133 research outputs found
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
Utilisation de la grille pour la simulation de température de brillance dans une atmosphère nuageuse composée de cirrus
Utilisation de la grille pour la simulation de température de brillance dans une atmosphère nuageuse composée de cirru
Scale dependence of cirrus heterogeneity effects. Part II: MODIS NIR and SWIR channels
In a context of global climate change, the understanding of the radiative
role of clouds is crucial. On average, ice clouds such as cirrus have a
significant positive radiative effect, but under some conditions the effect
may be negative. However, many uncertainties remain regarding the role of ice
clouds on Earth's radiative budget and in a changing climate. Global
satellite observations are particularly well suited to monitoring clouds,
retrieving their characteristics and inferring their radiative impact. To
retrieve ice cloud properties (optical thickness and ice crystal effective
size), current operational algorithms assume that each pixel of the observed
scene is plane-parallel and homogeneous, and that there is no radiative
connection between neighboring pixels. Yet these retrieval assumptions are
far from accurate, as real radiative transfer is 3-D. This leads to the
plane-parallel and homogeneous bias (PPHB) plus the independent pixel
approximation bias (IPAB), which impacts both the estimation of
top-of-the-atmosphere (TOA) radiation and the retrievals. An important factor
that determines the impact of these assumptions is the sensor spatial
resolution. High-spatial-resolution pixels can better represent cloud
variability (low PPHB), but the radiative path through the cloud can involve
many pixels (high IPAB). In contrast, low-spatial-resolution pixels poorly
represent the cloud variability (high PPHB), but the radiation is better
contained within the pixel field of view (low IPAB). In addition, the solar
and viewing geometry (as well as cloud optical properties) can modulate the
magnitude of the PPHB and IPAB. In this, Part II of our study, we simulate
TOA 0.86 and 2.13 µm solar reflectances over a cirrus uncinus
scene produced by the 3DCLOUD model. Then, 3-D
radiative transfer simulations are performed with the 3DMCPOL
code at spatial resolutions ranging from 50 m to 10 km, for 12
viewing geometries and nine solar geometries. It is found that, for simulated
nadir observations taken at resolution higher than 2.5 km, horizontal
radiation transport (HRT) dominates biases between 3-D and 1-D reflectance
calculations, but these biases are mitigated by the side illumination and
shadowing effects for off-zenith solar geometries. At resolutions coarser
than 2.5 km, PPHB dominates. For off-nadir observations at resolutions
higher than 2.5 km, the effect that we call THEAB (tilted and homogeneous
extinction approximation bias) due to the oblique line of sight passing
through many cloud columns contributes to a large increase of the
reflectances, but 3-D radiative effects such as shadowing and side
illumination for oblique Sun are also important. At resolutions coarser than
2.5 km, the PPHB is again the dominant effect. The magnitude and resolution
dependence of PPHB and IPAB is very different for visible, near-infrared and
shortwave infrared channels compared with the thermal infrared channels
discussed in Part I of this study. The contrast of 3-D radiative effects
between solar and thermal infrared channels may be a significant issue for
retrieval techniques that simultaneously use radiative measurements across a
wide range of solar reflectance and infrared wavelengths.</p
The Prospect of Detecting Volcanic Signatures on an ExoEarth Using Direct Imaging
The James Webb Space Telescope (JWST) has provided the first opportunity to
study the atmospheres of terrestrial exoplanets and estimate their surface
conditions. Earth-sized planets around Sun-like stars are currently
inaccessible with JWST however, and will have to be observed using the next
generation of telescopes with direct imaging capabilities. Detecting active
volcanism on an Earth-like planet would be particularly valuable as it would
provide insight into its interior, and provide context for the commonality of
the interior states of Earth and Venus. In this work we used a climate model to
simulate four exoEarths over eight years with ongoing large igneous province
eruptions with outputs ranging from 1.8-60 Gt of sulfur dioxide. The
atmospheric data from the simulations were used to model direct imaging
observations between 0.2-2.0 m, producing reflectance spectra for every
month of each exoEarth simulation. We calculated the amount of observation time
required to detect each of the major absorption features in the spectra, and
identified the most prominent effects that volcanism had on the reflectance
spectra. These effects include changes in the size of the O, O, and
HO absorption features, and changes in the slope of the spectrum. Of these
changes, we conclude that the most detectable and least ambiguous evidence of
volcanism are changes in both O absorption and the slope of the spectrum.Comment: 13 pages, 5 figures, 4 tables, Accepted for publication in AJ
(September 26, 2023
Sensitive Probing of Exoplanetary Oxygen via Mid Infrared Collisional Absorption
The collision-induced fundamental vibration-rotation band at 6.4 um is the
most significant absorption feature from O2 in the infrared (Timofeyev and
Tonkov, 1978; Rinslandet al., 1982, 1989), yet it has not been previously
incorporated into exoplanet spectral analyses for several reasons. Either CIAs
were not included or incomplete/obsolete CIA databases were used. Also, the
current version of HITRAN does not include CIAs at 6.4 um with other collision
partners (O2-X). We include O2-X CIA features in our transmission spectroscopy
simulations by parameterizing the 6.4 um O2-N2 CIA based on Rinsland et
al.(1989) and the O2-CO2 CIA based on Baranov et al. (2004). Here we report
that the O2-X CIA may be the most detectable O2 feature for transit
observations. For a potentialTRAPPIST-1e analogue system within 5 pc of the
Sun, it could be the only O2 detectable signature with JWST (using MIRI LRS)
for a modern Earth-like cloudy atmosphere with biological quantities of O2.
Also, we show that the 6.4 um O2-X CIA would be prominent for O2-rich
desiccated atmospheres (Luger and Barnes, 2015) and could be detectable with
JWST in just a few transits. For systems beyond 5 pc, this feature could
therefore be a powerful discriminator of uninhabited planets with
non-biological "false positive" O2 in their atmospheres - as they would only be
detectable at those higher O2 pressures.Comment: Published in Nature Astronom
Evaluating the Plausible Range of N2O Biosignatures on Exo-Earths: An Integrated Biogeochemical, Photochemical, and Spectral Modeling Approach
Nitrous oxide (N2O) -- a product of microbial nitrogen metabolism -- is a
compelling exoplanet biosignature gas with distinctive spectral features in the
near- and mid-infrared, and only minor abiotic sources on Earth. Previous
investigations of N2O as a biosignature have examined scenarios using Earthlike
N2O mixing ratios or surface fluxes, or those inferred from Earth's geologic
record. However, biological fluxes of N2O could be substantially higher, due to
a lack of metal catalysts or if the last step of the denitrification metabolism
that yields N2 from N2O had never evolved. Here, we use a global biogeochemical
model coupled with photochemical and spectral models to systematically quantify
the limits of plausible N2O abundances and spectral detectability for Earth
analogs orbiting main-sequence (FGKM) stars. We examine N2O buildup over a
range of oxygen conditions (1%-100% present atmospheric level) and N2O fluxes
(0.01-100 teramole per year; Tmol = 10^12 mole) that are compatible with
Earth's history. We find that N2O fluxes of 10 [100] Tmol yr would lead
to maximum N2O abundances of ~5 [50] ppm for Earth-Sun analogs, 90 [1600] ppm
for Earths around late K dwarfs, and 30 [300] ppm for an Earthlike TRAPPIST-1e.
We simulate emission and transmission spectra for intermediate and maximum N2O
concentrations that are relevant to current and future space-based telescopes.
We calculate the detectability of N2O spectral features for high-flux scenarios
for TRAPPIST-1e with JWST. We review potential false positives, including
chemodenitrification and abiotic production via stellar activity, and identify
key spectral and contextual discriminants to confirm or refute the biogenicity
of the observed N2O.Comment: 22 pages, 17 figures; ApJ, 937, 10
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