From W. R. Corliss's book Mysterious Universe, a handbook of astronomical anomalies (1979), I know about certain telescopic observations performed while Mars was at opposition. During such circumstances several bright spots were seen at the limb of Mars. Could it be that the halo phenomenon Countersun was responsible for these bright spots? (read: sunlight reflected by ice crystals in the thin Cirrus clouds of the Martian atmosphere). See also: retro-reflection of sunlight shining in corner-cube (or cube-corner?) shaped ice crystals.
The Countersun effect (a bright diffuse white spot at 180° from the true sun) is sometimes visible from airliners flying above Cirrus clouds, and perhaps also from spacecraft in orbit around Earth (from the I.S.S. for example).

Are such bright spots also observed at the limb of Jupiter? (Jupiter in opposition).
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