SPEX Science

Science case SPEX: general

Light is fully described by its (total) flux (also referred to as radiance) and its degree and direction of polarization. The degree of polarization is defined as the ratio of the polarized flux to the total (unpolarized plus polarized) flux. The degree of polarization of sunlight that is incident on a planet can be assumed to equal zero (when integrated over the solar disk). However, sunlight that has been reflected by a planet will usually be polarized because it has been scattered in the planetary atmosphere and/or it has been reflected by the planetary surface (if there is any). Note that this reflected sunlight is predominantly linearly polarized; its degree of circular polarization is usually very small and we will ignore it in the following.

It has long been known that the degree of polarization of the sunlight that has been reflected by a planet is very sensitive to the microphysical properties (i.e. size, shape, and composition / refractive index) of the scatterers in the atmosphere and/or to the reflection properties of the surface (i.e. its roughness, and the microphysical properties of the particles on the surface).

The degree of polarization of the reflected sunlight is also sensitive to the vertical distribution of the atmospheric constituents, such as the vertical extend of dust storms or the altitudes of cloud tops. In addition, the degree of polarization varies with wavelength and the illumination and viewing geometries. Because the degree of polarization of the reflected sunlight has a different, and usually higher sensitivity to the characteristics of a planetary atmosphere and/or surface than the flux of the reflected sunlight, polarimetry is a strong and often the only tool for disentangling the many parameters that describe planetary atmospheres and surfaces. In particular the combination of flux and polarization measurements is a very powerful remote-sensing tool (see Mishchenko and Travis [1997]).

The strengths of polarimetry for studying planetary atmospheres and surfaces have long been recognized. As an early example, Hansen and Hovenier [1974] successfully derived the composition and size of Venus' cloud particles, and the altitude of the main cloud deck from Earth-based polarimetry at a few wavelengths and a range of phase angles. Note that because Venus is an inner planet, it can be observed at phase angles ranging from almost zero to almost 180 degrees.

Earth-based polarimetry of the outer planets has not been very popular, because at the small phase angles under which these planets can be seen, the observable degree of polarization is always very small. However, there have been many planetary missions to these planets that carried instruments with polarimetric capabilities, such as the Pioneers 10 and 11, the Voyager 1 and 2, the Galileo and Cassini missions, and the Huygens lander.

In Earth remote-sensing, various versions of the POLDER instrument (CNES), with dedicated polarimetric capabilities in a number of broad spectral bands, specifically for aerosol, cloud and surface characterisation, have successfully been flown on satellites. NASA's Glory mission, which unfortunately failed for launch in 2011, carried with it APS, the Aerosol Polarimeter Sensor, a dedicated polarimeter for the characterisation of aerosol and cloud particles.

Science Case SPEX: Details  

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SPEX Instrument

Spectral modulation

High precision polarimetry is performed using the principle of spectral modulation, thus by encoding the degree and angle of linear polarization of the incoming light in a sinusoidal modulation of the intensity (flux) spectrum. This is achieved by using an achromatic quarter-wave retarder, an athermal multiple-order retarder and a polarizing beam splitter behind the entrance pupil. Measuring a single intensity spectrum thus provides the spectral dependence of the degree and angle of linear polarization. This modulation principle is covered by a preliminary patent application to Utrecht University.

The principle of operation is shown in Figure 1. Spectral flux enters from the left, and passes through the achromatic quarter-wave plate (QWP). When this light is linearly polarized along the QWP optical axis (vertical in Figure 1), it passes the QWP unchanged.

The next element is the multiple order retarder (MOR), which is the key element of the polarization optics. This MOR is a composite bi-refringent crystals, meaning that the crystals have two orthogonal optical axis. Along these directions, light propagates at different speeds. At the exit-face of the retarder, the two components have accumulated a phase shift. The magnitude of the phase shift, Δφ, is proportional to Δn·d/λ, with Δn the amount of birefringence, d the thickness of the MOR, and λ the wavelength of the incident light. Thus, depending on the wavelength the total phase shift can be (2n+¼)·π , (2n+½)·π , (2n+¾)·π , (2n+1)·π , or some value in between, leading to left-circular, horizontal linear, right-circular, (unchanged) vertical linear, or elliptical polarization, respectively.

The polarizing beam splitter projects the polarization state of the beam onto two orthogonal axes. Therefore, the intensity spectrum of the two output beams is modulated, with for one output beam maxima occurring at wavelengths at which the total phase shift Δφ = (2n+1)·π and minima when Δφ = (2n+½)·π (and vice versa for the other output beam). The amplitude of the modulation is a direct measure for the degree of linear polarization.

The function of the QWP is to transform light linearly polarized along 45 degrees w.r.t. the vertical direction (which would pass the MOR unchanged and would leave its intensity spectrum unmodulated) into circularly polarized light. This circularly polarized light is affected by the MOR, and it can be shown that the resulting modulation of the intensity spectrum is shifted by π/2 with respect to vertical linearly polarized light. The phase shift of the modulation is directly linked to the angle of the incident linear polarization, and this way SPEX can measure both the degree as well as the angle of linear polarization.

Figure 2 shows an example of an input (solar) spectrum and the output-spectra of SPEX for two cases: the input spectrum is partially and fully polarized. The figure illustrates that the amplitude of the modulation scales with the degree of linear polarization.