DPS 2001 meeting, November 2001
Session 47. Icy Galilean Satellites II: Surface Composition
Oral, Chair: D. Domingue, Friday, November 30, 2001, 3:00-4:00pm, Regency GH

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[47.04] New Observations of Molecular Oxygen on Europa and Ganymede

J. R. Spencer (Lowell Observatory), A. Klesman (M.I.T.)

One of the recent surprises in studies of the icy Galilean satellites has been the discovery of numerous volatile species on their surfaces, including CO2, O2, O3, H2O2, SO2, and others. The first of these to be discovered, and the only one with absorptions in the visible part of the spectrum, is O2, which was first identified on the trailing hemisphere of Ganymede (Spencer et al. 1995, J. Geophys. Res. 100, 19049). The O2 absorption bands, at 5773 and 6250 Å, have a maximum depth of only 1.7%, and require high-density, condensed O2 for their formation.

New observations with the HST STIS spectrograph have provided near-global mapping of O2 band strength. The new observations have shown that O2 is concentrated at low latitudes, away from the polar caps, at all longitudes on Ganymede, extending previous observations of low-latitude O2 concentration on the trailing side (Calvin and Spencer 1998, Icarus 130, 505). One surface location shows stronger O2 band depth in the afternoon than in the morning, due either to temporal variability or effects of viewing geometry. Recent modeling of plasma bombardment of Ganymede (Cooper et al. 2001, Icarus 149 133) suggests that its magnetosphere may not prevent significant trailing hemisphere concentration in plasma bombardment, so irradiation may explain the trailing-hemisphere concentration of the O2.

We have also detected a much weaker molecular O2 band on Europa. Maximum band depth is only about 0.3%, making the band difficult to study, but (in contrast to Ganymede), the band appears to be present in similar strength on both leading and trailing hemispheres of Europa. The presence of O2 on Europa confirms the highly oxidizing nature of its surface, and provides a potential source of chemical energy for any life in the Europan ocean, if there is a pathway for surface materials to reach that ocean (Chyba 2000, Nature 403 381).

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