Temporal behavior of the SO 1.707 μm ro-vibronic emission band in Ioʹs atmosphere

We report observations of the ro-vibronic a 1 Δ → X 3 Σ − transition of SO at 1.707 μm on Io. These data were taken while Io was eclipsed by Jupiter, on four nights between July 2000 and March 2003. We analyze these results in conjunction with a previously published night to investigate the temporal behavior of these emissions. The observations were all conducted using the near-infrared spectrometer NIRSPEC on the W.M. Keck II telescope. The integrated emitted intensity for this band varies from 0.8 × 10 27 to 2.4 × 10 27   photons / s , with a possible link to variations in Lokiʹs infrared brightness. The band-shapes imply rotational temperatures of 550–1000 K for the emitting gas, lending further evidence to a volcanic origin for sulfur monoxide. An attempt to detect the B 1 Σ → X 3 Σ − transition of SO at 0.97 μm was unsuccessful; simultaneous detection with the 1.707 μm band would permit determination of the SO column abundance.