Water Planets in the Habitable Zone: Atmospheric Chemistry, Observable Features, and the case of Kepler-62e and -62f

Planets composed of large quantities of water that reside in the habitable zone are expected to have distinct geophysics and geochemistry of their surfaces and atmospheres. We explore these properties motivated by two key questions: whether such planets could provide habitable conditions and whether they exhibit discernable spectral features that distinguish a water-planet from a rocky Earth-like planet. We show that the recently discovered planets Kepler-62e and -62f are the first viable candidates for habitable zone water-planet. We use these planets as test cases for discussing those differences in detail. We generate atmospheric spectral models and find that potentially habitable water-planets show a distinctive spectral fingerprint in transit depending on their position in the habitable zone.Comment: 8 pages, 4 figures, ApJ, 775, L4

UV Surface Environment of Earth-like Planets Orbiting FGKM Stars Through Geological Evolution

The UV environment of a host star affects the photochemistry in the atmosphere, and ultimately the surface UV environment for terrestrial planets and therefore the conditions for the origin and evolution of life. We model the surface UV radiation environment for Earth-sized planets orbiting FGKM stars at the 1AU equivalent distance for Earth through its geological evolution. We explore four different types of atmospheres corresponding to an early Earth atmosphere at 3.9 Gyr ago and three atmospheres covering the rise of oxygen to present day levels at 2.0 Gyr ago, 0.8 Gyr ago and modern Earth (Following Kaltenegger et al. 2007). In addition to calculating the UV flux on the surface of the planet, we model the biologically effective irradiance, using DNA damage as a proxy for biological damage. We find that a pre-biotic Earth (3.9 Gyr ago) orbiting an F0V star receives 6 times the biologically effective radiation as around the early Sun and 3520 times the modern Earth-Sun levels. A pre-biotic Earth orbiting GJ 581 (M3.5V) receives 300 times less biologically effective radiation, about 2 times modern Earth-Sun levels. The UV fluxes calculated here provide a grid of model UV environments during the evolution of an Earth-like planet orbiting a range of stars. These models can be used as inputs into photo-biological experiments and for pre-biotic chemistry and early life evolution experiments.Comment: 10 pages, 5 figure

Effect of UV Radiation on the Spectral Fingerprints of Earth-like Planets Orbiting M dwarfs

We model the atmospheres and spectra of Earth-like planets orbiting the entire grid of M dwarfs for active and inactive stellar models with $T_{eff}$ = 2300K to $T_{eff}$ = 3800K and for six observed MUSCLES M dwarfs with UV radiation data. We set the Earth-like planets at the 1AU equivalent distance and show spectra from the VIS to IR (0.4$\mu$m - 20$\mu$m) to compare detectability of features in different wavelength ranges with JWST and other future ground- and spaced-based missions to characterize exo-Earths. We focus on the effect of UV activity levels on detectable atmospheric features that indicate habitability on Earth, namely: H$_2$O, O$_3$, CH$_4$, N$_2$O and CH$_3$Cl. To observe signatures of life - O$_2$/O$_3$ in combination with reducing species like CH$_4$, we find that early and active M dwarfs are the best targets of the M star grid for future telescopes. The O$_2$ spectral feature at 0.76$\mu$m is increasingly difficult to detect in reflected light of later M dwarfs due to low stellar flux in that wavelength region. N$_2$O, another biosignature detectable in the IR, builds up to observable concentrations in our planetary models around M dwarfs with low UV flux. CH$_3$Cl could become detectable, depending on the depth of the overlapping N$_2$O feature. We present a spectral database of Earth-like planets around cool stars for directly imaged planets as a framework for interpreting future lightcurves, direct imaging, and secondary eclipse measurements of the atmospheres of terrestrial planets in the HZ to design and assess future telescope capabilities.Comment: in press, ApJ (submitted August 18, 2014), 16 pages, 12 figure

Water-planets in the habitable zone: atmospheric chemistry, observable features, and the case of kepler-62e and -62f

Planets composed of large quantities of water that reside in the habitable zone are expected to have distinct geophysics and geochemistry of their surfaces and atmospheres. We explore these properties motivated by two key questions: whether such planets could provide habitable conditions and whether they exhibit discernable spectral features that distinguish a water-planet from a rocky Earth-like planet. We show that the recently discovered planets Kepler-62e and -62f are the first viable candidates for habitable zone water-planets. We use these planets as test cases for discussing those differences in detail. We generate atmospheric spectral models and find that potentially habitable water-planets show a distinctive spectral fingerprint in transit depending on their position in the habitable zone.Publisher PDFPeer reviewe

Finding Signs of Life on Earth-like Planets: High-resolution Transmission Spectra of Earth through time around FGKM stars

The search for life in the universe mainly uses modern Earth as a template. However, we know that Earth's atmospheric composition changed significantly through its geological evolution. Recent discoveries show that transiting, potentially Earth-like exoplanets orbit a wide range of host stars, which strongly influence their atmospheric composition and remotely detectable spectra. Thus, a database for transiting terrestrial exoplanet around different host stars at different geological times is a crucial missing ingredient to support observational searches for signs of life in exoplanet atmospheres. Here, we present the first high-resolution transmission spectra database for Earth-like planets, orbiting a wide range of host stars, throughout four representative stages of Earth's history. These correspond to a prebiotic high CO2-world - about 3.9 billion years ago in Earth's history - and three epochs through the rise of oxygen from 0.2% to modern atmospheric levels of 21%. We demonstrate that the spectral biosignature pairs O2 + CH4 and O3 + CH4 in the atmosphere of a transiting Earth-like planet would show a remote observer that a biosphere exists for oxygen concentrations of about 1% modern Earth's - corresponding to about 1 to 2 billion years ago in Earth's history - for all host stars. The full model and high-resolution transmission spectra database, covering 0.4 to 20microns, for transiting exoplanets - from young prebiotic worlds to modern Earths-analogs - orbiting a wide range of host stars is available online. It can be used as a tool to plan and optimize our observation strategy, train retrieval methods, and interpret upcoming observations with ground- and space-based telescopes.Comment: 9 pages, accepted in Ap

Photochemical modelling of atmospheric oxygen levels confirms two stable states

This work was supported by the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation programme (grant no. 678812 awarded to M.W.C).Various proxies and numerical models have been used to constrain O2 levels over geological time, but considerable uncertainty remains. Previous investigations using 1-D photochemical models have predicted how O3 concentrations vary with assumed ground-level O2 concentrations, and indicate how the ozone layer might have developed over Earth history. These classic models have utilised the numerical simplification of fixed mixing ratio boundary conditions. Critically, this modelling assumption requires verification that predicted fluxes of biogenic and volcanic gases are realistic, but also that the resulting steady states are in fact stable equilibrium solutions against trivial changes in flux. Here, we use a 1-D photochemical model with fixed flux boundary conditions to simulate the effects on O3 and O2 concentrations as O2 (and CH4) fluxes are systematically varied. Our results suggest that stable equilibrium solutions exist for trace- and high-O2/O3 cases, separated by a region of instability. In particular, the model produces few stable solutions with ground O2 mixing ratios between 6×10-7 and 2×10-3 (3×10-6 and 1% of present atmospheric levels). A fully UV-shielding ozone layer only exists in the high-O2 states. Our atmospheric modelling supports prior work suggesting a rapid bimodal transition between reducing and oxidising conditions, and proposes Proterozoic oxygen levels higher than some recent proxies suggest. We show that the boundary conditions of photochemical models matter, and should be chosen and explained with care.Publisher PDFPeer reviewe

Oxidised micrometeorites as evidence for low atmospheric pressure on the early Earth

Reconstructing a record of the partial pressure of molecular oxygen in Earth’s atmosphere is key for understanding macroevolutionary and environmental change over geological history. Recently, the oxidation state of iron in micrometeorites has been taken to imply the presence of modern Earth concentrations of oxygen in the upper atmosphere at 2.7 Ga, and therefore a highly chemically stratified atmosphere (Tomkins et al., 2016). We here explore the possibility that the mixing ratio of oxygen in Earth’s upper atmosphere, that probed by micrometeorites, may instead be sensitive to the surface atmospheric pressure. We find that the concentrations of oxygen in the upper atmosphere required for micrometeorite oxidation are achieved for a 0.3 bar atmosphere. In this case, significant water vapour reaches high up in the atmosphere and is photodissociated, leading to the formation of molecular oxygen. The presence of oxidised iron in micrometeorites at 2.7 Ga may therefore be further evidence that the atmospheric pressure at the surface of the early Earth was substantially lower than it is today

Oxidised micrometeorites as evidence for low atmospheric pressure on the early Earth

Reconstructing a record of the partial pressure of molecular oxygen in Earth’s atmosphere is key for understanding macroevolutionary and environmental change over geological history. Recently, the oxidation state of iron in micrometeorites has been taken to imply the presence of modern Earth concentrations of oxygen in the upper atmosphere at 2.7 Ga, and therefore a highly chemically stratified atmosphere (Tomkins et al., 2016). We here explore the possibility that the mixing ratio of oxygen in Earth’s upper atmosphere, that probed by micrometeorites, may instead be sensitive to the surface atmospheric pressure. We find that the concentrations of oxygen in the upper atmosphere required for micrometeorite oxidation are achieved for a 0.3 bar atmosphere. In this case, significant water vapour reaches high up in the atmosphere and is photodissociated, leading to the formation of molecular oxygen. The presence of oxidised iron in micrometeorites at 2.7 Ga may therefore be further evidence that the atmospheric pressure at the surface of the early Earth was substantially lower than it is today.PBR thanks the Simons Foundation and Kavli Foundation for funding, specifically Simons Foundation SCOL awards 59963

The MUSCLES Treasury Survey. V. FUV Flares on Active and Inactive M Dwarfs

M dwarf stars are known for their vigorous flaring. This flaring could impact the climate of orbiting planets, making it important to characterize M dwarf flares at the short wavelengths that drive atmospheric chemistry and escape. We conducted a far-ultraviolet flare survey of 6 M dwarfs from the recent MUSCLES (Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems) observations, as well as 4 highly-active M dwarfs with archival data. When comparing absolute flare energies, we found the active-M-star flares to be about 10$\times$ more energetic than inactive-M-star flares. However, when flare energies were normalized by the star's quiescent flux, the active and inactive samples exhibited identical flare distributions, with a power-law index of -$0.76^{+0.1}_{-0.09}$ (cumulative distribution). The rate and distribution of flares are such that they could dominate the FUV energy budget of M dwarfs, assuming the same distribution holds to flares as energetic as those cataloged by Kepler and ground-based surveys. We used the observed events to create an idealized model flare with realistic spectral and temporal energy budgets to be used in photochemical simulations of exoplanet atmospheres. Applied to our own simulation of direct photolysis by photons alone (no particles), we find the most energetic observed flares have little effect on an Earth-like atmosphere, photolyzing $\sim$0.01% of the total O$_3$ column. The observations were too limited temporally (73 h cumulative exposure) to catch rare, highly energetic flares. Those that the power-law fit predicts occur monthly would photolyze $\sim$1% of the O$_3$ column and those it predicts occur yearly would photolyze the full O$_3$ column. Whether such energetic flares occur at the rate predicted is an open question.Comment: Accepted to ApJ. v2 fixed some transposed errors, added PDF To

Variability due to climate and chemistry in observations of oxygenated Earth-analogue exoplanets

The Great Oxidation Event was a period during which Earth’s atmospheric oxygen (O2) concentrations increased from ∼10−5 times its present atmospheric level (PAL) to near modern levels, marking the start of the Proterozoic geological eon 2.4 billion years ago. Using WACCM6, an Earth System Model, we simulate the atmosphere of Earth-analogue exoplanets with O2 mixing ratios between 0.1 and 150 per cent PAL. Using these simulations, we calculate the reflection spectra over multiple orbits using the Planetary Spectrum Generator. We highlight how observer angle, albedo, chemistry, and clouds affect the simulated observations. We show that inter-annual climate variations, as well short-term variations due to clouds, can be observed in our simulated atmospheres with a telescope concept such as LUVOIR or HabEx. Annual variability and seasonal variability can change the planet’s reflected flux (including the reflected flux of key spectral features such as O2 and H2O) by up to factors of 5 and 20, respectively, for the same orbital phase. This variability is best observed with a high-throughput coronagraph. For example, HabEx (4 m) with a starshade performs up to a factor of two times better than a LUVOIR B (6 m) style telescope. The variability and signal-to-noise ratio of some spectral features depends non-linearly on atmospheric O2 concentration. This is caused by temperature and chemical column depth variations, as well as generally increased liquid and ice cloud content for atmospheres with O2 concentrations of <1 per cent PAL