DPS 2001 meeting, November 2001
Session 24. Io, Tori, and Satellite Atmospheres
Oral, Chairs: I. de Pater, W. Smythe, Wednesday, November 28, 2001, 5:00-6:40pm, Regency E

## [24.02] Io's 1-2.5\mum Spectrum in Eclipse with SpeX at IRTF

J. D. Goguen, D. L. Blaney (JPL/Caltech)

We observed Io in eclipse on 12 October 2000 UT to investigate the shape of the emission spectrum from 1-2.5\mum at \lambda/\delta\lambda~1200 using SpeX at the NASA IRTF. Our spectra show a narrow emission feature centered at 1.707\mum that de Pater et al. (2001) first observed on 24 February 1999 UT and identified as the forbidden a1 \Delta arrow X3 \Sigma - 0-0 transition in SO. The spatial distribution of the SO emission is asymmetric with respect to the center of Io's disk with the spatial center for the emission band wavelengths offset towards E on the sky-plane compared to the center for the adjacent continuum wavelengths. This is consistent with the SO emission concentrated in localized low-latitude volcanic plumes above sources located at longitudes W of Io's disk center (344 W), e.g. Kanehekili or Janus.

The overall shape of the spectra from 1-2.5\mum suggests excess" emission and spectral structure towards shorter wavelengths that is not easily explained with lava flow models. Io is known to be a prodigous source of \leq 0.01\mum dust likely originating in volcanic plumes. Collisions with oxygen and sulfur ions corotating with Jupiter's magnetic field can briefly heat the smallest dust grains to high temperatures. This energetic ion heating of small dust may enhance the short wavelength emission and also result in spectral structure due to the dependence of the emissivity on the optical constants of the dust (e.g. Dwek, 1986). A small mass of hot dust is sufficient to produce the observed level of excess emission at short wavelengths. \\ \\ de Pater, I. et al. (2001). ( Jupiter: Planet, Satellites and Magnetosphere, Boulder, CO). \\ Dwek, E. 1986. Astrophys. J. 302, 363-370. \\ This research was supported in part by the NASA Planetary Astronomy Program.