589 research outputs found
Looking for the rainbow on exoplanets covered by liquid and icy water clouds
Looking for the primary rainbow in starlight that is reflected by exoplanets
appears to be a promising method to search for liquid water clouds in
exoplanetary atmospheres. Ice water clouds, that consist of water crystals
instead of water droplets, could potentially mask the rainbow feature in the
planetary signal by covering liquid water clouds. Here, we investigate the
strength of the rainbow feature for exoplanets that have liquid and icy water
clouds in their atmosphere, and calculate the rainbow feature for a realistic
cloud coverage of Earth. We calculate flux and polarization signals of
starlight that is reflected by horizontally and vertically inhomogeneous
Earth--like exoplanets, covered by patchy clouds consisting of liquid water
droplets or water ice crystals. The planetary surfaces are black. On a planet
with a significant coverage of liquid water clouds only, the total flux signal
shows a weak rainbow feature. Any coverage of the liquid water clouds by ice
clouds, however, dampens the rainbow feature in the total flux, and thus the
discovery of liquid water in the atmosphere. On the other hand, detecting the
primary rainbow in the polarization signal of exoplanets appears to be a
powerful tool for detecting liquid water in exoplanetary atmospheres, even when
these clouds are partially covered by ice clouds. In particular, liquid water
clouds covering as little as 10%-20% of the planetary surface, with more than
half of these covered by ice clouds, still create a polarized rainbow feature
in the planetary signal. Indeed, calculations of flux and polarization signals
of an exoplanet with a realistic Earth--like cloud coverage, show a strong
polarized rainbow feature.Comment: accepted for publication in Astronomy & Astrophysic
Symmetry and structure of reflection matrices of celestial bodies with particulate surfaces
The polarization of electromagnetic radiation reflected by a particulate surface is determined by a four by four reflection matrix. Symmetry relations are quite common for such reflection matrices. The reciprocity and mirror symmetry relations are combined to derive a third symmetry relation. These three relations are used to simplify reflection matrices for a variety of special directions of incident and reflected radiation. We show that some elements of the reflection matrix can vanish or equal other elements that have the same or opposite sign. Several applications of the results for studies of particulate surfaces and atmospheres above them are pointed out.The work of Olga Munoz has been supported by the Plan Nacional de Astronomia y Astrofisica contract AYA2015-67152-R.Peer reviewe
The composition and size distribution of the dust in the coma of comet Hale-Bopp
We discuss the composition and size distribution of the dust in the coma of
comet Hale-Bopp. We do this by fitting simultaneously the infrared emission
spectrum measured by the infrared space observatory (ISO) and the measured
degree of linear polarization of scattered light at various phase angles and 12
different wavelengths. The effects of particle shape on the modeled optical
properties of the dust grains are taken into account. We constrain our fit by
forcing the abundances of the major rock forming chemical elements to be solar.
The infrared spectrum at long wavelengths reveals that large grains are needed
in order to fit the spectral slope. The size and shape distribution we employ
allows us to estimate the sizes of the crystalline silicates. The ratios of the
strength of various forsterite features show that the crystalline silicate
grains in Hale-Bopp must be submicron sized. We exclude the presence of large
crystalline silicate grains in the coma. Because of this lack of large
crystalline grains combined with the fact that we do need large amorphous
grains to fit the emission spectrum at long wavelengths, we need only
approximately 4% of crystalline silicates by mass. After correcting for
possible hidden crystalline material included in large amorphous grains, our
best estimate of the total mass fraction of crystalline material is
approximately 7.5%, significantly lower than deduced in previous studies in
which the typical derived crystallinity is 20-30%. The implications of this on
the possible origin and evolution of the comet are discussed. The crystallinity
we observe in Hale-Bopp is consistent with the production of crystalline
silicates in the inner solar system by thermal annealing and subsequent radial
mixing to the comet forming region.Comment: Accepted for publication in Icaru
Experimental phase functions of mm-sized cosmic dust grains
We present experimental phase functions of three types of millimeter-sized
dust grains consisting of enstatite, quartz and volcanic material from Mount
Etna, respectively. The three grains present similar sizes but different
absorbing properties. The measurements are performed at 527 nm covering the
scattering angle range from 3 to 170 degrees. The measured phase functions show
two well defined regions i) soft forward peaks and ii) a continuous increase
with the scattering angle at side- and back-scattering regions. This behavior
at side- and back-scattering regions are in agreement with the observed phase
functions for the Fomalhaut and HR 4796A dust rings. Further computations and
measurements (including polarization) for millimeter sized-grains are needed to
draw some conclusions about the fluffy or compact structure of the dust grains
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