Polarization-phase curves of martian simulants and mixtures of Mg-Fe silica with carbon from the PROGRA² experiment database. Sample names in the legend are the same as those in the database. Data in panel a are measurements at 545 nm. Filled symbols in panel b are the same as those in panel a (i.e., the Martian simulant (<100 µm) and Mg·Fe-silica with carbon at 545 nm) but their open counterparts indicate measurements at 630 nm.