Spectropolarimetry of Mars: Why and how?

Polarimetry of sunlight that is reflected by a planet or that is transmitted through its atmosphere is a powerful tool for the characterisation of the planetary atmosphere and, if present, the surface. The main reason for the power of polarimetry is that the angular distribution of the degree of linear polarisation of sunlight that has been singly scattered by particles in the atmosphere or on the surface is very sensitive to the microphysical properties of these particles (their size, shape, and composition), indeed much more sensitive than the total flux is [see 1, 2, 3]. And because multiple scattered light usually has a low degree of polarisation, it might decrease the overall degree of polarisation of the light that emerges from the planetary atmosphere, but the angular pattern, which holds the crucial information about the scatterers, will remain. A classic example of the power of polarimetry was the derivation of the size distribution, composition, and altitude of the particles constituting Venus’s main cloud deck from Earth-based, disk-integrated polarimetry across a wide phase angle range and at a few wavelengths [4]. Since then, spectrometers with some polarimetric capabilities have flown on, for example, the Pioneer Venus, Galileo, and Cassini missions. The POLDER polarimeter has flown on various Earth observing missions, and NASA’s Earth remote-sensing PACE mission with SPEXone [5] onboard is scheduled for launch in 2024.

Mars, with its dust storms, its water and carbon-dioxide ice clouds, and dusty surface, appears to be an ideal target for polarimetry. The polarimetric attention for this planet has, however, been surprisingly limited. Two linear polarimeters have flown onboard the Soviet spacecraft MARS-5 and provided some information about mostly ice cloud particle shapes, sizes, and composition, even though they encountered a very clear Martian atmosphere during their short active measurement period [6, 7]. And, indeed, HST observations show linear polarisation variations that correlate with the presence of clouds and dust [8]. However, with HST orbiting the Earth, these observations were necessarily done with Mars at a small phase angle; the measured degrees of polarization are therefore very small and the angular range extremely limited. To truly enjoy the advantages of polarimetry for Mars remote-sensing, a polarimeter should either orbit the planet or be landed on the surface, because only then a range of scattering angles holding most of the information would be within reach.

We will describe the case for martian spectropolarimetry from an orbiter or a lander/rover, highlighting the potential for characterisation of the atmospheric dust and clouds and of the surface, and also covering the use of circular polarimetry for identifying chiral signatures [9,10] that on Earth are typical for life.  Traditionally, polarimeters have been based on polarisers in rotating filter wheels. Such designs are neither robust nor do they achieve the accuracy that fully unlocks the power of polarimetry. We will present an innovative, compact, robust (no moving parts), and accurate type of spectropolarimeter [11] whose flexible design allows incorporation on various vehicles. Finally, the analysis and interpretation of martian spectropolarimetric measurements requires radiative transfer algorithms that fully include linear (and circular) polarimetry. We will describe a few available algorithms and discuss laboratory measurements of light scattering properties of various types of atmospheric particles and surfaces that would improve the radiative transfer computations and with that the interpretation of (future) spectropolarimetry of Mars.       

References

[1] J. E. Hansen and L. D. Travis. Light scattering in planetary atmospheres. Space Sci. Rev., 16:527–610, 1974. 

[2] M. I. Mishchenko and L. D. Travis. Satellite retrieval of aerosol properties over the ocean using polarization as well as intensity of reflected sunlight. J. Geophys. Res., 102:16989–17014. doi: 10.1029/96JD02425, 1997

[3] Y. Shkuratov, N. Opanasenko, E. Zubko, Y. Grynko, V. Korokhin, C. Pieters, G. Videen, U. Mall, and A. Opanasenko. Multispectral polarimetry as a tool to investigate texture and chemistry of lunar regolith particles. Icarus, 187:406–416. doi: 10.1016/j.icarus. 2006.10.012, 2007 

[4] J. E. Hansen and J. W. Hovenier. Interpretation of the Polarization of Venus. J. Atmos. Sci., 31:1137–1160, 1974. 

[5] F. Snik, J. H. Rietjes, G. Van Harten, D. M. Stam, C. U. Keller, J. M. Smit. SPEX: the spectropolarimeter for planetary exploration. Proc. SPIE 7731:77311B. doi: 10.1117/12.857941, 2010.

[6] R. Santer, M. Deschamps, L. V. Ksanfomaliti, and A. Dollfus. Photopolarimetric analysis of the Martian atmosphere by the Soviet MARS-5 orbiter. I - White clouds and dust veils. Astron. Astrophys., 150:217–228, 1985. 

[7] R. Santer, M. Deschamps, L. V. Ksanfomaliti, and A. Dollfus. Photopolarimetry of Martian aerosols. II - Limb and terminator measurements. Astron. Astrophys., 158:247–258, 1986.

[8] Y. Shkuratov, M. Kreslavsky, V. Kaydash, G. Videen, J. Bell, M. Wolff, M. Hubbard, K. Noll, and A. Lubenow. Hubble Space Telescope imaging polarimetry of Mars during the 2003 opposition. Icarus, 176:1–11. doi: 10.1016/j.icarus.2005.01.009, 2005

[9] W. Sparks, J. H. Hough, Th. A. Germer, F. Robb, and L. Kolokolova, Remote sensing of chiral signatures on Mars, Planet. Space Sci. 72, doi: 10.1016/j.pss.2012.08.010, 2012

[10] W. Sparks, J. H. Hough, and L. E. Bergeron, The search for chiral signatures on Mars, Astrobiology, 5, doi: 10.1089/ast.2005.5.737, 2005

[11] D. M. Stam, E. Laan, F. Snik, T. Karalidi, C. Keller, R. ter Horst, R. Navarro, C. Aas, J. De Vries, G. Oomen, R. Hoogeveen, Polarimetry of Mars with SPEX, an innovative spectropolarimeter, Third International Workshop on The Mars Atmosphere: Modeling and Observations, LPI Contributions, Vol. 1447, 2008.