I was reading this text where they state: "Many cosmic masers are observed to have both circular and linear polarization although the reasons for linear polarization are not well understood." Just curious, aren't there any molecules such as OH, CH4, or water masers that have transitions that emit linearly polarized waves? Why is the mechanism behind linear polarization in masers an outstanding question? Perhaps on a related note, have astrophysical masers ever had measurements of rotation measure (RM)? Although these masers can arise in a variety of conditions (e.g. near stars, pulsar), they are typically in environments with strong EM fields, correct? So would we be able to observe an RM associated with a maser?