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Build a scale model of the Solar System on a city map., AstroEdu, No. 3 (2015)

Clouds play an important role in the radiative transfer of planetary atmospheres: they are key elements of the climate system and influence the planet’s spectral appearance. Given the thousands of exoplanets discovered so far, including some dozens of Earth-sized exoplanets, the feasibility of remote sensing of exoplanet atmospheres is attracting increasing attention. Here we present a study of the thermal emission of cloud-covered Earth-like exoplanets orbiting in the habitable zone of F, G, K, and M-type stars. A line-by-line model for molecular absorption has been coupled to a discrete ordinate multiple scattering radiative transfer solver. Pressure, temperature, and molecular concentration profiles were taken from a consistent radiative-convective climate model including a parameterized cloud description (Kitzmann et al., A&A, 2010). The main focus of the current work is the impact of multi-layer clouds on emission spectra in the thermal infrared. The effects of low-level water clouds and high level ice clouds simultaneously on signatures of H2 O, CO2 , O3 , etc will be studied for various resolutions. Furthermore, comparisons with spectra resulting from a low-resolution code will be shown.

This work addresses the main question regarding how much can we learn from spectroscopic observations of Earth-like exoplanets along with what can we identify in terms of atmospheric conditions and biosignatures that may be present on the planet and serve as evidence of life. Using a high resolution infrared radiative transfer model, we calculate the emission spectra of Earth-like exoplanets and their partial derivatives with respect to the unknowns of the inverse problem (i.e. the Jacobians). Sensitivity analysis based on Jacobian matrices revealed which parameters can be retrieved independently and which parameters have to be set a priori, depending on the type of observation and spectral domain. A set of few optimized state vectors were used for extensive retrieval simulations investigating impact of the initial guess and a priori data information content. The calculations provided information on data quality depending on the spectral range, resolution, and number of molecular species considered in the atmosphere. Larger correlations were found between certain parameters, what provided information about the difficulty of retrieving a certain parameter independently. A sensitivity study has also been conducted in order to assess the feasibility of retrieving CO2 and H2O molecular concentrations from an observed spectrum with a 1% noise level. Also, comparisons between analytical and numerical Jacobians were studied.

GARLIC has been developed with emphasis on efficient and reliable numerical algorithms and a modular approach appropriate for simulation and/or retrieval in a variety of applications. It is suitable for arbitrary observation geometry, instrumental field-of-view, and line shape. The core of GARLIC's subroutines constitutes the basis of forward models used to implement inversion codes to retrieve atmospheric state parameters from limb and nadir sounding instruments. In view of the computational burden of lbl modeling considerable effort has been devoted to optimized algorithms. Collision induced absorption has been implemented as a complement of the H2O, CO2, etc continua. Furthermore, the DISORT multiple-scattering solver has been added to GARLIC for an assessment of the impact of clouds on exoplanet spectra. As a recent upgrade Jacobians with respect to temperature have been implemented utilizing algorithmic differentiation. In this contribution we summarize GARLIC's main features and give some examples from Earth and planetary science.

More than one thousand exoplanets have been discovered in the past two decades, with some dozen of them in the host stars’ habitable zone and with size and mass similar to Earth. Furthermore, spectra of exoplanets become available with reasonable quality (resolution and noise) that trigger the question of remote sensing of the planet’s atmosphere. The objective of this sensitivity study is to identify the optimal state vector representing the atmosphere in the inverse problem. Solving the inverse problem will ultimately allow to characterize the planet and determine its habitability. Using a high resolution infrared radiative transfer code with a line-by-line molecular absorption model, we calculate synthetic spectra of exoplanets orbiting dwarf stars. Key parameters describing the atmosphere (i.e., molecular abundances, temperature, pressure) are identified and the Jacobians (i.e. partial derivatives of the spectra) are evaluated to investigate the feasibility to retrieve the state of the planetary atmosphere.

The Space mission DWARFS is designed to determine atmospheric composition and physical properties of rocky extrasolar planets in the habitable zone around M-Type stars. Most importantly, we want to research the atmospheric composition with a special emphasize on O3, H2O, CH, CH4, CO2, CH3Cl lines, as well as the evolution of the planetary systems. For some of the stars DWARFS will be able to detect in addition surface features, moons or further planets in the system. All this will be done with two spectrographs in the NIR/MIR and a photometer (r, i SDSS) in a 4m space telescope.