LATITUDINAL DISTRIBUTION OF O-2 ON GANYMEDE - OBSERVATIONS WITH THE HUBBLE-SPACE-TELESCOPE

To help constrain the spatial variation of oxygen on Jupiter's satelli te Ganymede, and hence have more clues to its mode of production and s tability, we have obtained spectral data from the Faint Object Spectro graph (FOS) for a single pole-to-pole latitudinal strip, along with se veral Wide Field Planetary Camera 2 (WFPC2) images in three narrow ban d visible filters. All observations were made of the trailing hemisphe re. In the FOS data we observe both visible absorptions at 0.577 and 0 .627 mu m, associated with dense-phase oxygen (compressed gases, liqui ds, or solids). Filter options limited the WFPC2 observations to wavel engths near the weaker oxygen absorption at 0.627 mu m. These observat ions suggest that the dense-phase or dimer oxygen form is predominantl y found in equatorial and mid-latitudes. The spectroscopic absorption feature appears in both bright and dark terrains but may be somewhat w eaker in dark regions, which is consistent with the smaller mean photo n path length in the surface in darker areas. Therefore, the abundance of oxygen appears more dependent on latitude and longitude constraint s than surface albedo, At the highest latitudes, where the ratio spect ra have a strong upturn toward the blue, the oxygen bands do not appea r. This relation suggests that dimer oxygen and ozone (as seen by Gali leo) have opposite trends with latitude. Possible causes include compe tition or variation in the preferred stable form, which depends on tem perature, solar ultraviolet flux, and/or surface age; enhancement of O -3, at the poles due to plasma interactions; or viewing geometry effec ts that reduce the oxygen features at the poles when observed from Ear th. The predominantly equatorial feature supports the production of O- 2, through plasma bombardment and favors defect trapping over physical adsorption of the dimer molecules in the surface. We briefly consider the implications of Ganymede's magnetosphere for our understanding of O-2, and O-3 distribution on Ganymede. (C) 1997 Academic Press.