Abstract

Some of the clouds observed on Mars are composed of carbon dioxide ice and appear as uniform thin layers¹. On Earth, similar layers of water ice clouds produce a 22 degree halo and associated parhelia, known as sundogs. Will the CO₂ ice clouds of Mars produce halos and sundogs? If so, at what angles? To calculate the angles at which halos might appear, the shape of CO₂ ice crystals on Mars and their index of refraction are needed. Crystals of CO₂ ice were grown in a laboratory chamber under Martian atmospheric conditions; atmospheric pressure² of 0.73 kPa, and CO₂ sublimation temperature³ of 148 K. A literature search produced an estimate of the index of refraction of CO₂ ice under these same conditions. Angles of CO₂ ice crystal halos were then calculated to be 26±3 and 38±5 degrees from the sun.

© 1986 Optical Society of America