  Jupiter's moon Io hosts a dynamic atmosphere that is continually stripped off
and replenished through frost sublimation and volcanic outgassing. We observed
an emission band at 1.707 $\mu$m thought to be produced by hot SO molecules
directly ejected from a volcanic vent; the observations were made the NIRSPEC
instrument on the Keck II telescope while Io was in eclipse by Jupiter on three
nights in 2012-2016, and included two observations with 10x higher spectral
resolution than all prior observations of this band. These high-resolution
spectra reveal a contribution to the SO emission from gas reservoirs at both
high and low rotational temperatures. The scenario preferred by de Pater et al.
(2002) for the source of the SO gas - direct volcanic emission of SO in the
excited state - is consistent with these two temperature components if the
local gas density is high enough that rotational energy can be lost
collisionally before the excited electronic state spontaneously decays. Under
this scenario, the required bulk atmospheric gas density and surface pressure
are $n\sim 10^{11}$ cm$^{-3}$ and 1-3 nbar, consistent with observations and
modeling of Io's dayside atmosphere at altitudes below 10 km. The
low-temperature gas component is warmer for observations in the first 20
minutes of eclipse (in Dec 2015) than after Io had been in shadow for 1.5 hours
(in May 2016), suggesting cooling of the atmosphere during eclipse. Excess
emission is consistently observed at 1.69 $\mu$m, which cannot be matched by
two-temperature gas models but can be matched by models that over-populate high
rotational states. Finally, a comparison of the total band strengths observed
across eight dates from 1999-2016 reveals no significant dependence on thermal
hot spot activity (including Loki Patera), on the time since Io has been in
shadow, nor on the phase of Io's orbit at the time of observation.
