Daphne M. Stam

We report observations of faint (10 ⁻⁶ ), concentric, planet-wide rings in the polarized flux of sunlight that is reflected by Venus, obtained during a serendipitous, 36-minute run in 2010, with the highly sensitive Extreme Polarimeter (ExPo) on the William Herschel Telescope. The rings appear to be centered slightly downwind of the subsolar point, are visible in different filters across the visible, and are not obvious in the simultaneous total flux observations. ExPo’s dual-beam exchange and double-differencing design strongly suppresses first-order instrumental polarization, and we could not identify an instrumental cause of the observed pattern. Because ExPo was dismantled before the rings were identified in the data, this is the only set of observations of these rings. We are therefore careful in claiming the detection of a new atmospheric phenomenon on Venus. However, numerical radiative transfer simulations show that planet-wide rings in polarization can arise owing to density variations of 5%–10% in the gas above the clouds, consistent with a gravity wave. Our simulations also show that such density variations would not show up in total flux observations. By presenting our observations and numerical simulations, we hope to motivate new polarimetric observations of Venus that could confirm or refute the presence of such planet-wide waves.

Context. The detection and characterization of exorings (rings around exoplanets) will help us to better understand the origin and evolution of planetary rings in the Solar System and beyond. However, exorings are still elusive, and new and clever methods for identifying them need to be developed and tested. Aims. We explore the potential of polarimetry as a tool for discovering and characterizing exorings. Methods. For this purpose, we improved the general publicly available photometric code Pryngles by adding the results of radiative transfer calculations with an adding-doubling algorithm that fully includes polarization. With this improved code, we computed the total and polarized fluxes and the degree of polarization of model gas giant planets with or without rings. Additionally, we demonstrate the versatility of our code by predicting the polarimetric signal of the puffed-up planet HIP 41378 f as if it had an exoring. Results. Spatially unresolved dusty rings can significantly modify the flux and polarization signals of the light that is reflected by a gas giant exoplanet along its orbit. Rings are expected to have a low polarization signal, but they will decrease the degree of polarization of reflected light when they cast a shadow on the planet and/or block part of the planet. The most diagnostic feature of a ring occurs around the ring-plane crossings when sharp changes in the flux and degree of polarization curves are predicted by our model. When we applied our methods to HIP 41378 f, we found that if it is surrounded by a ring, noticeable changes in the degree of polarization of reflected light will arise. Although the reflected light on the planet cannot yet be directly imaged, the addition of polarimetry to future observations would aid in the characterization of the system.

Context. Terrestrial-type exoplanets in or near stellar habitable zones appear to be ubiquitous. It is, however, unknown which of these planets have temperate, Earth-like climates or for example, extreme Venus-like climates. Aims. Technical tools to distinguish different kinds of terrestrial-type planets are crucial for determining whether a planet could be habitable or incompatible with life as we know it. We aim to investigate the potential of spectropolarimetry for distinguishing exo-Earths from exo-Venuses. Methods. We present numerically computed fluxes and degrees of linear polarization of starlight that is reflected by exoplanets with atmospheres in evolutionary states ranging from similar to the current Earth to similar to the current Venus, with cloud compositions ranging from pure water to 75% sulfuric acid solution, for wavelengths between 0.3 and 2.5 µm. We also present flux and polarization signals of such planets in stable but spatially unresolved orbits around the star Alpha Centauri A. Results. The degree of polarization of the reflected starlight shows larger variations with the planetary phase angle and wavelength than the total flux. Across the visible, the largest degree of polarization is reached for an Earth-like atmosphere with water clouds due to Rayleigh scattering above the clouds and the rainbow feature at phase angles near 40°. At near-infrared wavelengths, the planet with a Venus-like CO₂ atmosphere and thin water cloud shows the most prominent polarization features due to Rayleigh-like scattering by the small cloud droplets. A planet in a stable orbit around Alpha Centauri A would leave temporal variations on the order of 10⁻¹³ W m s⁻¹ in the total reflected flux and 10⁻¹¹ in the total degree of polarization as the planet orbits the star and assuming a spatially unresolved star-planet system. Star-planet contrasts are on the order of 10⁻¹⁰ and vary proportionally with planetary flux. Conclusions. Current polarimeters appear to be incapable to distinguish between the possible evolutionary phases of spatially unresolved terrestrial exoplanets, as a sensitivity close to 10⁻¹⁰ would be required to discern the planetary signal given the background of unpolarized starlight. A telescope or instrument capable of achieving planet-star contrasts lower than 10⁻⁹ should be able to observe the large variation of the planets resolved degree of polarization as a function of its phase angle and thus be able to discern an exo-Earth from an exo-Venus based on their clouds unique polarization signatures.

In the JWST, Extremely Large Telescopes, and LUVOIR era, we expect to characterize a number of potentially habitable Earth-like exoplanets. However, the characterization of these worlds depends crucially on the accuracy of theoretical models. Validating these models against observations of planets with known properties will be key for the future characterization of terrestrial exoplanets. Due to its sensitivity to the micro- and macro-physical properties of an atmosphere, polarimetry will be an important tool that, in tandem with traditional flux-only observations, will enhance the capabilities of characterizing Earth-like planets. In this paper we benchmark two different polarization-enabled radiative-transfer codes against each other and against unique linear spectropolarimetric observations of the earthshine that cover wavelengths from ∼0.4 to ∼2.3 μm. We find that while the results from the two codes generally agree with each other, there is a phase dependency between the compared models. Additionally, with our current assumptions, the models from both codes underestimate the level of polarization of the earthshine. We also report an interesting discrepancy between our models and the observed 1.27 μm O₂ feature in the earthshine, and provide an analysis of potential methods for matching this feature. Our results suggest that only having access to the 1.27 μm O₂ feature coupled with a lack of observations of the O₂ A and B bands could result in a mischaracterization of an Earth-like atmosphere. Providing these assessments is vital to aid the community in the search for life beyond the solar system.

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...

The polarization state of starlight reflected by a planetary atmosphere uniquely reveals coverage, particle size, and composition of aerosols as well as changing cloud patterns. It is not possible to obtain a comparable level of detail from flux-only observations. It is therefore a powerful tool to better understand the crucial role played by clouds and aerosols in the chemistry, dynamics, and radiative balance of a planet. Furthermore, polarization observations can probe the atmosphere of planets independently of the orbital geometry (hence it applies to both transiting and non-transiting exoplanets). A high-resolution spectropolarimeter with a broad wavelength coverage, particularly if attached to a large space telescope, would enable simultaneous study of the polarimetric planetary properties of the continuum and to look for and characterize the polarimetric signal due to scattering from single molecules, providing detailed information about the composition and vertical structure of the atmosphere.

NASA’s EPOXI mission used the Deep Impact spacecraft to observe the disk-integrated Earth as an analog to terrestial exoplanets’ appearance. The mission took five 24 hr observations in 2008–2009 at various phase angles (57.°7–86.°4) and ranges (0.11–0.34 au), of which three equatorial (E1, E4, E5) and two polar (P1, North and P2, South). The visible data taken by the HRIV instrument ranges from 0.3 to 1.0 μm, taken trough seven spectral filters that have spectral widths of about 100 nm, and which are centered about 100 nm apart, from 350 to 950 nm. The disk-integrated, 24 hr averaged signal is used in a phase angle analysis. A Lambertian-reflecting, spherical planet model is used to estimate geometric albedo for every observation and wavelength. The geometric albedos range from 0.143 (E1, 950 nm) to 0.353 (P2, 350 nm) and show wavelength dependence. The equatorial observations have similar values, while the polar observations have higher values due to the ice in view. Therefore, equatorial observations can be predicted for other phase angles, but (Earth-like) polar views (with ice) would be underestimated.