The Role of Thermal Emission in Determining SO2 Frost Band Depths in the Galileo NIMS G2 data

Lopes-Gautier et al. (1997) derived areas and temperatures for 14 volcanos in the NIMS G1 nighttime observation. This volcanic flux, as noted by Lopes-Gautier and Davies et al. (1997), does not represent all the thermal emission coming from Io's disk in the Galileo nighttime observations. However, the disk integrated total night-side thermal emission from Io is consistent with ground-based observations at 4.8 um (Smythe et al. 1997). Using observations collected at the NASA IRTF (Goguen et al. 1996), we estimate that the thermal emission from volcanos on Io at the longitude and time of the G1 observations should be about 0.027 Wm-2um-1 using the method of Veeder et al. (1994). The total emission from the 14 identified volcanic regions in Lopes-Gautier et al. is 0.007 Wm-2um-1, < 1/3 of the total expected flux at 4.8 um. The analysis of Carlson et al. (1997) for SO2 frost distribution avoids the known locations of volcanos in mapping band depth. However, given the large amount of emission from currently undefined regions on Io, thermal emission from small volcanos may be affecting the band depth maps. In our examination of NIMS G2 spectra, we have found multiple spectra from the G2 encounter that have easily recognized thermal emission but are not designated NIMS hot spots. We also see evidence of thermal emission at wavelengths shorter than 4.8 um affecting the SO2 frost band depths at 4.07 and 3.35 um. References: Carlson, et al., Geophys. Res. Let., 24,2479-2482, 1997. Goguen, et al.,(abstract), Bull. Am. Astron. Soc., 27, 1153, 1996. Davies et al., LPSC XXVIII (abstract), p283, 1997. Lopez-Gautier et al., Geophys. Res. Let., 24, 2439-2442, 1997. Smythe et al., (abstract), AGU, San Francisco, Fall, 1997. Veeder et al., J. Geophys. Res., Planets 99, 17095-17162, 1994. This work was done at JPL/ CALTECH, under contract to NASA.