Hexagonal Prisms Form in Water-ice Clouds on Mars, Producing Halo
Displays Seen by Perseverance Rover
Abstract
Observations by several cameras on the Perseverance rover showed a 22°
scattering halo around the Sun over several hours on the morning of sol
292 (15 December 2021). Such a halo has not previously been seen off
Earth. The halo occurred during the aphelion cloud belt season and the
cloudiest time yet observed from the Perseverance site. The halo
required crystalline water-ice cloud particles in the form of hexagonal
columns large enough for refraction to be significant, at least 11 µm in
diameter and length. Near 44 km altitude, fall speeds would have been
0.3-1 m/s for the smallest allowed particles. Over the 3.3-hour duration
of the halo, particles could have fallen 3-12 km, causing downward
transport of water and dust. Halo-forming clouds are likely rare due to
the high supersaturation of water that is required but may be more
common in northern subtropical regions during mid-northern summer.