... The seeing had improved considerably by then and the moon looked like a cream colored three dimensional orb. Once it cleared the limb, it reverted to appearing like a 2-D disk...
Perhaps this is due to the line of sight specular reflection of Jupiter limb "light" illuminating Europa's limb on one side, while Europa's other side doesn't get the same contribution, being backed by a greater percentage of space?
One test of this hypothesis would be that decreasing aperture would yield a briefer period of the effect, due to lack of angular resolution necessary to perceive such.
The human eye is incredibly sensitive to "differential" effects, both in the subjective (time invariant cognitive) and perhaps other (time varying subconscious). The first case is like shading on a ball, the second is that our vision detect that a pattern is changing yet not in a catagorizable / "above threshold" way.
A lot of time with DSO's near the threshold, you know somethings there but can't quite perceive it.
Anyways, I've seen this effect quite a lot with Jupiter. Have often thought that because of Jupiter's surface patterns being so variable, specular variation in reflection might make it easiest to see here among any of the planets.
Thank you Ed for the observation and the nice sketch.