we observe a global enrichment in photochemical compounds in the mesosphere and stratosphere and a hot stratopause located around 0.01 mbar, both linked to downwelling in a pole-to-pole circulation cell. After the northern spring equinox, between December 2009 and April 2010, a stronger enhancement of photochemical compounds occurred at the north pole above the 0.01-mbar region, likely due to combined photochemical and dynamical effects. During the southern autumn in 2015, above the South pole, we also observed a strong enrichment in photochemical compounds that contributed to the cooling of the stratosphere above 0.2 mbar (∼300 km). Close to the northern spring equinox, in December 2009, the thermal profile at 74°N exhibits an oscillation that we interpret in terms of an inertia-gravity wave.
•Temperature and composition monitoring over the whole Cassini mission from CIRS limb spectra•Northern polar region presents an unexpected enrichment at high altitudes shortly after equinox.•Highest mixing ratios ever observed in Titan were localized at the south pole in March 2015.•An inertia-gravity wave has been possibly detected in December 2009 at 74°N.
We have analyzed data recorded by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft during the Titan flybys T0–T10 (July 2004–January 2006). The spectra characterize various ...regions on Titan from 70° S to 70° N with a variety of emission angles. We study the molecular signatures observed in the mid-infrared CIRS detector arrays (FP3 and FP4, covering roughly the 600–1500 cm
−1 spectral range with apodized resolutions of 2.54 or 0.53 cm
−1). The composite spectrum shows several molecular signatures: hydrocarbons, nitriles and CO
2. A firm detection of benzene (C
6H
6) is provided by CIRS at levels of about
3.5
×
10
−9
around 70° N. We have used temperature profiles retrieved from the inversion of the emission observed in the methane
ν
4
band at 1304 cm
−1 and a line-by-line radiative transfer code to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere. No longitudinal variations were found for these gases. Little or no change is observed generally in their abundances from the south to the equator. On the other hand, meridional variations retrieved for these trace constituents from the equator to the North ranged from almost zero (no or very little meridional variations) for C
2H
2, C
2H
6, C
3H
8, C
2H
4 and CO
2 to a significant enhancement at high northern (early winter) latitudes for HCN, HC
3N, C
4H
2, C
3H
4 and C
6H
6. For the more important increases in the northern latitudes, the transition occurs roughly between 30 and 50 degrees north latitude, depending on the molecule. Note however that the very high-northern latitude results from tours TB–T10 bear large uncertainties due to few available data and problems with latitude smearing effects. The observed variations are consistent with some, but not all, of the predictions from dynamical-photochemical models. Constraints are set on the vertical distribution of C
2H
2, found to be compatible with 2-D equatorial predictions by global circulation models. The D/H ratio in the methane on Titan has been determined from the CH
3D band at 1156 cm
−1 and found to be
1.17
−0.28
+0.23
×
10
−4
. Implications of this deuterium enrichment, with respect to the protosolar abundance on the origin of Titan, are discussed. We compare our results with values retrieved by Voyager IRIS observations taken in 1980, as well as with more recent (1997) disk-averaged Infrared Space Observatory (ISO) results and with the latest Cassini–Huygens inferences from other instruments in an attempt to better comprehend the physical phenomena on Titan.
•We analyze Huygens/DISR spectra to determine the CH4 abundance in Titan’s stratosphere.•We use recently published methane linelist for the 1.4-μm band.•Derived methane mole fraction agrees with ...Huygens/GCMS measurement.•Result is not consistent with recent Cassini/CIRS determination at low latitudes.
We present a determination of the methane mole fraction in Titan’s stratosphere using spectral measurements by the Upward Looking Infrared Spectrometer (ULIS) of the Descent Imager/Spectral Radiometer (DISR) aboard the Huygens probe. We analyzed the 1.4-μm band of methane for which a complete linelist, down to very low intensities, was recently made available. The DISR/ULIS measurements were used to derive the methane transmittance along the path from the probe to the Sun during the descent from 135 to 29km. Fitting the transmittance ratios to remove residual instrumental effects, we derived a stratospheric methane mole fraction of 1.44 (+0.27/−0.11)%, taking into account random errors and uncertainties in the spectroscopic parameters. This value is fully consistent with the simultaneous measurement of the methane profile by the Huygens Gas Chromatograph/Mass Spectrometer (GCMS). It disagrees with the ∼1% mixing ratio recently inferred from Cassini/CIRS spectra at low latitudes near 85km. Possible reasons for the discrepancy and uncertainties in the methane spectroscopic parameters are discussed.
ABSTRACT
Atmospheric characterization of exoplanets from the ground is an actively growing field of research. In this context, we have created the ATMOSPHERIX consortium: a research project aimed at ...characterizing exoplanets atmospheres using ground-based high-resolution spectroscopy. This paper presents the publicly available data analysis pipeline and demonstrates the robustness of the recovered planetary parameters from synthetic data. Simulating planetary transits using synthetic transmission spectra of a hot Jupiter that were injected into real SPIRou observations of the non-transiting system Gl 15 A, we show that our pipeline is successful at recovering the planetary signal and input atmospheric parameters. We also introduce a deep learning algorithm to optimize data reduction which proves to be a reliable, alternative tool to the commonly used principal component analysis. We estimate the level of uncertainties and possible biases when retrieving parameters such as temperature and composition and hence the level of confidence in the case of retrieval from real data. Finally, we apply our pipeline onto two real transits of HD 189733 b observed with SPIRou and obtain similar results than in the literature. In summary, we have developed a publicly available and robust pipeline for the forthcoming studies of the targets to be observed in the framework of the ATMOSPHERIX consortium, which can easily be adapted to other high resolution instruments than SPIRou (e.g. VLT-CRIRES, MAROON-X, ELT-ANDES).
ABSTRACT
In a companion paper, we introduced a publicly available pipeline to characterize exoplanet atmospheres through high-resolution spectroscopy. In this paper, we use this pipeline to study the ...biases and degeneracies that arise in atmospheric characterization of exoplanets in near-infrared ground-based transmission spectroscopy. We inject synthetic planetary transits into sequences of SPIRou spectra of the well known M dwarf star Gl 15 A, and study the effects of different assumptions on the retrieval. We focus on (i) mass and radius uncertainties, (ii) non-isothermal vertical profiles, and (iii) identification and retrieval of multiple species. We show that the uncertainties on mass and radius should be accounted for in retrievals and that depth-dependent temperature information can be derived from high-resolution transmission spectroscopy data. Finally, we discuss the impact of selecting wavelength orders in the retrieval and the issues that arise when trying to identify a single species in a multispecies atmospheric model. This analysis allows us to understand better the results obtained through transmission spectroscopy and their limitations in preparation to the analysis of actual SPIRou data.
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