The origin and evolution of the atmospheres of Earth, Venus and Mars are reviewed from the time when their protoplanets were released from the protoplanetary disk a few million years after the Sun came into being. The early disk-embedded phase of the evolution of protoplanetary cores that can accumulate gas from the disk and form thin planetary H 2-envelopes is also discussed. This scenario is compared to cases of late stage planet formation, where terrestrial planets accrete from large planetary embryos after the protoplanetary disk already disappeared. The differences between these two scenarios are discussed by investigating non-radiogenic noble gas isotope anomalies observed in the present atmospheres of the three planets. The role of the efficiency of the young Sun's EUV radiation and solar wind to the escape of early atmospheres is also discussed. The catastrophic outgassing of volatiles and the formation and cooling of steam atmospheres after the solidification of magma oceans is addressed together with the geochemical evidence of additional delivery of volatile-rich chondritic materials during the main stages of planetary formation. Unlike early Venus and Earth, no nebula-based H 2-envelope could be accumulated on early Mars due to its low planetary mass. According to the young Sun's luminosity and EUV flux history, Mars' magma ocean related outgassed steam atmosphere could have been lost during the first hundred Myrs. Depending on the young Sun's EUV flux, the presence of greenhouse gases, impacts, and the amount of greenhouse gases outgassed additional to that from the magma ocean, Mars could have developed episodically standing bodies of liquid water

Atmospheric temperature and mixing ratio profiles of terrestrial planets vary with the spectral energy flux distribution for different types of M-dwarf stars and the planetary gravity. We investigate the resulting effects on the spectral appearance of molecular absorption bands, which are relevant as indicators for potential planetary habitability during primary and secondary eclipse for transiting terrestrial planets with Earth-like biomass emissions. Atmospheric profiles are computed using a plane-parallel, 1D climate model coupled with a chemistry model. We then calculate simulated spectra using a line-by-line radiative transfer model. We find that emission spectra during secondary eclipse show increasing absorption of methane, water, and ozone for planets orbiting quiet M0-M3 dwarfs and the active M-type star AD Leo compared with solar-type central stars. However, for planets orbiting very cool and quiet M dwarfs (M4 to M7), increasing temperatures in the mid-atmosphere lead to reduced absorption signals, which impedes the detection of molecules in these scenarios. Transmission spectra during primary eclipse show strong absorption features of CH 4 , N 2 O and H 2 O for planets orbiting quiet M0-M7 stars and AD Leo. The N 2 O absorption of an Earth-sized planet orbiting a quiet M7 star can even be as strong as the CO 2 signal. However, ozone absorption decreases for planets orbiting these cool central stars owing to chemical effects in the atmosphere. To investigate the effect on the spectroscopic detection of absorption bands with potential future satellite missions, we compute signal-to-noise-ratios (SNR) for a James Webb Space Telescope (JWST)-like aperture telescope.

Context. The characterisation of the atmosphere of exoplanets is one of the main goals of exoplanet science in the coming decades. Aims. We investigate the detectability of atmospheric spectral features of Earth-like planets in the habitable zone (HZ) around M dwarfs with the future James Webb Space Telescope (JWST). Methods. We used a coupled 1D climate-chemistry-model to simulate the influence of a range of observed and modelled M-dwarf spectra on Earth-like planets. The simulated atmospheres served as input for the calculation of the transmission spectra of the hypothetical planets, using a line-by-line spectral radiative transfer model. To investigate the spectroscopic detectability of absorption bands with JWST we further developed a signal-to-noise ratio (S/N) model and applied it to our transmission spectra. Results. High abundances of methane (CH 4 ) and water (H 2 O) in the atmosphere of Earth-like planets around mid to late M dwarfs increase the detectability of the corresponding spectral features compared to early M-dwarf planets. Increased temperatures in the middle atmosphere of mid-to late-type M-dwarf planets expand the atmosphere and further increase the detectability of absorption bands. To detect CH 4 , H 2 O, and carbon dioxide (CO 2 ) in the atmosphere of an Earth-like planet around a mid to late M dwarf observing only one transit with JWST could be enough up to a distance of 4 pc and less than ten transits up to a distance of 10 pc. As a consequence of saturation limits of JWST and less pronounced absorption bands, the detection of spectral features of hypothetical Earth-like planets around most early M dwarfs would require more than ten transits. We identify 276 existing M dwarfs (including GJ 1132, TRAPPIST-1, GJ 1214, and LHS 1140) around which atmospheric absorption features of hypothetical Earth-like planets could be detected by co-adding just a few transits. Conclusions. The TESS satellite will likely find new transiting terrestrial planets within 15 pc from the Earth. We show that using transmission spectroscopy, JWST could provide enough precision to be able to partly characterise the atmosphere of TESS findings with an Earth-like composition around mid to late M dwarfs.

Understanding whether M dwarf stars may host habitable planets with Earth-like atmospheres and biospheres is a major goal in exoplanet research. If such planets exist, the question remains as to whether they could be identified via spectral signatures of biomarkers. Such planets may be exposed to extreme intensities of cosmic rays that could perturb their atmospheric photochemistry. Here, we consider stellar activity of M dwarfs ranging from quiet up to strong flaring conditions and investigate one particular effect upon biomarkers, namely, the ability of secondary electrons caused by stellar cosmic rays to break up atmospheric molecular nitrogen (N 2 ), which leads to production of nitrogen oxides (NO x ) in the planetary atmosphere, hence affecting biomarkers such as ozone (O 3 ). We apply a stationary model, that is, without a time dependence; hence we are calculating the limiting case where the atmospheric chemistry response time of the biomarkers is assumed to be slow and remains constant compared with rapid forcing by the impinging stellar flares. This point should be further explored in future work with time-dependent models. We estimate the NO x production using an air shower approach and evaluate the implications using a climate-chemical model of the planetary atmosphere. O 3 formation proceeds via the reaction O + O 2 + M/O 3 + M. At high NO x abundances, the O atoms arise mainly from NO 2 photolysis, whereas on Earth this occurs via the photolysis of molecular oxygen (O 2 ). For the flaring case, O 3 is mainly destroyed via direct titration, NO + O 3 /NO 2 + O 2 , and not via the familiar catalytic cycle photochemistry, which occurs on Earth. For scenarios with low O 3 , Rayleigh scattering by the main atmospheric gases (O 2 , N 2 , and CO 2 ) became more important for shielding the planetary surface from UV radiation. A major result of this work is that the biomarker O 3 survived all the stellar-activity scenarios considered except for the strong case, whereas the biomarker nitrous oxide (N 2 O) could survive in the planetary atmosphere under all conditions of stellar activity considered here, which clearly has important implications for missions that aim to detect spectroscopic biomarkers.