Mars Atmosphere: Modeling and Observations (2008) 9078.pdf Polarimetry of Mars with SPEX, an Innovative Spectropolarimeter Daphne M. Stam1,2 ([email protected]), Erik Laan3, Frans Snik4, Theodora Karalidi4, Christoph Keller4, Rik ter Horst5, Ramon Navarro5, Christina Aas1, Johan de Vries3, Gijs Oomen6, and Ruud Hoogeveen2, 1 DEOS, Aerospace Engineering, Technical University, Delft, 2 SRON Netherlands Institute for Space Research, Utrecht, 3 TNO Science & Industry, Delft, 4 Sterrekundig Instituut Utrecht, Utrecht, 5 ASTRON Netherlands Foundation for Research in Astronomy, Dwingeloo, 6 Dutch Space, Leiden, all in the Netherlands We present SPEX, an innovative, compact, and ro- bust Spectropolarimeter for Planetary EXploration, that measures fluxes F and polarization P of sunlight re- flected by Mars from 400 to 800 nm with 2 nm (F ) to 20 nm (P ) spectral resolution, in 9 fixed viewing direc- tions. With SPEX, dust and ice cloud particles in the atmosphere and the surface can be studied. To illustrate the power of spectropolarimetry, we present sample sim- ulations of F and P of Mars for different dust scenarios. Introduction Polarimetry of sunlight that is reflected by a planet has proven to be a powerful tool to characterize the planetary atmosphere and surface. The main reason is that the state of polarization of scattered sunlight is more sensitive to the microphysical properties of particles (size, shape, and Figure 1: With 7 fixed viewing directions, SPEX will composition) than the flux [1, 2, 3]. An early example of measure flux and polarization phase functions of scenes the power of polarimetry was the derivation of the size along its ground track (the 2 limb viewers are not shown). and composition of Venus cloud particles from Earth- based polarimetry at two wavelengths [4]. Since then, polarimeters have flown on the Pioneer Venus, Galileo, SPEX instrument concept and Cassini missions. The POLDER polarimeter has Sunlight that has been scattered within the Mars atmo- flown on various Earth observing missions, and NASA’s sphere and/or that has been reflected by the surface can Glory mission with the Aerosol Polarimetry Sensor on- be described by a (column) vector F [see e.g. 1] board will be launched at the end of 2008. Mars, with its dust storms and ice clouds and dusty F = [F, Q, U, V ] , (1) surface, is an ideal polarimetry target. Indeed, HST- with F the total flux, Q and U the linearly polarized flux, observations show polarization variations correlating and V the circularly polarized flux. We’ll ignore V be- with clouds and dust [5]. However, these observations cause it is very small [see e.g. 8]. The degree of (linear) were done with Mars at opposition; and the degrees of polarization P of the reflected sunlight is defined as polarization are therefore very small. To truly enjoy the p advantages of polarimetry for Mars remote-sensing, a P = Q2 + U 2/F. (2) polarimeter should orbit the planet, because only then the scattering angles that hold most of the information The polarization direction χ is given by tan 2χ = U/Q are within reach. Two polarimeters have flown onboard (defined with respect to the local meridian plane) [see 1]. the Soviet spacecraft MARS-5 [6, 7] providing informa- In planetary (including Earth) remote-sensing, fluxes tion about particle shapes, sizes, and composition, even F , Q, and U of scattered sunlight are usually determined though they encountered a very clear Mars atmosphere. from flux measurements through 3 polarization filters We present SPEX, an innovative (patented), compact with their optical axes under different angles, or through (1 liter), and robust (no moving parts) spectropolarime- a single rotating filter [see e.g. 1]. The advantage of this ter in a design tailored for observing Mars from orbit, method is its simplicity. There are, however, also dis- together with sample simulations of the observable flux advantages: 1. the time lapse between the flux measure- and degree of polarization of reflected sunlight. ments can yield polarization errors of several percents, Mars Atmosphere: Modeling and Observations (2008) 9078.pdf 2 depending on the observed scene and on the change of Sample simulations the geometries during the integration time; 2. different Both F and P (and χ) of the reflected sunlight depend on filters have different transmission properties and may de- the composition of Mars’ atmosphere, the optical prop- grade differently during flight, which will lead to errors; erties of the dust and ice cloud particles, the reflective 3. when combining color filters with polarization filters, properties of the surface, and the illumination and view- P as is usually done, you have limited information on ’s ing angles. Flux F of course also depends on the incom- spectral behaviour; and 4. filter wheels can get stuck. ing solar flux F0, hence on the distance between Mars With SPEX, fluxes F , Q and U in a given view- and the sun, but in the following, we assume F0 = π, ing direction are measured simultaneously, from 400 to independent of the wavelength. Both P and χ of the re- 800 nm. As explained by Snik et al. [9], P and χ of the flected sunlight are independent of F0, since Q, U, and reflected sunlight are encoded in the flux spectrum of the 2 F are all proportional to F0. light. The encoding is a spectral modulation: the flux For our sample simulations of F and P of reflected spectrum is multiplied with a sinusoidal function. The sunlight, we describe the atmosphere as a stack of ho- modulation amplitude yields P , and the phase, χ. The mogeneous layers containing gas and dust particles. The amplitude and phase will depend on λ, like P (see the surface pressure is 6 mbars, and an average temperature next section) and χ. The advantage of the modulation is profile is chosen. We ignore the few, shallow absorp- that the full polarization information is obtained with just tion bands of CO2 in the SPEX wavelength region. The one flux spectrum measurement. surface reflects Lambertian (isotropically and depolariz- The spectral modulation is achieved with [see 9] an ing), and has a realistic, wavelength dependent albedo. achromatic quarter-wave retarder, followed by an ather- The dust particles are homogeneous palagonite spheres, mal multiple-order retarder and a polarizing beamsplit- sized according to the standard distribution of Hansen ter. The modulation frequency (i.e. the width of the mod- and Travis [1], with an effective variance of 1.5, and ef- ulations) depends on the optical properties of the com- fective radii of 0.5 or 1.0 µm. For each wavelength, the ponents, such as their refractive index and geometrical optical properties of the dust particles are calculated us- thickness. SPEX’ components have been chosen such ing Mie-theory. Although real dust particles probably [see 9] that the modulated flux is measured with a reso- have irregular shapes, spherical particles are fine to illus- lution of about 2 nm (by combining two simultaneously trate the effects of dust on the flux and polarization. We measured spectra, the flux itself can be retrieved with this choose two values for the dust optical thickness τD: 0.2 same resolution), The polarization can be retrieved with and 1.0 (at 630 nm). The dust is located in the lower part a resolution of about 20 nm. of the atmosphere (up till 1.4 mbars). SPEX has 9 apertures and measures modulated flux The simulations are performed using an adding- spectra simultaneously in 9 fixed viewing directions doubling radiative transfer code, which fully includes 1 along its flight track . Two apertures look at the limb multiple scattering and polarization [12, 13, 14]. (forward and backward) for studying ice clouds. The Figure 2 shows F and P for the 2 particle sizes and ◦ other viewing angles are approximately: 0 (nadir), the 2 dust optical thicknesses for the 7 viewing directions ◦ ◦ ◦ ±20 , ±40 , and ±55 (the + indicates the forward, of SPEX (the limb directions have been excluded). The and the − the backward direction). With these apertures, solar zenith angle is 60◦ and the sun is in the orbital plane SPEX obtains the flux and polarization phase functions of the satellite. The surface albedo strongly determines of the scenes it flies over (see Fig. 1). Each apertures ◦ F , especially when τD = 0.2 and for nadir and ±20 ◦ ◦ field-of-view is 1 × 7 (along × across track). Ground viewing angles. With increasing viewing angle and/or for pixel sizes will depend on the satellite’s orbit. τ = 1.0, F increases because of the increased scattering A provisional patent application for SPEX’ passive of light by the dust. Some sensitivity of F to the dust polarimetric method (the concept of which has been particle size is seen, especially for the largest viewing proven with a single aperture optical train in an optical angles, but the spectral shape of F changes little. laboratory) has been submitted. The project to build a P appears to be more sensitive to the particle size breadboard of SPEX with its 9 apertures has started in than F is. The variation in P with the viewing angle September 2008. The flight model will have a volume reflects the dust particles’ single scattering polarization of about 1 liter, a mass smaller than 5 kg, and a power phase function, which depends stronger on the particle budget smaller than 5 W. size than the flux phase function. The spectral features 1 2 with a slight off-set to compensate for Mars’ rotation except possibly across Fraunhofer lines in F0 [see e.g. 10, 11] Mars Atmosphere: Modeling and Observations (2008) 9078.pdf 3 of the surface albedo also show up in P , especially at the References longer wavelengths, where the albedo is highest: because [1] J.