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.
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.
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
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