155 research outputs found
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 =
2300K to = 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 (0.4m - 20m) 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: HO, O, CH, NO and
CHCl.
To observe signatures of life - O/O in combination with reducing
species like CH, we find that early and active M dwarfs are the best
targets of the M star grid for future telescopes. The O spectral feature at
0.76m is increasingly difficult to detect in reflected light of later M
dwarfs due to low stellar flux in that wavelength region. NO, another
biosignature detectable in the IR, builds up to observable concentrations in
our planetary models around M dwarfs with low UV flux. CHCl could become
detectable, depending on the depth of the overlapping NO 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 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 - (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 0.01% of the total O
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 1% of the O column and those it
predicts occur yearly would photolyze the full O 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
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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
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