4,919 research outputs found
Spectropolarimetric signatures of Earth-like extrasolar planets
We present results of numerical simulations of the flux (irradiance), F, and
the degree of polarization (i.e. the ratio of polarized to total flux), P, of
light that is reflected by Earth-like extrasolar planets orbiting solar-type
stars, as functions of the wavelength (from 0.3 to 1.0 micron, with 0.001
micron spectral resolution) and as functions of the planetary phase angle. We
use different surface coverages for our model planets, including vegetation and
a Fresnel reflecting ocean, and clear and cloudy atmospheres. Our
adding-doubling radiative transfer algorithm, which fully includes multiple
scattering and polarization, handles horizontally homogeneous planets only; we
simulate fluxes and polarization of horizontally inhomogeneous planets by
weighting results for homogeneous planets. Like the flux, F, the degree of
polarization, P, of the reflected starlight is shown to depend strongly on the
phase angle, on the composition and structure of the planetary atmosphere, on
the reflective properties of the underlying surface, and on the wavelength, in
particular in wavelength regions with gaseous absorption bands. The sensitivity
of P to a planet's physical properties appears to be different than that of F.
Combining flux with polarization observations thus makes for a strong tool for
characterizing extrasolar planets. The calculated total and polarized fluxes
will be made available through the CDS.Comment: 31 pages text, 17 figures, 1 table Submitted to A&
AGRICULTURE IN NORTHWEST MINNESOTA: ITS ROLE IN THE FUTURE OF THE REGION
Community/Rural/Urban Development,
Modeled flux and polarisation signals of horizontally inhomogeneous exoplanets, applied to Earth--like planets
We present modeled flux and linear polarisation signals of starlight that is
reflected by spatially unresolved, horizontally inhomogeneous planets and
discuss the effects of including horizontal inhomogeneities on the flux and
polarisation signals of Earth-like exoplanets. Our code is based on an
efficient adding--doubling algorithm, which fully includes multiple scattering
by gases and aerosol/cloud particles. We divide a model planet into pixels that
are small enough for the local properties of the atmosphere and surface (if
present) to be horizontally homogeneous. Given a planetary phase angle, we sum
up the reflected total and linearly polarised fluxes across the illuminated and
visible part of the planetary disk, taking care to properly rotate the
polarized flux vectors towards the same reference plane. We compared flux and
polarisation signals of simple horizontally inhomogeneous model planets against
results of the weighted sum approximation, in which signals of horizontally
homogeneous planets are combined. Apart from cases in which the planet has only
a minor inhomogeneity, the signals differ significantly. In particular, the
shape of the polarisation phase function appears to be sensitive to the
horizontal inhomogeneities. The same holds true for Earth-like model planets
with patchy clouds above an ocean and a sandy continent. Our simulations
clearly show that horizontal inhomogeneities leave different traces in flux and
polarisation signals. Combining flux with polarisation measurements would help
retrieving the atmospheric and surface patterns on a planet
Circular polarization signals of cloudy (exo)planets
The circular polarization of light that planets reflect is often neglected
because it is very small compared to the linear polarization. It could,
however, provide information on a planet's atmosphere and surface, and on the
presence of life, because homochiral molecules that are the building blocks of
life on Earth are known to reflect circularly polarized light.
We compute , the degree of circular polarization, for light that is
reflected by rocky (exo)planets with liquid water or sulfuric acid solution
clouds, both spatially resolved across the planetary disk and, for planets with
patchy clouds, integrated across the planetary disk, for various planetary
phase angles .
The optical thickness and vertical distribution of the atmospheric gas and
clouds, the size parameter and refractive index of the cloud particles, and
all influence . Spatially resolved, varies between (the sign indicates the polarization direction). Only for small gas
optical thicknesses above the clouds do significant sign changes (related to
cloud particle properties) across the planets' hemispheres occur. For patchy
clouds, the disk--integrated is typically smaller than ,
with maximums for between and , and
to . As expected, the disk--integrated is virtually zero at
and 180. The disk--integrated is also very small
at .
Measuring circular polarization signals appears to be challenging with
current technology. The small atmospheric circular polarization signal could,
however, allow the detection of circular polarization due to homochiral
molecules. Confirmation of the detectability of such signals requires better
knowledge of the strength of circular polarization signals of biological
sources.Comment: 15 pages, 11 figures, Accepted for publication in Astronomy and
Astrophysic
The O2 A-band in fluxes and polarization of starlight reflected by Earth-like exoplanets
Earth-like, potentially habitable exoplanets are prime targets in the search
for extraterrestrial life. Information about their atmosphere and surface can
be derived by analyzing light of the parent star reflected by the planet. We
investigate the influence of the surface albedo , the optical
thickness and altitude of water clouds, and the mixing ratio
of biosignature O on the strength of the O A-band (around 760
nm) in flux and polarization spectra of starlight reflected by Earth-like
exoplanets. Our computations for horizontally homogeneous planets show that
small mixing ratios ( < 0.4) will yield moderately deep bands in flux and
moderate to small band strengths in polarization, and that clouds will usually
decrease the band depth in flux and the band strength in polarization. However,
cloud influence will be strongly dependent on their properties such as optical
thickness, top altitude, particle phase, coverage fraction, horizontal
distribution. Depending on the surface albedo, and cloud properties, different
O mixing ratios can give similar absorption band depths in flux and
band strengths in polarization, in particular if the clouds have moderate to
high optical thicknesses. Measuring both the flux and the polarization is
essential to reduce the degeneracies, although it will not solve them, in
particular not for horizontally inhomogeneous planets. Observations at a wide
range of phase angles and with a high temporal resolution could help to derive
cloud properties and, once those are known, the mixing ratio of O or any
other absorbing gas.Comment: 21 pages, 20 figures, accepted for publication in Ap
Blue, white, and red ocean planets - Simulations of orbital variations in flux and polarization colors
An exoplanet's habitability will depend strongly on the presence of liquid
water. Flux and/or polarization measurements of starlight that is reflected by
exoplanets could help to identify exo-oceans. We investigate which broadband
spectral features in flux and polarization phase functions of reflected
starlight uniquely identify exo-oceans. We compute total fluxes F and polarized
fluxes Q of starlight reflected by cloud-free and (partly) cloudy exoplanets,
for wavelengths from 350 to 865 nm. The ocean surface has waves composed of
Fresnel reflecting wave facets and whitecaps, and scattering within the water
body is included. Total flux F, polarized flux Q, and degree of polarization P
of ocean planets change color from blue, through white, to red at phase angles
alpha ranging from 134-108 deg for F, and from 123-157 deg for Q, with cloud
coverage fraction fc increasing from 0.0 to 1.0 for F, and to 0.98 for Q. The
color change in P only occurs for fc ranging from 0.03-0.98, with the color
crossing angle alpha ranging from 88-161 deg. The total flux F of a cloudy,
zero surface albedo planet can also change color, and for fc=0.0, an ocean
planet's F will not change color for surface pressures ps > 8 bars. Polarized
flux Q of a zero surface albedo planet does not change color for any fc. The
color change of P of starlight reflected by an exoplanet, from blue, through
white, to red with increasing alpha above 88 deg, appears to identify a
(partly) cloudy exo-ocean. The color change of polarized flux Q with increasing
alpha above 123 deg appears to uniquely identify an exo-ocean, independent of
surface pressure or cloud fraction. At the color changing phase angle, the
angular distance between a star and its planet is much larger than at the phase
angle where the glint appears in reflected light. The color change in
polarization thus offers better prospects for detecting exo-oceans.Comment: Accepted for publication in Astron. Astrophys; multicolumn versio
The influence of forward-scattered light in transmission measurements of (exo)planetary atmospheres
[Abridged] The transmission of light through a planetary atmosphere can be
studied as a function of altitude and wavelength using stellar or solar
occultations, giving often unique constraints on the atmospheric composition.
For exoplanets, a transit yields a limb-integrated, wavelength-dependent
transmission spectrum of an atmosphere. When scattering haze and/or cloud
particles are present in the planetary atmosphere, the amount of transmitted
flux not only depends on the total optical thickness of the slant light path
that is probed, but also on the amount of forward-scattering by the scattering
particles. Here, we present results of calculations with a three-dimensional
Monte Carlo code that simulates the transmitted flux during occultations or
transits. For isotropically scattering particles, like gas molecules, the
transmitted flux appears to be well-described by the total atmospheric optical
thickness. Strongly forward-scattering particles, however, such as commonly
found in atmospheres of Solar System planets, can increase the transmitted flux
significantly. For exoplanets, such added flux can decrease the apparent radius
of the planet by several scale heights, which is comparable to predicted and
measured features in exoplanet transit spectra. We performed detailed
calculations for Titan's atmosphere between 2.0 and 2.8 micron and show that
haze and gas abundances will be underestimated by about 8% if
forward-scattering is ignored in the retrievals. At shorter wavelengths, errors
in the gas and haze abundances and in the spectral slope of the haze particles
can be several tens of percent, also for other Solar System planetary
atmospheres. We also find that the contribution of forward-scattering can be
fairly well described by modelling the atmosphere as a plane-parallel slab.Comment: Icarus, accepted for publicatio
PyMieDAP: a Python--Fortran tool to compute fluxes and polarization signals of (exo)planets
PyMieDAP (the Python Mie Doubling-Adding Programme) is a Python--based tool
for computing the total, linearly, and circularly polarized fluxes of incident
unpolarized sun- or starlight that is reflected by, respectively, Solar System
planets or moons, or exoplanets at a range of wavelengths. The radiative
transfer computations are based on an adding--doubling Fortran algorithm and
fully include polarization for all orders of scattering. The model (exo)planets
are described by a model atmosphere composed of a stack of homogeneous layers
containing gas and/or aerosol and/or cloud particles bounded below by an
isotropically, depolarizing surface (that is optionally black). The reflected
light can be computed spatially--resolved and/or disk--integrated.
Spatially--resolved signals are mostly representative for observations of Solar
System planets (or moons), while disk--integrated signals are mostly
representative for exoplanet observations. PyMieDAP is modular and flexible,
and allows users to adapt and optimize the code according to their needs.
PyMieDAP keeps options open for connections with external programs and for
future additions and extensions. In this paper, we describe the radiative
transfer algorithm that PyMieDAP is based on and the code's principal
functionalities. And we provide benchmark results of PyMieDAP that can be used
for testing its installation and for comparison with other codes. PyMieDAP is
available online under the GNU GPL license at
http://gitlab.com/loic.cg.rossi/pymiedapComment: 15 pages, 7 figures, 4 tables. Accepted for publication in Astronomy
and Astrophysic
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