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

Abstract

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.

Simultaneous optical and near-infrared linear spectropolarimetry of the earthshine

“…All optical data display a qualitatively similar pattern (previously discussed), but they differ quantitatively in the amount of polarization per wavelength and the spectral slope. The spectral slope of Sterzik et al (2012) and Bazzon et al (2013) data is steeper than the ALFOSC spectrum, while our measurements and those of Takahashi et al (2013, see their Fig. 3) display related declivity. These differences may be understood in terms of distinct lunar areas explored by the various groups and different observing dates.…”

“…We used observations of the Moon's dark side to characterize the integrated polarimetric properties of the Earth. These observations extend our current understanding of the optical linear polarimetric data of the earthshine (Bazzon et al 2013, and references therein) toward red wavelengths. A&A 562, L5 (2014) …”

“…Various groups (Dollfus 1957;Wolstencroft & Breon 2005;Sterzik et al 2012;Takahashi et al 2013;Bazzon et al 2013) have reported a similar property. In our data, we observe that the linear polarization intensity remains nearly constant at about 5.7% between ∼0.8 μm and 0.9 μm.…”

“…The extrapolation of Eq. (9) by Bazzon et al (2013) toward the NIR yields corrections of ∼×2.2− × 3.3 for the wavelength interval 0.9−2.3 μm, implying that the true linear polarization intensity of the Earth may be ∼9−12% for the NIR continuum, and ∼12−36% at the peak of the O 2 (1.25 μm) and H 2 O (1.12 μm) bands. The extrapolation of Dollfus (1957) depolarization wavelength dependency toward the NIR would yield even higher true polarization values by a factor of ∼4.…”

Bio-signatures of Planet Earth from Spectropolarimetry

“…For a given spectral resolution, the amplitude of these features are sensitive to the top of the cloud layer, and to the O 2 mixing ratio. Bazzon et al (2013) measured the fractional polarisation of Earthshine P es in the BV RI filters for Earth phase-angles α between 30 and 110 degree with a new, specially designed wide field polarimeter. Their data were taken in March and October 2011.…”