Latitudinal Distribution of O2 on Ganymede

John Spencer, Lowell Observatory; Wendy Calvin, USGS Flagstaff; Robert Johnson, U. Virginia

In 1993 we discovered two weak absorption bands in the visible spectrum of Ganymede, at 5770 and 6250 A, and identified them as due to condensed O2 (Calvin and Spencer 1994, Spencer et al. 1995). The O2 feature is strongly concentrated on the trailing hemisphere, suggesting an origin by magnetospheric bombardment (Calvin et al. 1996), and suggesting that bombarding ions can reach the surface despite the presence of a magnetosphere (Kivelson et al. 1996). The bands require pairs of molecules in close proximity and thus are an indicator of high-density O2. Oxygen's high vapor pressure makes it unlikely to be stable as a surface frost, so its presence on Ganymede is surprising.

Groundbased observations cannot investigate the latitudinal distribution of the O2, which might hold clues to its formation and stability on Ganymede's surface. In 1995 we therefore used HST to map the distribution of O2 in latitude on the trailing side of Ganymede, using both the faint-object spectrograph and narrowband imaging with the Wide-Field Planetary Camera. Both experiments showed that the oxygen bands are concentrated at low latitudes, and are absent at latitudes greater than 60 degrees north or south (Figure 1, and Calvin and Spencer 1997). This is in contrast to the polar concentration of O3 seen in initial Galileo observations (Barth et al. 1996). The presence of the O2 at warm low latitudes makes it unlikely that it is adsorbed on the surface, and trapping in subsurface bubbles is more likely.

Figure 1. HST WFPC2 narrowband images of Ganymede's trailing side, showing the equatorial concentration of the 6250 A O2 absorption band. North is at the top. A. Continuum image of Ganymede. B. 5894 / 6590 A continuum ratio image, showing the correlation between low albedo regions and red regions, which appear darker in this ratio. The bright limb is an artifact. C and D. Two independent (6306 A / continuum) ratio images. Darker areas have a stronger 6250 A O2 absorption band. E. The average of frames C and D, smoothed to reduce noise. Black (low latitudes) corresponds to an 6250 A O2 band depth of 0.014, and white (polar regions) corresponds to a band depth of zero. From Calvin and Spencer 1997.

It is possible that O2 occurs everywhere in Ganymede's ice, produced by magnetospheric bombardment or some other process such as UV photolysis, but that it only becomes visible in regions where bubbles form in the ice (Johnson and Jesser, 1997). Bubble formation would provide sites for concentration of the O2, bringing molecules close enough together to generate the 5770 and 6250 A bands. Bubbles may be produced by magnetospheric bombardment, and form most readily at higher temperatures (Johnson and Jesser 1997), and so bubble trapping may explain both the equatorial and trailing-hemisphere concentration of the O2 feature.

List of References

Contact Information:

John Spencer
Lowell Observatory
Flagstaff, AZ 86001
Telephone: (520) 774-3358 x229
email: spencer@lowell.edu