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Saturn's Moon Mimas


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Reappearance of Saturn's Satellite Mimas

On 10 August 1995 HST observed the second passage of the Earth through Saturn's ring plane in the 1995/96 series of triple crossings. This pair of 100 sec exposures was taken with the Wide Field and Planetary Camera (WFPC2) in planetary (high resolution) mode 2.5 hours prior to the Earth's passage through the ring plane. An 8922 A methane band filter was used to reduce the scattered light from the planet.

The rings appear as a razor-thin line extending from the planet's eastern limb out to the usually faint F Ring, at a radius of 140,000 km. Unlike ground-based images taken a few days earlier, the rings show no sign of clumps or bright spots at this time. The brightness of the rings in this view implies an effective `thickness' of about 1.5 km, although the actual ring thickness is much less than this - probably as little as 10 meters. The edge-on brightness of the rings is due to a combination of vertical waves in the rings, the F Ring, and perhaps other unidentified sources.

In August, the sun was 1.5 degrees above (ie., on the north side of) the ring plane. In the first image, the satellite Mimas is seen very faintly approximately 0.8 arcsec below the rings, in the shadow of the almost opaque B Ring. Three minutes later Mimas has brightened considerably as it is illuminated by sunlight shining through the relatively transparent Cassini Division between the A and B Rings. Epimetheus, the smaller of the two co-orbital satellites, is visible just above the ring plane and 0.6 arcsec from the eastern ansa.

Credit: Phil Nicholson (Cornell University), Mark Showalter (NASA-Ames/Stanford) and NASA

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