  We present a measurement of H2O ice crystallinity on the surface of
trans-neptunian objects (TNOs) with near-infrared narrow-band imaging. The
newly developed photometric technique allows us to efficiently determine the
strength of an 1.65-um absorption feature in crystalline H2O ice. Our data for
three large objects, Haumea, Quaoar, and Orcus, which are known to contain
crystalline H2O ice on the surfaces, show a reasonable result with high
fractions of the crystalline phase. It can also be pointed out that if the
H2O-ice grain size is larger than ~20 um, the crystallinities of these objects
are obviously below 1.0, which suggest the presence of the amorphous phase.
Especially, Orcus exhibits a high abundance of amorphous H2O ice compared to
Haumea and Quaoar, possibly indicating a correlation between bulk density of
the bodies and surface crystallization degree. We also found the presence of
crystalline H2O ice on Typhon and 2008 AP129, both of which are smaller than
the minimum size limit for inducing cryovolcanism as well as a transition from
amorphous to crystalline through the thermal evolution due to the decay of
long-lived isotopes.
