251 research outputs found
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
Transits of Earth-Like Planets
Transmission spectroscopy of Earth-like exoplanets is a potential tool for
habitability screening. Transiting planets are present-day "Rosetta Stones" for
understanding extrasolar planets because they offer the possibility to
characterize giant planet atmospheres and should provide an access to
biomarkers in the atmospheres of Earth-like exoplanets, once they are detected.
Using the Earth itself as a proxy we show the potential and limits of the
transiting technique to detect biomarkers on an Earth-analog exoplanet in
transit. We quantify the Earths cross section as a function of wavelength, and
show the effect of each atmospheric species, aerosol, and Rayleigh scattering.
Clouds do not significantly affect this picture because the opacity of the
lower atmosphere from aerosol and Rayleigh losses dominates over cloud losses.
We calculate the optimum signal-to-noise ratio for spectral features in the
primary eclipse spectrum of an Earth-like exoplanet around a Sun-like star and
also M stars, for a 6.5-m telescope in space. We find that the signal to noise
values for all important spectral features are on the order of unity or less
per transit - except for the closest stars - making it difficult to detect such
features in one single transit, and implying that co-adding of many transits
will be essential.Comment: 17 pages, 3 figures, 6 tables, to appear in ApJ (accepted) V2:
corrected transit times, corrected values for M4 star radiu
Spectral Evolution of an Earth-Like Planet
We have developed a characterization of the geological evolution of the
Earths atmosphere and surface in order to model the observable spectra of an
Earth-like planet through its geological history. These calculations are
designed to guide the interpretation of an observed spectrum of such a planet
by future instruments that will characterize exoplanets. Our models focus on
spectral features that either imply habitability or are required for
habitability. These features are generated by H2O, CO2, CH4, O2, O3, N2O, and
vegetation-like surface albedos. We chose six geological epochs to
characterize. These epochs exhibit a wide range in abundance for these
molecules, ranging from a CO2 rich early atmosphere, to a CO2/CH4-rich
atmosphere around 2 billion years ago to a present-day atmosphere. We analyzed
the spectra to quantify the strength of each important spectral feature in both
the visible and thermal infrared spectral regions, and the resolutions required
to unambiguously observe the features for each epoch. We find a wide range of
spectral resolutions required for observing the different features. For
example, H2O and O3 can be observed with relatively low resolution, while O2
and N2O require higher resolution. We also find that the inclusion of clouds in
our models significantly affects both the strengths and resolutions required to
observe all spectral features.Comment: 34 pages, 24 fig, pdf, ApJ, TB
Cryptic photosynthesis, Extrasolar planetary oxygen without a surface biological signature
On the Earth, photosynthetic organisms are responsible for the production of
virtually all of the oxygen in the atmosphere. On the land, vegetation reflects
in the visible, leading to a red edge that developed about 450 Myr ago and has
been proposed as a biosignature for life on extrasolar planets. However, in
many regions of the Earth, and particularly where surface conditions are
extreme, for example in hot and cold deserts, photosynthetic organisms can be
driven into and under substrates where light is still sufficient for
photosynthesis. These communities exhibit no detectable surface spectral
signature to indicate life. The same is true of the assemblages of
photosynthetic organisms at more than a few metres depth in water bodies. These
communities are widespread and dominate local photosynthetic productivity. We
review known cryptic photosynthetic communities and their productivity. We link
geomicrobiology with observational astronomy by calculating the disk-averaged
spectra of cryptic habitats and identifying detectable features on an exoplanet
dominated by such a biota. The hypothetical cryptic photosynthesis worlds
discussed here are Earth-analogs that show detectable atmospheric biomarkers
like our own planet, but do not exhibit a discernable biological surface
feature in the disc-averaged spectrum.Comment: 23 pages, 2 figures, Astrobiology (TBP) - updated Table 1, typo in
detectable O2 correcte
Clouds in the atmospheres of extrasolar planets. II. Thermal emission spectra of Earth-like planets influenced by low and high-level clouds
We study the impact of multi-layered clouds (low-level water and high-level
ice clouds) on the thermal emission spectra of Earth-like planets orbiting
different types of stars. Clouds have an important influence on such planetary
emission spectra due to their wavelength dependent absorption and scattering
properties. We also investigate the influence of clouds on the ability to
derive information about planetary surface temperatures from low-resolution
spectra.Comment: accepted for publication in A&
The influence of thermal evolution in the magnetic protection of terrestrial planets
Magnetic protection of potentially habitable planets plays a central role in determining their actual habitability and/or the chances of detecting atmospheric biosignatures. Here we develop a thermal evolution model of potentially habitable Earth-like planets and super-Earths (SEs). Using up-to-date dynamo-scaling laws, we predict the properties of core dynamo magnetic fields and study the influence of thermal evolution on their properties. The level of magnetic protection of tidally locked and unlocked planets is estimated by combining simplified models of the planetary magnetosphere and a phenomenological description of the stellar wind. Thermal evolution introduces a strong dependence of magnetic protection on planetary mass and rotation rate. Tidally locked terrestrial planets with an Earth-like composition would have early dayside magnetopause distances between 1.5 and 4.0 Rp , larger than previously estimated. Unlocked planets with periods of rotation ~1 day are protected by magnetospheres extending between 3 and 8 Rp . Our results are robust in comparison with variations in planetary bulk composition and uncertainties in other critical model parameters. For illustration purposes, the thermal evolution and magnetic protection of the potentially habitable SEs GL 581d, GJ 667Cc, and HD 40307g were also studied. Assuming an Earth-like composition, we found that the dynamos of these planets are already extinct or close to being shut down. While GL 581d is the best protected, the protection of HD 40307g cannot be reliably estimated. GJ 667Cc, even under optimistic conditions, seems to be severely exposed to the stellar wind, and, under the conditions of our model, has probably suffered massive atmospheric losses
Greenhouse gas implications of mobilizing agricultural biomass for energy: a reassessment of global potentials in 2050 under different food-system pathways
Global bioenergy potentials have been the subject of extensive research and continued controversy. Due to vast uncertainties regarding future yields, diets and other influencing parameters, estimates of future agricultural biomass potentials vary widely. Most scenarios compatible with ambitious climate targets foresee a large expansion of bioenergy, mainly from energy crops that needs to be kept consistent with projections of agriculture and food production. Using the global biomass balance model BioBaM, we here present an assessment of agricultural bioenergy potentials compatible with the Food and Agriculture Organization's (2018) 'Alternative pathways to 2050' projections. Mobilizing biomass at larger scales may be associated with systemic feedbacks causing greenhouse gas (GHG) emissions, e.g. crop residue removal resulting in loss of soil carbon stocks and increased emissions from fertilization. To assess these effects, we derive 'GHG cost supply-curves', i.e. integrated representations of biomass potentials and their systemic GHG costs. Livestock manure is most favourable in terms of GHG costs, as anaerobic digestion yields reductions of GHG emissions from manure management. Global potentials from intensive livestock systems are about 5 EJ/yr. Crop residues can provide up to 20 EJ/yr at moderate GHG costs. For energy crops, we find that the medium range of literature estimates (~40 to 90 EJ/yr) is only compatible with FAO yield and human diet projections if energy plantations expand into grazing areas (~4–5 million km2) and grazing land is intensified globally. Direct carbon stock changes associated with perennial energy crops are beneficial for climate mitigation, yet there are—sometimes considerable—'opportunity GHG costs' if one accounts the foregone opportunity of afforestation. Our results indicate that the large potentials of energy crops foreseen in many energy scenarios are not freely and unconditionally available. Disregarding systemic effects in agriculture can result in misjudgement of GHG saving potentials and flawed climate mitigation strategies
A scene model of exosolar systems for use in planetary detection and characterisation simulations
Instrumental projects that will improve the direct optical finding and
characterisation of exoplanets have advanced sufficiently to trigger organized
investigation and development of corresponding signal processing algorithms.
The first step is the availability of field-of-view (FOV) models. These can
then be submitted to various instrumental models, which in turn produce
simulated data, enabling the testing of processing algorithms. We aim to set
the specifications of a physical model for typical FOVs of these instruments.
The dynamic in resolution and flux between the various sources present in
such a FOV imposes a multiscale, independent layer approach. From review of
current literature and through extrapolations from currently available data and
models, we derive the features of each source-type in the field of view likely
to pass the instrumental filter at exo-Earth level.
Stellar limb darkening is shown to cause bias in leakage calibration if
unaccounted for. Occurrence of perturbing background stars or galaxies in the
typical FOV is unlikely. We extract galactic interstellar medium background
emissions for current target lists. Galactic background can be considered
uniform over the FOV, and it should show no significant drift with parallax.
Our model specifications have been embedded into a Java simulator, soon to be
made open-source. We have also designed an associated FITS input/output format
standard that we present here.Comment: 9 pages (+5 of appendices), 7 figures, accepted for publication in
Astronomy & Astrophysic
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