  Mid-IR albedo values of Neptune are derived from Spitzer Space Telescope
measurements reported by Stauffer et al. (2016). The method of this derivation
is described and the results indicate that the geometric albedo was about 1% or
less at the time of the observations in 2016. Short-term mid-IR variability of
Neptune, evidenced by the Spitzer observations themselves, indicates an albedo
at 3.6 microns ranging from 0.2% to 0.6% with a mean of 0.4%. The corresponding
albedos at 4.5 microns are 0.7%, 1.3% and 0.9%. Furthermore, the 60-year
history of visible-light brightness variations, which show that Neptune was
significantly fainter a few decades ago, suggests that the mid-IR albedo during
that earlier period of time may have been much less than 1%. The albedo values
reported here can have implications for models of Neptune's atmosphere.
However, the mid-IR brightness of Neptune cannot contribute very strongly to
its total albedo because the Sun emits only about 2% of its flux long-ward of 3
micron. By contrast, the Sun emits 42% of its flux at visible and near-UV
wavelengths where the planet's albedo is in the tens of percents.
