Wavelength dependence of linear polarization in the visible and near infrared domain for large levitating grains (PROGRA2 instruments)

•Wavelength dependence of the linear polarization between 545 nm and 1.5 μm for levitating grains.•The wavelength dependence of polarization is related to the refractive index of the particles.•PROGRA2 studies are involved in the ICAPS project onboard the International Space Station.

Remote sensing measurements of light scattered by dust in solar system objects can provide clues on their physical properties. Databases obtained in the laboratory with numerous samples are necessary to interpret these measurements. We present here first studies of the wavelength dependence of the linear polarization between 545 nm and 1.5 μm, using the imaging polarimeters PROGRA2 for large levitating compact grains (PROGRA2-VIS in the visible domain, and the new instrument PROGRA2-IR in the near infrared). The measurements are conducted in microgravity conditions during parabolic flights for glass beads, quartz, sands, silicon carbides, anthracite, and lunar and Martian simulants. Comparison between measurements on glass beads and Mie calculations with glass spheres provides an assessment of the quality of the instruments. The dependence of the polarization on the wavelength is related to the complex refractive index of the particles, i.e. to their composition and to the size of the grains. More laboratory measurements will be necessary, in particular with smaller grains in aggregates, to better reproduce the remote sensing observations of solar system bodies.