Detecting the glint of starlight on the oceans of distant planets

Williams, D. M.; Gaidos, Eric

doi:10.1016/j.icarus.2008.01.002

Cited by 150 publications

(146 citation statements)

References 28 publications

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“…Detailed calculations for planets with realistically inhomogeneous surfaces and inhomogeneous cloud decks, preferably including the variations of the cloud deck in time, would help to study the effects of the surface reflection. With an ocean surface with waves, one could also expect the glint of starlight to contribute to the reflected flux and polarization (Williams & Gaidos 2008). How often this glint would be visible through broken clouds and its effect on the rainbow of starlight that is scattered by cloud particles, when it is indeed visible, will be subject to further studies.…”

Section: Discussionmentioning

confidence: 99%

Looking for the rainbow on exoplanets covered by liquid and icy water clouds

Karalidi

Stam

Hovenier

2012

A&A

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Aims. 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. Methods. 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. Results. 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.

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Section: Discussionmentioning

confidence: 99%

Looking for the rainbow on exoplanets covered by liquid and icy water clouds

Karalidi

Stam

Hovenier

2012

A&A

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“…The (temporarily) ultimate step will be the direct imaging of surface features (oceans, continents). In this configuration one can search for the direct detection of the ocean's glint (Williams and Gaidos 2008). This approach is particularly interesting for imaging forests and savannahs in order to investigate at a moderate spectral resolution the equivalent of the "red edge" of terrestrial vegetation at 725 nm.…”

Section: Direct Imaging Approachmentioning

confidence: 99%

The Far Future of Exoplanet Direct Characterization

et al. 2010

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“…Model calculations have been made for the fractional polarization of the reflected light from Earth-like planets (Stam 2008), as well as the polarization produced by reflecting clouds (e.g., Karalidi et al 2011Karalidi et al , 2012Bailey 2007) or glint from ocean water surfaces (Williams & Gaidos 2008). The models provide an adequate description of the dominating scattering processes and the signatures of different surface types.…”

Section: Introductionmentioning

confidence: 99%

Measurement of the earthshine polarization in theB, V, R, andIbands as function of phase

Bazzon

Schmid

Gisler

2013

A&A

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Context. Earth-like, extrasolar planets may soon become observable with upcoming high contrast polarimeters. Therefore, the characterization of the polarimetric properties of the planet Earth is important for interpreting expected observations and planning of future instruments. Aims. Benchmark values for the polarization signal of integrated light from the planet Earth in broad band filters are derived from new polarimetric observations of the earthshine backscattered from the Moon's dark side. Methods. The fractional polarization of the earthshine p es is measured in the B, V, R, and I filters for Earth-phase angles α between 30• and 110• with a new, specially designed wide field polarimeter. In the observations, the light from the bright lunar crescent is blocked with focal plane masks. Because the entire Moon is imaged, the earthshine observations can be corrected for the stray light from the bright lunar crescent and twilight. The phase dependence of p es is fitted by a function p es = q max sin 2 α. Depending on wavelength λ and the lunar surface albedo a, the polarization of the backscattered earthshine is significantly reduced. To determine the polarization of the planet Earth, we correct our earthshine measurements by a polarization efficiency function for the lunar surface (λ, a) derived from measurements of lunar samples from the literature. Results. The polarization of the earthshine decreases toward longer wavelengths and is about a factor 1.3 lower for the higher albedo highlands. For mare regions the measured maximum polarization is about q max,B = 13% for α = 90• (half moon) in the B band. The resulting fractional polarizations for the planet Earth derived from our earthshine measurements and corrected by (λ, a) are 24.6% for the B band, 19.1% for the V band, 13.5% for the R band, and 8.3% for the I band. Together with the literature values for the spectral reflectivity, we obtain a contrast C p between the polarized flux of the planet Earth and the (total) flux of the Sun with an uncertainty of less than 20%, and we find that the best phase for detecting an Earth twin is around α = 65• . Conclusions. The obtained results provide a multiwavelength and multiphase set of benchmark values that are useful for assessing different instrument and observing strategies for the future high contrast polarimetry of extrasolar planetary systems. Polarimetric models of Earth-like planets are in qualitative agreement with our results, but there are also significant differences that might guide more detailed computations.

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Detecting the glint of starlight on the oceans of distant planets

Cited by 150 publications

References 28 publications

Looking for the rainbow on exoplanets covered by liquid and icy water clouds

Looking for the rainbow on exoplanets covered by liquid and icy water clouds

The Far Future of Exoplanet Direct Characterization

Measurement of the earthshine polarization in theB, V, R, andIbands as function of phase

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