  Reddish coloration and color diversity among icy bodies in the outer solar
system are one of the significant clues for understanding the status and
history of the solar system. However, the origin of color distribution remains
debatable. Here, we demonstrate reddish coloration that is stable only at
cryogenic temperatures in a laboratory experiment. The reddish coloration was
produced on methanol- and water-containing ice irradiated with
nitrogen-containing cryoplasma at 85 K. The reddish color visually faded and
disappeared at 120-150 K as the ice was heated, unlike well-known refractory
organic tholins that are stable even when heated to room temperature. This
temperature dependence of reddish coloration under cryogenic conditions could
provide a new possible explanation for the absence of ultra-red coloration
closer to the Sun in the outer solar system. Our result implies that a reddish
material specific to cryogenic environments is useful for the investigation of
color diversity and formation mechanism of the outer solar system.
