129 research outputs found
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
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
Spectral and Temporal Variability of Earth Observed in Polarization
We present a comprehensive set of spectropolarimetric observations of
Earthshine as obtained by FORS2 at the VLT for phase angles from 50degree to
135degree (Sun-Earth-Moon angle), covering a spectral range from 430nm to
920nm. The degree of polarization in BVRI passbands, the differential
polarization vegetation index, and the equivalent width of the O2A polarization
band around 760nm are determined with absolute errors around 0.1 percent in the
degree of polarization. Earthshine polarization spectra are corrected for the
effect of depolarization introduced by backscattering on the lunar surface,
introducing systematic errors of the order of 1 percent in the degree of
polarization. Distinct viewing sceneries such as observing the Atlantic or
Pacific side in Earthshine yield statistically different phase curves. The
equivalent width defined for the O2A band polarization is found to vary from
-5nm to +2nm. A differential polarized vegetation index is introduced and
reveals a larger vegetation signal for those viewing sceneries that contain
larger fractions of vegetated surface areas. We corroborate the observed
correlations with theoretical models from the literature, and conclude that the
Vegetation Red Edge(VRE) is a robust and sensitive signature in polarization
spectra of planet Earth. The overall behaviour of polarization of planet Earth
in the continuum and in the O2A band can be explained by existing models.
Biosignatures such as the O2A band and the VRE are detectable in Earthshine
polarization with a high degree of significance and sensitivity. An in-depth
understanding of Earthshines temporal and spectral variability requires
improved models of Earths biosphere, as a prerequisite to interpret possible
detections of polarised biosignatures in earthlike exoplanets in the future.Comment: 19 pages, 14 figures, 3 table
Traces of exomoons in computed flux and polarization phase curves of starlight reflected by exoplanets
Context: Detecting moons around exoplanets is a major goal of current and
future observatories. Moons are suspected to influence rocky exoplanet
habitability, and gaseous exoplanets in stellar habitable zones could harbour
abundant and diverse moons to target in the search for extraterrestrial
habitats. Exomoons will contribute to exoplanetary signals but are virtually
undetectable with current methods. Aims: We identify and analyse traces of
exomoons in the temporal variation of total and polarised fluxes of starlight
reflected by an Earth-like exoplanet and its spatially unresolved moon across
all phase angles, with both orbits viewed in an edge-on geometry. Methods: We
compute the total and linearly polarised fluxes, and the degree of linear
polarization P of starlight that is reflected by the exoplanet with its moon
along their orbits, accounting for the temporal variation of the visibility of
the planetary and lunar disks, and including effects of mutual transits and
mutual eclipses. Our computations pertain to a wavelength of 450 nm. Results:
Total flux F shows regular dips due to planetary and lunar transits and
eclipses. Polarization P shows regular peaks due to planetary transits and
lunar eclipses, and P can increase and/or slightly decrease during lunar
transits and planetary eclipses. Changes in F and P will depend on the radii of
the planet and moon, on their reflective properties, and their orbits, and are
about one magnitude smaller than the smooth background signals. The typical
duration of a transit or an eclipse is a few hours. Conclusions: Traces of an
exomoon due to planetary and lunar transits and eclipses show up in F and P of
sunlight reflected by planet-moon systems and could be searched for in
exoplanet flux and/or polarisation phase functions.Comment: Accepted for publication in Astronomy & Astrophysic
Chronic Thromboembolic Pulmonary Hypertension - What Have We Learned From Large Animal Models
Chronic thrombo-embolic pulmonary hypertension (CTEPH) develops in a subset of patients after acute pulmonary embolism. In CTEPH, pulmonary vascular resistance, which is initially elevated due to the obstructions in the larger pulmonary arteries, is further increased by pulmonary microvascular remodeling. The increased afterload of the right ventricle (RV) leads to RV dilation and hypertrophy. This RV remodeling predisposes to arrhythmogenesis and RV failure. Yet, mechanisms involved in pulmonary microvascular remodeling, processes underlying the RV structural and functional adaptability in CTEPH as well as determinants of the susceptibility to arrhythmias such as atrial fibrillation in the context of CTEPH remain incompletely understood. Several large animal models with critical clinical features of human CTEPH and subsequent RV remodeling have relatively recently been developed in swine, sheep, and dogs. In this review we will discuss the current knowledge on the processes underlying development and progression of CTEPH, and on how animal models can help enlarge understanding of these processes
Modelling reflected polarised light from close-in giant exoplanet WASP-96b using PolHEx (Polarisation of Hot Exoplanets)
We present the Polarisation of Hot Exoplanets (PolHEx) code for modelling the
total flux (F) and degree of linear polarisation (P) of light spectra reflected
by close-in, tidally locked exoplanets. We use the output from a global climate
model (GCM) combined with a kinetic cloud model of hot Jupiter WASP-96b as a
base to investigate effects of atmospheric longitudinal-latitudinal
inhomogeneities on these spectra. We model F and P-spectra as functions of
wavelength and planet orbital phase for various model atmospheres. We find
different materials and sizes of cloud particles to impact the reflected flux
F, and particularly the linear polarisation state P. A range of materials are
used to form inhomogeneous mixed-material cloud particles (Al2O3, Fe2O3,
Fe2SiO4, FeO, Fe, Mg2SiO4, MgO, MgSiO3, SiO2, SiO, TiO2), with Fe2O3, Fe, and
FeO the most strongly absorbing species. The cloud particles near the
relatively cool morning terminator are expected to have smaller average sizes
and a narrower size distribution than those near the warmer evening terminator,
which leads to different reflected spectra at the respective orbital phases .We
also find differences in the spectra of F and P as functions of orbital phase
for irregularly or spherically shaped cloud particles. This work highlights the
importance of including polarisation in models and future observations of the
reflection spectra of exoplanets.Comment: Accepted for publication in MNRA
- …