  Interstellar dust grains are often aligned. If the grain alignment direction
varies along the line of sight, the thermal emission becomes
circularly-polarized. In the diffuse interstellar medium, the circular
polarization at far-infrared and submm wavelengths is predicted to be very
small, and probably unmeasurable. However, circular polarization may reach
detectable levels in infrared dark clouds and protoplanetary disks. Measurement
of circular polarization could help constrain the structure of the magnetic
field in infrared dark clouds, and may shed light on the mechanisms responsible
for grain alignment in protoplanetary disks.
