Exoplanet atmospheres

Abstract:

Implicit in the definition of the classical circumstellar habitable zone (HZ) is the hypothesis that the carbonate-silicate cycle can maintain clement climates on exoplanets with land and surface water across a range of instellations by adjusting atmospheric CO2 partial pressure (pCO2). This hypothesis is made by analogy to the Earth system, but it is an open question whether silicate weathering can stabilize climate on planets in the outer reaches of the HZ, where instellations are lower than those received by even the Archean Earth and CO2 is thought likely to dominate atmospheres. Since weathering products are carried from land to ocean by the action of water, silicate weathering is intimately coupled to the hydrologic cycle, which intensifies with hotter temperatures under Earth-like conditions. Here, we use global climate model simulations to demonstrate that the hydrologic cycle responds counterintuitively to changes in climate on planets with CO2-H2O atmospheres at low instellations and high pCO2, with global evaporation and precipitation decreasing as pCO2 and temperatures increase at a given instellation. Within the Maher & Chamberlain (or MAC) weathering formulation, weathering then decreases with increasing pCO2 for a range of instellations and pCO2 typical of the outer reaches of the HZ, resulting in an unstable carbon cycle that may lead to either runaway CO2 accumulation or depletion of CO2 to colder (possibly snowball) conditions. While the behavior of the system has not been completely mapped out, the results suggest that silicate weathering could fail to maintain habitable conditions in the outer reaches of the nominal HZ.

Abstract:

Amateur observers (Hill et al., 2024) have shown that filter-averaged measurements of the reflectance of Jupiter in molecular absorption bands of ammonia and methane can be made with modest-sized telescopes and reduced to yield spatial maps of ammonia. We now create an empirical limb correction for the ammonia abundance and effective cloud-top pressure and assemble sets of synoptic maps with partial or complete longitude coverage taken in moderately close temporal proximity (fewer than 10 Jupiter days). We then examine the maps to evaluate the characteristics and changes seen in localized ammonia enhancements and gradients in the context of effective cloud-top pressure and visual features. The ammonia depletion associated with the Great Red Spot (GRS) and the enhancements in the NEZ show significant contrast and are the focus of the study. The GRS region shows generally persistent characteristics over the period of observation, including a small southward offset of the depleted regions from the visual centroid of the GRS. The NEZ enhancements and their relationship to visual features such as plumes and North Equatorial Dark Features (NEDFs) are presented along with changes seen on timescales of days to weeks. This work demonstrates the utility of frequent observations in following the evolution of the Jovian ammonia distribution.

Abstract:

Voyager 2 radio occultation measurements of Uranus and Neptune revealed a layer approximately 2-4 km thick near 1.2 and 1.6 bars, respectively, wherein the atmospheric refractivity exhibited a slope variation (1, 2). These findings were interpreted as indicating a region where methane gas was undergoing condensation, forming an ice cloud centered around this pressure level. While the formation of this putative cloud would explain the observed decrease in methane abundance with height above 1.2 and 1.6 bars, or the banded structure of Uranus through latitudinal variations in the opacity of this cloud, several recent works and observations do not provide direct evidence in favor of this cloud (3): (i) radiative transfer models show an enhancement in the scattering opacity at pressures near 4-6 bars, more consistent with the presence of H2S ice (4, 5); (ii) observations from ground-based telescopes (or observations from telescopes in orbit around the Earth) of methane clouds indicate cloud tops near 0.4 bars in both planets (6), approximately a scale height above the base of the putative methane cloud.To investigate the formation of methane clouds in the atmospheres of the Ice Giants, we employed a one-dimensional cloud microphysical model originally developed for Titan and Mars (7,8). This model includes the processes of nucleation, condensation, coagulation, evaporation, precipitation, and coalescence. In the model, vertical transport is parameterized using an eddy diffusion profile (Keddy). Figure 1 illustrates, as example, cloud microphysics simulations carried out for the atmosphere of Uranus, assuming a constant Keddy in the troposphere and a concentration of haze particles (cloud condensation nuclei-CCN) of 3 particles per cm3. For this scenario, the model indicates the formation of a cloud layer near the 1.2-bar level with high precipitation rates and haze scavenging.In this work, we will discuss the different scenarios that may lead to the formation of methane clouds in the Ice Giants, as well as the cloud properties (e.g., precipitation rates, particle radius, and opacity) derived from the model. These results will be compared against observations and previous works, and we will evaluate the differences with respect to the methane clouds observed in Titan’s atmosphere, focusing on parameters such as lifetime or particle radius. References: [1] G. F. Lindal, et al., Journal of Geophysical Research: Space Physics 92, 14987 (1987). [2] G. Lindal, et al., Geophysical Research Letters 17, 1733 (1990). [3]. L. Sromovsky, P. Fry, J. H. Kim, Icarus 215, 292 (2011). [4] P. G. Irwin, et al., Nature Astronomy 2, 420 (2018). [5] P. G. Irwin, et al., Icarus 321, 550 (2019). [6]. E. Karkoschka, Science 280, 570 (1998). [7] P. Rannou, F. Montmessin, F. Hourdin, S. Lebonnois, science 311, 201 (2006). [8] F. Montmessin, P. Rannou, M. Cabane, Journal of Geophysical Research: Planets 107, 4 (2002).

Abstract:

There is still some debate on the origin and distribution of Jupiter’s red colouration. Based on the red compound obtained in a laboratory by Carlson et al. (2016)[1] through the reaction of photolysed ammonia with acetylene, Stromovsky et al. (2017)[2] proposed the idea of a “universal chromophore” which was able to fit observations with Cassini/VIMS-V. Baines et al.(2019)[3] later concluded that the chromophore would be located in a thin layer above the ammonia clouds, which was known as the “Crème Brûlée” model. However, other models propose a different scheme, with a more extended and less blue absorbing chromophore layer, such as Pérez-Hoyos et al. (2020)[4] for a North Temperate Belt disturbance and Braude et al. (2020)[5] for the overall latitudinal structure, even without discarding the possible existence of a universal chromophore. More recently, Aguiano-Arteaga et al. (2023)[6] analysed HST/WFC3 images of Jupiter’s Great Red Spot, as well as its surroundings and the Oval BA. Their results suggest the presence of two colouring aerosols, one very similar to the “universal chromophore” proposed by Stromovsky et al. (2017)[2] and a new colouring species at tropospheric levels, below the main chromophore layer. This highlights that there is still some uncertainty on how the aerosols are vertically distributed and their properties as well as the temporal and spatial variability, which, at last instance, is linked to the unknowns related to the formation and the nature of the absorbing particles and their chemistry.In this work, we analysed Jupiter spectra obtained for the first time with CARMENES in Calar-Alto in 2019. The Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs is an instrument operating at the 3.5m telescope at the Calar Alto Observatory. It consists of two separated spectrographs covering the wavelength ranges from 0.52 to 0.96 µm and from 0.96 to 1.71 µm with spectral resolutions R = 80,000-100,000, each of which performs high-accuracy radial-velocity measurements (∼1 m s⁻¹) with long-term stability. While the observations in this work were taken for Doppler velocimetry purposes, we used here a downgraded resolution version (R = 173-570) so the observations match the available spectral data for methane, the dominant gaseous source of opacity in the Jovian spectra.To achieve flux calibration, 2017 first-time observations of Saturn with CARMENES were used, in particular, the observations of Saturn’s B ring (Figure 1). We used these to obtain the response function of the instrument, no other sources of calibration being available at the desired resolution. We used the spectrum of Saturn’s B ring in terms of absolute reflectivity (I/F) from Poulet et al. (2003)[7] as the flux calibrator to calculate the response function. We checked the flux calibration and its uncertainty comparing with the normalized albedo spectrum of Saturn from Karkoskcha (1994)[8]. Lastly, we applied the flux calibration to the Jupiter observations and compared them with MUSE/VLT Jupiter observations from Braude et al. (2020)[5].For this analysis, we used the NEMESIS (Nonlinear Optimal Estimator for MultivariatE Spectral analySIS) radiative transfer suite (Irwin et al., 2008 [9]). This tool allows coverage of both reflection and emission from any planetary atmosphere in scattering and non-scattering environments. The code utilizes an optimal estimator method (Rodgers, 2000) [10] to find the best plausible values of the parameters that define the atmospheric model, with an a priori parametrization of the atmosphere and the observational uncertainties as the starting point. Our goal is to retrieve aerosol properties from the Jovian atmosphere using the NEMESIS radiative transfer suite, in order to constrain the cloud and aerosol vertical distribution as well as the chromophore(s) properties that give Jupiter its reddish colouration in some bands and storms. In particular, we want to compare different competing vertical models, with an extended or concentrated chromophore layer. For doing so, we used spectra from both the centre of the disk as well as near the limb in order to highlight the effects of the aerosols when Jupiter is observed from various viewing angles. We present here our first exploratory results from this analysis. Figure 1: One of the CARMENES guiding camera images and visible and NIR spectra of Saturn's B ring used for flux calibration Figure 2: First model spectrum obtained (red) when compared with the observations (blue) after retrieving two aerosol density profiles. References: Carlson, R. W., et al. (2016). 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Compositions of Saturn's rings A, B, and C from high resolution near-infrared spectroscopic observations. A&A, 412, 1, 305–316. Karkoschka, E. (1994). Spectrophotometry of the Jovian Planets and Titan at 300- to 1000-nm Wavelength: The Methane Spectrum. Icarus, 111, 1, 174–192. Irwin, P., et al. (2008). The NEMESIS planetary atmosphere radiative transfer and retrieval tool. J. Quant. Spectrosc. Radiat. Transf., 109, 1136–1150. Rodgers CD. (2000). Inverse methods for atmospheric sounding: theory and practice. Singapore: World Scientific.