239 research outputs found
Theoretical Reflectance Spectra of Earth-Like Planets through Their Evolutions: Impact of Clouds on the Detectability of Oxygen, Water, and Methane with Future Direct Imaging Missions
In the near-future, atmospheric characterization of Earth-like planets in the
habitable zone will become possible via reflectance spectroscopy with future
telescopes such as the proposed LUVOIR and HabEx missions. While previous
studies have considered the effect of clouds on the reflectance spectra of
Earth-like planets, the molecular detectability considering a wide range of
cloud properties has not been previously explored in detail. In this study, we
explore the effect of cloud altitude and coverage on the reflectance spectra of
Earth-like planets at different geological epochs and examine the detectability
of , , and with test parameters
for the future mission concept, LUVOIR, using a coronagraph noise simulator
previously designed for WFIRST-AFTA. Considering an Earth-like planet located
at 5 pc away, we have found that for the proposed LUVOIR telescope, the
detection of the A-band feature (0.76 m) will take
approximately 100, 30, and 10 hours for the majority of the cloud parameter
space modeled for the atmospheres with 10\%, 50\%, and 100\% of modern Earth
O abundances, respectively. Especially, for {the case of \%} of
modern Earth O abundance, the feature will be detectable with integration
time hours as long as there are lower altitude ( km)
clouds with a global coverage of . For the 1\% of modern Earth
abundance case, however, it will take more than 100 hours for
all the cloud parameters we modeled.Comment: 16 pages, 10 figures, accepted for publication in A
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
Prebiosignature Molecules Can Be Detected in Temperate Exoplanet Atmospheres with JWST
The search for biosignatures on exoplanets connects the fields of biology and
biochemistry to astronomical observation, with the hope that we might detect
evidence of active biological processes on worlds outside the solar system.
Here we focus on a complementary aspect of exoplanet characterisation
connecting astronomy to prebiotic chemistry: the search for molecules
associated with the origin of life, prebiosignatures. Prebiosignature surveys
in planetary atmospheres offer the potential to both constrain the ubiquity of
life in the galaxy and provide important tests of current prebiotic syntheses
outside of the laboratory setting. Here, we quantify the minimum abundance of
identified prebiosignature molecules that would be required for detection by
transmission spectroscopy using JWST. We consider prebiosignatures on five
classes of terrestrial planet: an ocean planet, a volcanic planet, a
post-impact planet, a super-Earth, and an early Earth analogue. Using a novel
modelling and detection test pipeline, with simulated JWST noise, we find the
detection thresholds of hydrogen cyanide (HCN), hydrogen sulfide (H2S),
cyanoacetylene (HC3N), ammonia (NH3), methane (CH4), acetylene (C2H2), sulfur
dioxide (SO2), nitric oxide (NO), formaldehyde (CH2O), and carbon monoxide (CO)
in a variety of low mean molecular weight (<5) atmospheres. We test the
dependence of these detection thresholds on M dwarf target star and the number
of observed transits, finding that a modest number of transits (1-10) are
required to detect prebiosignatures in numerous candidate planets, including
TRAPPIST-1e with a high mean molecular weight atmosphere. We find that the
NIRSpec G395M/H instrument is best suited for detecting most prebiosignatures.Comment: 28 pages, 12 figures, accepted for publication in A
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Hues of Habitability: Characterizing Pale Blue Dots Around Other Stars
A wide range of potentially rocky transiting planets in the habitable zone (HZ) have been detected by Kepler as well as ground-based searches. The spectral type of the host star will influence our ability to detect atmospheric features with future space and ground based missions like JWST, GMT and E-ELT. For my thesis, I present a complete suit of stellar models with a stellar effective temperature ranging from Teff = 2300K to Teff = 7000K, sampling the entire FGKM stellar type range, for modeling extrasolar planets. I also have a grid of model atmospheres for an Earth-analogue planet orbiting stars and derive remotely detectable spectral atmospheric features.
The UV emission from a planet's host star dominates the photochemistry and thus the resultant observable spectral features. Using the latest UV spectra obtained by Hubble as well as IUE, I model Earth-like planets for a wide range of host stars. I detail the results of activity on the primary detectable atmospheric features that indicate habitability on Earth, namely: H2O, O3, CH4, N2O and CH3Cl. I model the emergent spectra of Earth-analogue planets orbiting our grid of FGKM stars in the VIS/NIR (0.4 - 4 microns) and the IR (5 - 20 microns) range in accordance with future mission design concepts like JWST and direct detection missions like HDST/LUVOIR in the more distant horizon. We also model the amount of UV flux reaching the surface of Earth-like planets at various geological epochs ranging from a pre-biotic world through the rise of oxygen and for Earth-like planets orbiting FGKM stars at equivalent stages of evolution.Astronom
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
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