Is H 2O Present on Io? The Detection of a New Strong Band Near 3590 cm -1 (2.79 μm)

A strong absorption band at 3590 ± 20 cm ^-1 (2.790 ± 0.015 μm) has been discovered in the spectrum of Io using the Kuiper Airborne Observatory (KAO). The 2 v₁ + v₃ combination mode of solid SO ₂ falls at this position. Since SO ₂ is abundant on Io it must contribute to the new band. However, a band due to H ₂O was predicted near this frequency in Io's spectrum based on laboratory experiments of H ₂O: SO ₂ mixed Io ice analogs which were used to assign the two weak, variable features at 3370 and 3170 cm ^-1 (2.97 and 3.15 μm) to trace amounts of H ₂O frozen in solid SO ₂ on Io. The new band probably originates from both SO ₂ and H ₂O. Unfortunately, the spectral resolution of the data is insufficient to settle the issue of whether there are two resolvable components.