formed by clouds of frozen carbon dioxide shine in a dusty Martian
sky. Familiar water-ice crystals form the inner 22° halo. CO2
cuboctahedral crystals produce the next of 26° radius and the outer
39° halo is from octahedra and cuboctahedra. Cuboctahedral platelike
crystals generate multiple parhelia and the bright uppermost arc.
Painting & HaloSim3 simulation by Les Cowley
The thin Martian atmosphere is mostly carbon dioxide with only minute
traces of water vapour. Each Martian year at the winter pole it gets
cold enough (minus 123°C) that some of the atmosphere condenses into
tall clouds of CO2, 'dry ice' crystals.
As winter deepens, CO2 snow falls and covers
the polar surface until springtime with a dense 2m thick layer. The
wax and wane of the 'ice' caps can be watched from Earth with a small
Could Mars have CO2 crystal halos? CO2
crystals are sufficiently transparent. They would also have to be
reasonably large enough and optically good. Clouds over the winter
pole are mostly in darkness but elsewhere, high in the atmosphere,
it is sufficiently cold for cirrus like CO2
clouds. Many conditions need be met but Mars might sometimes
have exotic and colourful halos quite unlike those of Earth.
Michael Schroeder and myself used an early version of the HaloSim
software to predict them and the Mars Polar Lander launched in January
'99 was scheduled to search for halos as part of its multi-spectral
sky imaging program. Observation of even fragments of halos would
tell us more about Martian clouds and climate. However, after an 11
month voyage, contact with the Lander was lost after it entered the
Martian atmosphere. We shall have to wait longer to see halos in the
skies of another world ...
More details in:
Martian Halos by Les Cowley & Michael Schroeder,
& Telescope, Dec '99. pp 60-4.
'Vorhersage von Mars-Halos',
Meteoros, Jahrgang 3 Nr.6/2000