Cover image is the Q polarization of Jupiter taken with ZIMPOL in Gisler & Schmid 2003
Exoplanet polarimetry is in its infancy but in our own Solar System polarimetry has been vital to understanding the nature of the Sun, our planets, their moons, and minor bodies like comets and asteroids.
Here are some key discoveries in our own Solar System made with polarimetry:
Titan is Hazy (from hydrocarbons)
Zellner 1973 - This paper showed—with polarimetry—that Titan was enshrouded in thick cloud (haze), that its unresolved appearance was not from atmospheric molecules nor the surface
Veverka 1973 - The atmosphere is optically thick with a thin Rayleigh layer over thick cloud
West & Smith 1991 This paper determined that not only was the albedo due to haze, but that that haze was probably comprised of fractal aggregates of hydrocarbons.
Hazy gas giants
Schmid 2011 - The spectropolarimetry of Jupiter and Saturn, exposing the haze present at the poles.
Schmid 2006 - Imaging polarimetry of Uranus and Neptune, which showed they are dominated by polarization from their hazes
Joos 2006 - Spectropolarimetry for Uranus and Neptune indicates stratification with Rayleigh scattering condensates
Venus is no tropical paradise
Hansen & Hovenier 1974 - the famous paper analyzing a collection of historic polarimetric data of Venus to show that its clouds were probably comprised of an 85% sulfuric acid, 15% water mix
One of the Galilean moons in not like the others
Dollfus 1975 - the polarimetric phase curves of the four moons (all near full phase of course) have a distinct negative component… but Callisto looks different (we now know the moon probably has more silicates at its surface and is less differentiated)
Mandeville 1980 - investigated the asymmetry in the polarization across Callisto’s surface from major craters
Rosenbush 2002 - a more recent paper returning to that asymmetry and finding the low fractional polarization may be predominantly from the Valhalla crater
Mars dust
Petrova 1999 - the size and shape of Mars’ solid aerosols/dust particles is measured, and the importance of not assuming solids are spheroidal is shown
Ebisawa 1992 - the dust of Mars’ dust storms is characterized in comparison to longer-lived atmospheric dust
On Earth…
polarimetry is frequently used for retrieving gas abundances (for example to monitor Carbon Dioxide sources and sinks) and characterizing clouds for weather observations and predictions (high cold icy clouds look very different from low warm liquid clouds). As on Mars, dust on Earth from haboobs has been characterized using polarimetry (see: Bailey 2008 )
Asteroids and Comets
Polarimetry is frequently used to determine the composition and shape of small bodies in our Solar System. For example, Mukai+ 1994 used the technique to determine that the asteroid 4179 Toutatis is a rocky, rubbly S-type, which may indicate its origin. In comets a negative polarization at small angles indicates the porosity of the aggregates on their surface (or in their “dust”) similar to that seen on the icy Galilean moons. Circular polarization of at least some comets show a preference for left-handed chiral molecules, the same handedness life on Earth uses, lending credence to the idea that stellar environments may impart the small preference in molecule orientation that life runs with to make a more extreme excess in one handedness (Bailey+ 1998).
Sun (and other stars)
Our home star has been studied with polarimeters extensively. A great overview of the many discoveries of its nature is available in Harvey 2015. While polarimeters have been used to study other stars too for quite some time, some interesting discoveries very recently have come from very high sensitivity visible light polarimeters designed for exoplanet observations being pointed at solo stars (e.g. a star oblate from rotation, an active region on a star monitored from Earth, and stars in a binary system reflecting light off each other).
Outside our Solar System Bonus: Exoplanets
While no confirmed detection of polarized light from an exoplanet has been found yet, there is a disputed detection which may have discovered a planet’s color, and a non-detection which ruled out certain cloud scenarios:
Berdyugina 2011 - may have determined the blue color of HD 189733b via polarimetry, although this is disputed by Wiktorowicz 2015 and Bott 2016.
Bott 2018 - a non-detection of polarization places an upper limit ruling out a fine (Rayleigh) haze on WASP-18b