  Saturn's moon Iapetus, which is in synchronous rotation, is covered by an
optically dark material mainly on its leading side, while its trailing side is
significantly brighter. Because longer wavelengths probe deeper into the
subsurface, observing both sides at a variety of wavelengths brings to light
possible changes in thermal, compositional, and physical properties with depth.
We have observed Iapetus's leading and trailing hemispheres at 1.2 and 2.0 mm,
using the NIKA2 camera mounted on the IRAM 30-m telescope, and compared our
observations to others performed at mm to cm wavelengths. We calibrate our
observations on Titan, which is simultaneously observed within the field of
view. Due to the proximity of Saturn, it is sometimes difficult to separate
Iapetus's and Titan's flux from that of Saturn, detected in the telescope's
side lobes. Preliminary results show that the trailing hemisphere brightness
temperatures at the two wavelengths are equal within error bars, unlike the
prediction made by Ries (2012). On the leading side, we report a steep spectral
slope of increasing brightness temperature (by 10 K) from 1.2 to 2.0 mm, which
may indicate rapidly varying emissivities within the top few centimeters of the
surface. Comparison to a diffuse scattering model and a thermal model will be
necessary to further constrain the thermophysical properties of the subsurface
of Iapetus's two faces.
