Acknowledgement. We thank Science for their permission to use an excerpt from:

Broadfoot, A. L., et al. 1981. Extreme Ultraviolet Observations from Voyager 1 Encounter with Saturn. Science 212 (4491), 206-211. (Excerpt from pp. 208-209.)

Copyright AAAS, April 10, 1981.

Extreme Ultraviolet Observations from Voyager 1 Encounter with Saturn

Rings. Resonance-scattered Ly alpha emission was detected from neutral hydrogen gas associated with Saturn's rings. No other emissions have been detected in the examined data sets. Upper limits for several strong oxygen lines that are reasonable candidates for excitation in any electrical discharge type phenomena (11) associated with the rings are O I (989 Angstroms), O II (834 Angstroms), O III (703, 834 Angstroms) < 0.6 R, and O I (1304 Angstroms) < 2.6 R.

A scan across the rings (Fig. 3) shows a highly variable Ly alpha signal from the sky background transmitted by the rings and a neutral hydrogen ring atmosphere. Contamination by the sky background can be avoided by using observations of the B ring taken at grazing aspect so that the effective optical depth is greater than 3 (13, 14). The Ly alpha albedo of the B ring does not contribute significantly; the B ring's sunlit and dark sides have almost identical brightnesses. Thus the Ly alpha brightness of 360 R measured under these conditions must be resonance-scattered by H atoms between the spacecraft and the rings.

The limb drift observations show no Ly alpha emission (over the level of the sky background plus torus) far above the limb of Saturn, so this hydrogen probably comes from the ring particles. If the hydrogen is distributed in a torus having a circular cross section of radius 1 R_S, then an H column density of 1e13 cm is required for optically thin conditions. For a uniformly distributed cloud, the number density is 600 cm^-3 and the content of the cloud is about 5e33 atoms.

This density is similar to previous measurements. A density of 400 cm^-3 as derived by interpretation (15) of a rocket observation (2). This large amount of hydrogen is difficult to explain by proposed source mechanisms (16, 17), suggesting that it may be necessary to reexamine the H loss mechanisms, in particular the sticking coefficients for H on H_2 O ice.

At present the Ly alpha signature of the rings is difficult to reconcile with the Pioneer 11 ultraviolet photometer observations (8). This is probably due in part to the different nature of the instruments, the observational techniques, and the viewing geometry provided by the different trajectories.

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