Millimetric thermal emission from the two faces of Iapetus

Saturn's moon Iapetus, which is in synchronous rotation, travels through the dusty Phoebe ring. The leading face of Iapetus is thus covered by a dark material, making it darker at visible wavelengths and increasing its microwave emissivity. Ries (2012) observed Iapetus' two faces at wavelengths varying from 3 to 10.8 mm. We observed the millimetric thermal emission of the two faces of Iapetus at wavelengths of 1.2 and 2.0 mm. On Iapetus' trailing side, we confirm the prediction of Ries (2012): there is a scattering feature centered near 3 mm, consistent with 1-2 mm particle sizes. On the leading side, however, we find that the brightness temperature is much lower at 1.2 and 2.0 mm than that observed at 3 mm by Ries (2012), which may be due to physical properties quickly changing with depth in the top few cm of the subsurface.