816 research outputs found
Analytic Scattering and Refraction Models for Exoplanet Transit Spectra
Observations of exoplanet transit spectra are essential to understanding the
physics and chemistry of distant worlds. The effects of opacity sources and
many physical processes combine to set the shape of a transit spectrum. Two
such key processes - refraction and cloud and/or haze forward scattering - have
seen substantial recent study. However, models of these processes are typically
complex, which prevents their incorporation into observational analyses and
standard transit spectrum tools. In this work, we develop analytic expressions
that allow for the efficient parameterization of forward scattering and
refraction effects in transit spectra. We derive an effective slant optical
depth that includes a correction for forward scattered light, and present an
analytic form of this correction. We validate our correction against a
full-physics transit spectrum model that includes scattering, and we explore
the extent to which the omission of forward scattering effects may bias models.
Also, we verify a common analytic expression for the location of a refractive
boundary, which we express in terms of the maximum pressure probed in a transit
spectrum. This expression is designed to be easily incorporated into existing
tools, and we discuss how the detection of a refractive boundary could help
indicate the background atmospheric composition by constraining the bulk
refractivity of the atmosphere. Finally, we show that opacity from Rayleigh
scattering and collision induced absorption will outweigh the effects of
refraction for Jupiter-like atmospheres whose equilibrium temperatures are
above 400-500 K.Comment: ApJ accepted; submitted Feb. 7, 201
Exploring and Validating Exoplanet Atmospheric Retrievals with Solar System Analog Observations
Solar System observations that serve as analogs for exoplanet remote sensing
data can provide important opportunities to validate ideas and models related
to exoplanet environments. Critically, and unlike true exoplanet observations,
Solar System analog data benefit from available high-quality ground- or
orbiter-derived "truth" constraints that enable strong validations of exoplanet
data interpretation tools. In this work, we first present a versatile
atmospheric retrieval suite, capable of application to reflected light, thermal
emission, and transmission observations. The tool -- dubbed rfast -- is
designed, in part, to enable exoplanet mission concept feasibility studies.
Following model validation, the retrieval tool is applied to a range of Solar
System analog observations for exoplanet environments. Retrieval studies using
Earth reflected light observations from NASA's EPOXI mission provide a key
proof-of-concept for under-development exo-Earth direct imaging concept
missions. Inverse modeling applied to an infrared spectrum of Earth from the
Mars Global Surveyor Thermal Emission Spectrometer achieves good constraints on
atmospheric gases, including many biosignature gases. Finally, retrieval
analysis applied to a transit spectrum of Titan derived from the Cassini Visual
and Infrared Mapping Spectrometer provides a proof-of-concept for interpreting
more feature-rich transiting exoplanet observations from NASA's James Webb
Space Telescope (JWST). In the future, Solar System analog observations for
exoplanets could be used to verify exoplanet models and parameterizations, and
future exoplanet analog observations of any Solar System worlds from planetary
science missions should be encouraged.Comment: submitted to PSJ; community comments and feedback welcome
Scattering Transparency of Clouds in Exoplanet Transit Spectra
The presence of aerosols in an exoplanet atmosphere can veil the underlying
material and can lead to a flat transmission spectrum during primary transit
observations. In this work, we explore forward scattering effects from
super-micron sized aerosol particles present in the atmosphere of a transiting
exoplanet. We find that the impacts of forward scattering from larger aerosols
can significantly impact exoplanet transits and the strength of these effects
can be dependent on wavelength. In certain cloud configurations, the
forward-scattered light can effectively pass through the clouds unhindered,
thus rendering the clouds transparent. The dependence of the aerosol scattering
properties on wavelength can then lead to a positive slope in the transit
spectrum. These slopes are characteristically different from both Rayleigh and
aerosol absorption slopes. As examples, we demonstrate scattering effects for
both a rocky world and a hot Jupiter. In these models, the predicted spectral
slopes due to forward scattering effects can manifest in the transit spectrum
at the level of 10s to 100s of parts per million and, hence, could
be observable with NASA's James Webb Space Telescope.Comment: 9 pages, 7 figures, published in MNRA
Titan solar occultation observations reveal transit spectra of a hazy world
High altitude clouds and hazes are integral to understanding exoplanet
observations, and are proposed to explain observed featureless transit spectra.
However, it is difficult to make inferences from these data because of the need
to disentangle effects of gas absorption from haze extinction. Here, we turn to
the quintessential hazy world -- Titan -- to clarify how high altitude hazes
influence transit spectra. We use solar occultation observations of Titan's
atmosphere from the Visual and Infrared Mapping Spectrometer (VIMS) aboard
NASA's Cassini spacecraft to generate transit spectra. Data span 0.88-5 microns
at a resolution of 12-18 nm, with uncertainties typically smaller than 1%. Our
approach exploits symmetry between occultations and transits, producing transit
radius spectra that inherently include the effects of haze multiple scattering,
refraction, and gas absorption. We use a simple model of haze extinction to
explore how Titan's haze affects its transit spectrum. Our spectra show strong
methane absorption features, and weaker features due to other gases. Most
importantly, the data demonstrate that high altitude hazes can severely limit
the atmospheric depths probed by transit spectra, bounding observations to
pressures smaller than 0.1-10 mbar, depending on wavelength. Unlike the usual
assumption made when modeling and interpreting transit observations of
potentially hazy worlds, the slope set by haze in our spectra is not flat, and
creates a variation in transit height whose magnitude is comparable to those
from the strongest gaseous absorption features. These findings have important
consequences for interpreting future exoplanet observations, including those
from NASA's James Webb Space Telescope.Comment: Updated journal reference; data available via
http://sites.google.com/site/tdrobinsonscience/science/tita
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