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. 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.

The study of polarized sunlight scattered from planetary atmospheres provides diagnostic tools that can help explore the possible composition and size distribution of clouds and aerosol particles. Previous studies have shown the potential of this technique in studying Water clouds on Earth and in the discovery of Sulphuric Acid clouds on Venus. The atmosphere of Mars is unique as it hosts three different types of aerosols: water (H₂O) ice, carbon dioxide (CO₂) ice and dust. We considered scenarios analogous to Martian conditions and calculated the single and multiple scattering polarization for Martian dust, water ice and carbon dioxide ice with the help of a Radiative Transfer algorithm for Nadir and Limb spacecraft observation positions. The polarization features (-Q/I) of Mars atmosphere have low amplitude and have been found to be within the range of ±0.1 in the Nadir/Limb geometry for the spherical, spheroidal and cylindrical shapes considered here. We study its dependence upon the observation geometry, shape, size and composition of the scatterer. Future spacecraft studies of microphysical properties of dust and clouds through polarization will reveal the nature of condensation processes active in the Martian atmosphere.

Planet-wide stationary gravity waves have been observed with the thermal camera on the Akatsuki spacecraft. These waves have been attributed to the underlying surface topography and have successfully been reproduced using the Institut Pierre Simon Laplace (IPSL) Venus Mesoscale Model (VMM). Here, we use numerical radiative transfer computations of the total and polarized fluxes of the sunlight that is reflected by Venus under the conditions of these gravity waves to show that the waves could also be observed in polarimetric observations. To model the waves, we use the density perturbations computed by the IPSL VMM. We show the computed wave signatures in the polarization for nadir-viewing geometries observed by a spacecraft in orbit around Venus and as they could be observed using an Earth-based telescope. We find that the strength of the signatures of the atmospheric density waves in the degree of polarization of the reflected sunlight depends not only on the density variations themselves, but also on the wavelength and the cloud top altitude. Observations of such wave signatures on the dayside of the planet would give insight into the occurrence of the waves and possibly into the conditions that govern their onset and development. The computed change in degree of polarization due to these atmospheric density waves is about 1000 ppm at a wavelength of 300 nm. This signal is large enough for an accurate polarimeter to detect.