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   Other Worlds  

  
Halos from octahedral ammonia crystals as might exist in the cold high level clouds of Jupiter and Saturn. The 42° circular halo has four associated sundogs. The inner halos are produced by rays reflected within the crystal.
The skies of other planets are too often portrayed with Earthlike halos - they are likely to be more outlandish! Can we say whether there are halos and what they would look like? If we know the crystal structures and optical properties of the crystals in their clouds we can indeed predict them.

Outwards from the sun, Earth is the first planet with significant clouds containing large crystals. Mars has clouds of potentially halo forming carbon dioxide crystals plus traces of water-ice cirrus. The gas giants Jupiter and Saturn have multidecked exotic clouds. There, ammonia crystals as well as ordinary water-ice could make halos shine. The colder giants, Uranus and Neptune, have methane-ice clouds and tiny Pluto might occasionally see halos from crystalline nitrogen and methane.

Moons too have atmospheres or eruptions of possible halo forming crystals. Volcanos on Jupiter's Io could produce monoclinic sulfur and other strange entities. The liquid nitrogen geysers on Neptune's Triton might give rare showers of nitrogen crystals. Far fetched? The exploration of the Solar System has so far shown it to be stranger than was ever imagined.